The pressure of a gas is directly proportional to its temperature in accordance with the basic principle of the universal/perfect gas law:  PV = nRT where P = absolute pressure, V = volume, T = absolute temperature, R = universal gas constant and n = number of gas particles (moles).

Description

All filled-system temperature measurement instruments consists of a bulb, connecting tubing known as “capillary,” and a pressure sensing element, usually a bourdon tube.  All commercially available filled system thermometers have been classified by ASME B40.200 (ASME B40.4).  The standard classifies filled-system thermometers by the type of fill fluid used (liquid, vapor, gas) and further subdivided by the type of temperature compensation.  The different types of filled systems are identified by “Class Numbers”, ranging from 1 through 5, refer ASME B40.200 for more details.

Bulb Design

The bulb volume varies over a range of 100 to 1 depending on the fill fluid, the temperature span, and the capillary length.  Long bulbs give an average temperature and are sometimes used in stretched-out form for gas ducts.

Different bulb materials are available. However when used with a Thermowell, standard material such as copper, bronze, or stainless steel can be used. In case   atmospheric corrosion, Stainless steel is preferred. If a well is not used (not recommended), the bulb must suitable for process fluid.

Capillary Tubing & Armoring

Capillary tubing is small-diameter tubing, usually of stainless steel. Armor should always be specified not to provide only mechanical strength but also distinguish with other tubing. Armor material shall be stainless steel. However in a corrosive atmosphere, the armor should be plastic coated. The length of capillary shall be carefully selected and specified so that instrument can be installed as per requirement, since it cannot be stretched or spliced.

Filled System Temperature Measurement

Mounting the Bulb

Of the many mounting styles available for installing a bulb, the one generally recommended has a bulb with an adjustable union and bendable extension.  The bendable extension is usually of smaller diameter than the bulb, thus reducing conduction error. The extension with adjustable union permits sliding the bulb against the bottom of its well to make solid contact and give improved thermal performance.

Temperature Compensation

Since the thermal sensing fluid extends from the bulb to the pressure element, it is affected by the temperatures existing everywhere in the system – bulb, capillary, and pressure element.  The temperature sensitivity of the fluid in the bulb is the essence of the measuring system; in other parts of the system, the sensitivity is a characteristic that may or may not cause significant error. Vapor-pressure systems, Class 2, are not subject to errors from this cause because the system pressure depends only on the temperature of the liquid/vapor interface, which is in the bulb; the volume of the bulb chamber is temperature-sensitive but to a negligible extent.

For Class 1, 3, and 5 systems, errors of varying extent can be caused by ambient temperature.  The need for compensation is a function of ratio of bulb volume to total system volume, length of capillary, ambient temperature, measuring range, and need for accuracy.  Case compensation corrects only for variations of case temperature.  Full compensation corrects for variations of the temperature of the capillary and case. Case compensation is usually achieved by installing a bimetallic strip into the instrument case and attaching it to the pressure sensing element. Full compensation is accomplished by installing an auxiliary capillary without bulb parallel to the primary/main capillary and connecting it to the measuring mechanism.

The decisions as to whether to specify compensation and which type to use may be handled in either of the following ways:

• The preferred and most reliable way is to order Class 1 or 3 instruments with full compensation, that is, as Classes 1A or 3A.  Class 2 instruments do not require compensation
• For economy:  In addition to the usual information included in an instrument specification, state (1) the ambient temperature ranges for the capillary and the case, and (2) the required accuracy.  The Manufacturer can then select the proper compensation, if any is required

Hydrostatic/Head Errors

In as much as a filled-system instrument works through the medium of a fluid, it is subject to pressure variations, therefore zero-shift errors, plus or minus, occur merely by changing the relative elevations of the bulb and pressure element.  The magnitude of the error is a function of the filling pressure, the type of filling fluid, the phase of the filling fluid, and the elevation offset, and it may vary from zero, for Class 2B, to large for some other classes.  Reference should be made to Table 1 and manufacturer’s literature so that the specification for an instrument may include information about elevations, if needed, so that proper compensation may be incorporated in the instrument.  It is best to avoid the need for head compensation, everything else being equal.

External / Atmosphere Pressure Errors

External pressure may cause measuring errors because the pressure element measures the gage pressure of the filling fluid.  Barometric pressure change has negligible effect on the liquid-filled systems, Classes 1 and 5, which are at relatively high internal pressure.  Vapor and gas systems, Classes 2, 3 and 4, are affected by barometric pressure change but the resulting error usually does not exceed 0.4 percent of span.

Thermowells are used to protect temperature sensing elements such as Thermocouples, RTD’s, Bimetallic Thermometers, and Filled System Bulbs from breaking if they are installed in a tank, vessel, or pipe.  Thermowells also allow a temperature measuring element such as a thermocouple, RTD, or bulb to be removed for servicing without the requirement that the plant be shutdown. Sometimes “Protection Tubes” instead of thermowells are used to protect thermocouples. These protection tubes are usually fabricated of ceramic materials and they are installed in high temperature flue gas and duct applications.  Protection tubes are also available made of Inconel and with their ends open in order to improve speed of response and heat transfer.

Thermowells are used for almost all applications except for some specific applications (that is, for very-low-pressure air systems, for ventilating systems, bearing temperature measurement, lube oil drain in compressor and so on) where faster response required or space limitation such as casing drain in compressor. Temperature elements can be installed without Thermowells in following applications or condition exist,

• The process fluid is not corrosive or otherwise hazardous.
• The process is not under significant pressure.
• Air-in leakage is permissible.
• The primary element has the necessary static and dynamic mechanical strength for the application.
• Either a failure of the element may be tolerated until the process can conveniently be shut down, or operation can continue without the use of the element.

There is no personnel hazard if the thermocouple is inadvertently removed

In case if test Thermowells are required, then devices with suitable plugs shall be provided. Design of all the Thermowells should be same for all elements (i.e. for bimetallic gauges, RTDs or thermocouples) so that interchangeability can be obtained. Factors to be reviewed while choosing Thermowells are resistance to corrosion, flow induced vibrations, erosion, ease of installation, preferred installation point of Thermowell, insertion length, material, velocity considerations, lagging extension etc.

Standard Thermowells

The Thermowells are available with following types of connections:

• Weld-in Type (W) Thermowell
• Socket Weld Type (S) Thermowell
• Threaded Type (T) Thermowell
• Flanged Type (F) Thermowell

Flanged type Thermowells are most commonly used in all Oil & Gas industries. The drawing for this type of Thermowell is shown in the figure below.

Standard Thermowells should be stamped with the ASTM material designation, “U” (insertion length) dimension, Rating and the tag number.

Thermowell Installation

Thermowells installation shall be per Piping & Instrumentation Interface Diagram or as specified in the piping specification. The location for a Thermowell should also be checked with respect to accessibility.  In particular, there should be sufficient space to allow removal of the Thermowell assembly.

In-case if Thermowells are installed in pipe lines which are less than 4”, then a section of this pipe line shall be expanded to 4”. This expansion (3” or 4”) shall be decided based on specific project requirement.

Thermowells can be installed in Tee at a 90-degree pipe elbow in case of smaller pipe sizes.

The above typical flanged Thermowell drawing is only for reference. Construction of he device shall be as per specific project requirement. Care shall be taken while specifying teak weld or full penetration weld for flanged Thermowells based on flange ratings/pipe ratings.

Different variation of construction is available. In case if there is no specific and clear requirement, API construction can be followed.

Thermowell Insertion Length

Thermowell insertion length should be sufficient to measure the bulk temperature of the fluid without conduction effect. A Thermowell installed perpendicular or at a 45-degree angle to the pipe wall should have a minimum immersion length of 2” and a maximum distance of 5” from the wall of the pipe.

If the Thermowell is installed at an angle or in an elbow, the tip should point toward the flow in the process line.

Thermowells installed in lines through which fluid flows at a high velocity may be subject to vibration, which can rupture the well below the mounting. Tapered stems and U lengths established by means of stress analysis are recommended for high velocity.

On Equipment or Vessels minimum immersion length should be between 300 to 500mm based on requirement.

While deciding insertion length, nozzle stand out, insulation thickness, pipe thickness etc shall be taken in account. This activity should be coordinated with mechanical/ Piping to avoid any mismatch during selection and installation.

Special applications such as agitated vessels, separation columns, etc., with internals, may require shorter Thermowell lengths and should be checked for interference.

Thermowell Material

The Thermowell material is selected as per piping class and specification. For better accuracy in temperature measurement, Thermowells must have a smooth, shiny surface with no pitting and corrosion.

Insulation

Pipe insulation or lagging extension is the extra length provided between the Thermowell piping connection and the temperature element connection. This extra length is provided to ensure the temperature element connection is located outside the insulation and for ease of temperature element installation or removal. This extra lagging extension length shall be taken into account while deciding Thermowell insertion length.

Thermowell Applications

• Thermowell Flange Material Different than Pipe Material: In case, if material of construction (MOC) of Thermowell is different than the pipe class material, the flange rating of Thermowell shall be suitable and withstand pipe line design condition that is, pressure temperature. For example, if pipe class is Carbon Steel (CS) and MOC for Thermowell is SS316, then Instrument Engineer should check the rating required for Thermowell based on line design pressure & temperature.
• Incase if the flange rating of Thermowell is different than the pipe rating, it should be communicated to Piping and Process to take necessary action at their end (provide nozzle with same rating as Thermowell rating).
• Requirement of Insulating Gasket shall be communicated to Mechanical & Process in case material of construction for Thermowell is different than piping material.
• In case if Thermowells are with collar to protect them from failure due to high vibration, the nozzle ID shall be checked with respective to collar diameter. This information should be communicated to Mechanical to avoid any mismatch during installation.
• Care shall be taken when Thermowells are selected with collars for lined vessels & cladded vessels. In these cases nozzle ID shall be checked with equipment drawings to avoid mismatch of Thermowell during installation.
• Requirement of Thermowell connections (Flanged, Threaded or Welded) shall be checked as per specific project requirement.
• NDE requirement for the device shall be checked and specify as per piping requirement.
• When the Thermowells are installed on the equipment & vessel, care shall be taken while deciding insertion length of these Thermowells to avoid fouling with equipment/vessel internals. Equipment/vessel internal arrangement drawing & nozzle location shall be reviewed by Instrument engineer to avoid any mismatch. 

Multipoint Temperature Element with Thermowell

Multipoint temperature elements (RTDs or Thermocouples) are used for tank gauging system & process reactors. The purpose of these elements is to measure the temperature at different location or different levels.

In the tank gauging system, the requirement is to measure fluid temperature at different level, which can be obtained by installing multipoint temperature elements assembly, which consist of number of elements of different length in one Thermowell. This device is prefabricated and installed in the tank by tank supplier. The numbers of temperature elements (i.e. 4, 8,10,12,16 etc) shall depend upon tank height & process requirement. The figure below shows a typical drawing for this type temperature element.

In case of process reactors, the device made from pipe is used to installed 3 or 4 temperature element of different length (to measure temperature at different location on reactor diameter) to measure temperature of chemical bed at specific location.

In some reactors requirement is to measure temperature of chemical bed at different location and different levels. In this case thermocouple/coupling assembly of required elements (8, 10,12,14,16 etc) inserted/installed on reactor by clamping the elements inside reactor. Care shall be taken while deciding the insertion length of element. The length also depends upon routing of these elements in reactor.

A resistance temperature detector (RTD) can also be called a resistance thermometer as the temperature measurement will be a measure of the output resistance.

The main principle of operation of an RTD is that when the temperature of an object increases or decreases, the resistance also increases or decreases proportionally. The main difference between a RTD and a Thermistor is that the sensing element used in a RTD is a metal and a thermistor uses ceramic or polymer material. As platinum is the most commonly used metal for making RTD’s, the device can also be called Platinum Resistance Thermometers (PRT’s).

RTD Types

RTD types are broadly classified according to the different sensing elements used. Platinum, Nickel and Copper are the most commonly used sensing elements.  Platinum is considered the best as it has the widest temperature range. This is shown in the resistance versus temperature graph below. Platinum type RTD is also known for its best interchange ability than copper and nickel. It also has the highest time stability. PRT’s can also be used in unsuitable environments where it can reduce atmospheric metallic vapours and also catalizable vapours if the element is bare.  It can also be used in radioactive environments. In industrial applications, a PRT is known to measure temperatures as high as 1500 degree Fahrenheit while copper and Nickel can measure only to a maximum of 400 degree Fahrenheit.

RTD-Resistance Versus Temperature Graph

RTD Styles

RTD’s are available with single, double, or triple windings, each electrically separated.  Use of more than one winding enables two independent measuring circuits to measure the same temperature, and also permits more than one measurement to be made with only one sensor installation.  However, the additional mass introduced to the sensor by adding windings and their associated support and encapsulating materials increases both the response time and the conduction error.  Using separate sensors provides mechanical independence of the sensors for maintenance.

RTDs should generally be of spring-loaded, tip-sensitive construction, with a 1/4-inch-diameter sheath.

RTD Wiring Arrangements

RTD’s are available with either two, three, or four output wires for connection to the secondary instrument as shown in the figure below.  The various wiring arrangements are designed to reduce and/or eliminate any errors introduced due to resistance changes of the lead wires when they also undergo temperature changes.  RTDs used for electrical equipment generally use either a three-wire system or a four-wire system having paired lead wires.

Copper lead wires are satisfactory for all the arrangements.  For a given RTD, all the lead-wires should be of the same gauge and the same length, and should be run in the same conduit.

The four wire system is little affected by temperature induced resistance changes in lead-wires, and, of all the arrangements, it is affected least by stray currents.  It, therefore, is used to measure temperature differences and is used generally for making very accurate measurements. The three-wire system is generally satisfactory for industrial measurement using a secondary instrument that is remote, say, more than 3 meters distant from the RTD.  Although the error caused by temperature change in the leads is virtually eliminated in a 3-wire arrangement, a slight non-linearity in the resistance change is introduced with this scheme.

Resistance Temperature Detector-(RTD)-2 Wire,3 Wire,4 Wire Systems

Power Supply for RTD

An electric dc power supply is required to provide current for the resistance­ measuring circuit.  The power supply is normally applied through the secondary instrument.  If the secondary instrument is a transmitter having a current output of (4-20) mA, then the power is carried by the two output wires of the transmitter.

RTD Connection Head

Unless a transmitter is mounted on the Thermowell, the sensor should be connected to a connection head generally like that for thermocouples except as follows:

• For a single RTD, the terminal block should be able to handle four lead wires.
• The head shall be explosion proof where and as needed to conform to a hazardous area rating.  However, explosion proofing will not be required if the system is intrinsically safe. In this case the thermocouple head should be specified to be weatherproof.

RTD Grounding

The principles for grounding that are stated in “Grounding” for Thermocouples apply to RTDs, with the exception that the sensitive portion, the resistance wire, of a RTD is never grounded because it must not be shorted.  A RTD in a power device, such as a transformer, should be grounded locally; otherwise, RTDs are normally grounded at the power supply.  A power supply and all its associated RTDs should be grounded at only one point.  If local grounding is required for a RTD, then an individual power supply is required for this RTD.

RTD Shielding

The RTD shielding principle is the same as that of Thermocouple Shielding.

Transmission of RTD signals

The transmitter is the most commonly used instrument for transmission of RTD signals. A transmitter may be mounted either on an enclosed rack or locally.  A local transmitter may be mounted on a Thermowell and supplied with it as a complete assembly. The most commonly used RTD transmitter is the so-called “Smart” transmitter.  A typical “Smart” temperature transmitter is remarkably versatile:  It is suitable for Platinum and Nickel RTDs; 2, 3, or 4 lead wire arrangements; 100, 200, or 500 ohm Platinum sensors, etc.  This same instrument can also be used as a thermocouple transmitter, suitable for every thermocouple combination commercially available.

Advantages

• Very high accuracy
• Excellent stability and reproducibility
• Interchangeability
• Ability to be matched to close tolerances for temperature difference measurements.
• Ability to measure narrow spans
• Suitability for remote measurement

Disadvantages

• Susceptibility to mechanical damage
• Need for lead wire resistance compensation
• Sometimes expensive
• Susceptibility to self-heating error
• Susceptibility to signal noise
• Unsuitability for bare use in electrically conducting substance
• Generally not repairable
• Need for power supply

As discussed earlier, an Optical Pyrometer can be not only be used for temperature measurement, but also can be used to see the heat that is measured. The observer is actually able to calculate the infrared wavelength of the heat produced and also see the heat patterns by the object. But the amount of heat that the device can sense is limited to 0.65 microns. This is why the radiation pyrometer is more useful, as it can be used to measure all temperatures of wavelengths between 0.70 microns and 20 microns.

Radiation Pyrometer

The wavelengths measured by the device are known to be pure radiation wavelengths, that is, the common range for radioactive heat. This device is used in places where physical contact temperature sensors like Thermocouple, RTD, and Thermistors would fail because of the high temperature of the source.

The main theory behind a radiation pyrometer is that the temperature is measured through the naturally emitted heat radiation by the body. This heat is known to be a function of its temperature. According to the application of the device, the way in which the heat is measured can be summarized into two:

1. Total Radiation Pyrometer – In this method, the total heat emitted from the hot source is measured at all wavelengths.
2. Selective Radiation Pyrometer – In this method, the heat radiated from the hot source is measured at a given wavelength.

As shown in the figure below, the radiation pyrometer has an optical system, including a lens, a mirror and an adjustable eye piece. The heat energy emitted from the hot body is passed on to the optical lens, which collects it and is focused on to the detector with the help of the mirror and eye piece arrangement. The detector may either be a thermistor or photomultiplier tubes. Though the latter is known for faster detection of fast moving objects, the former may be used for small scale applications. Thus, the heat energy is converted to its corresponding electrical signal by the detector and is sent to the output temperature display device.

Advantages

• The device can be used to measure very high temperatures without direct contact with the hot source (Molten metal).
• The biggest advantage is that the optical lens can be adjusted to measure temperature of objects that are even 1/15 inch in diameter and that too kept at a long s=distance from the measuring device.
• The sight path of the device is maintained by the construction of the instrument components, such as the lens and curved mirrors.

Pyrometer

A pyrometer is a device that is used for the temperature measurement of an object. The device actually tracks and measures the amount of heat that is radiated from an object. The thermal heat radiates from the object to the optical system present inside the pyrometer. The optical system makes the thermal radiation into a better focus and passes it to the detector. The output of the detector will be related to the input thermal radiation. The biggest advantage of this device is that, unlike a Resistance Temperature Detector (RTD) and Thermocouple, there is no direct contact between the pyrometer and the object whose temperature is to be found out.

Optical Pyrometer

In an optical pyrometer, a brightness comparison is made to measure the temperature. As a measure of the reference temperature, a color change with the growth in temperature is taken. The device compares the brightness produced by the radiation of the object whose temperature is to be measured, with that of a reference temperature. The reference temperature is produced by a lamp whose brightness can be adjusted till its intensity becomes equal to the brightness of the source object. For an object, its light intensity always depends on the temperature of the object, whatever may be its wavelength. After adjusting the temperature, the current passing through it is measured using a multimeter, as its value will be proportional to the temperature of the source when calibrated. The working of an optical pyrometer is shown in the figure below.

Optical Pyrometer - Working

As shown in the figure above, an optical pyrometer has the following components.

1. An eye piece at the left side and an optical lens on the right.
2. A reference lamp, which is powered with the help of a battery.
3. A rheostat to change the current and hence the brightness intensity.
4. So as to increase the temperature range which is to be measured, an absorption screen is fitted between the optical lens and the reference bulb.
5. A red filter placed between the eye piece and the reference bulb helps in narrowing the band of wavelength.

Working

The radiation from the source is emitted and the optical objective lens captures it. The lens helps in focusing the thermal radiation on to the reference bulb. The observer watches the process through the eye piece and corrects it in such a manner that the reference lamp filament has a sharp focus and the filament is super-imposed on the temperature source image. The observer starts changing the rheostat values and the current in the reference lamp changes. This in turn, changes its intensity. This change in current can be observed in three different ways.

1. The filament is dark. That is, cooler than the temperature source.

2. Filamnet is bright. That is, hotter than the temperature source.

3. Filament disappears. Thus, there is equal brightness between the filament and temperature source. At this time, the current that flows in the reference lamp is measured, as its value is a measure of the temperature of the radiated light in the temperature source, when calibrated.

Advantages

1. Simple assembling of the device enables easy use of it.
2. Provides a very high accuracy with +/-5 degree Celsius.
3. There is no need of any direct body contact between the optical pyrometer and the object. Thus, it can be used in a wide variety of applications.
4. As long as the size of the object, whose temperature is to measured fits with the size of the optical pyrometer, the distance between both of them is not at all a problem. Thus, the device can be used for remote sensing.
5. This device can not only be used to measure the temperature, but can also be used to see the heat produced by the object/source. Thus, optical pyrometers can be used to measure and view wavelengths less than or equal to 0.65 microns. But, a Radiation Pyrometer can be used for high heat applications and can measure wavelengths between 0.70 microns to 20 microns.

Disadvantages

1. As the measurement is based on the light intensity, the device can be used only in applications with a minimum temperature of 700 degree Celsius.
2. The device is not useful for obtaining continuous values of temperatures at small intervals.

Applications

1. Used to measure temperatures of liquid metals or highly heated materials.
2. Can be used to measure furnace temperatures.

A thermocouple is a temperature sensor that has a pair of dissimilar metals joined together at one end and terminated at the other end. The joined end is called the sensing junction or hot junction and the terminated end is called the reference junction or cold junction. The temperature at the reference junction is called reference temperature and is always maintained constant.  When the sensing junction and the reference junction are at different temperatures, a potential difference is obtained and this causes a flow of current in the circuit. The thermoelectric voltage produced is due to the different binding energies of the electrons to the metal ions. This voltage depends on the metals themselves, and in addition on the temperature. The thermal voltage is produced only because of the closed circuit between the two metals. This phenomenon is called “Seebeck Effect”.

The wires must be electrically separated beyond the measuring junction.  If the reference junction is kept at a standard temperature, usually 32°F, then a given pair of metals will have a unique variation of EMF as the measuring-junction temperature is changed (note that at 32^o F there is no EMF generated).  This variation can also be called thermocouple calibration, and is shown in the figure below for its different types.

Thermocouple Measurement

Thermocouple measurement is explained along with a figure shown below. The figure shows a thermocouple circuit with T2 at 32°F (0° C). This temperature is maintained with the help of an ice-bath reference junction.  The thermocouple circuit ends in the ice-bath,  the generated EMF flow through standard copper wire until it reaches its final destination, – a millivolt meter type instrument.  The millivolt value sensed by this instrument is then converted into a temperature T1. Tables are available for every commercially used thermocouple material combination and they are based on the reference junction temperature of 0°C/32°F.

Thermocouple Types

There are a lot of thermocouple material types and these combinations are approved and standardized by the American National Standard ISA MC 96.1: “Temperature Measurement Thermocouples” in USA.  The standards must be followed for the different device designations as well as its color coding.

According to ISA MC96.1 standards, the thermocouple can be designated with the different combinations of letters like “E”, “J”, “K”, “T”, ”S”, ”R”. Four of the most popular thermocouple wire combinations are almost always identified by their trade names:

• Thermocouple Type E    is a combination of Chromel (Nickel-Chromium) and Constantan (Copper-Nickel).
• Thermocouple Type J is a combination of Iron and Constantan.
• Thermocouple Type K    is a combination of Chromel (Nickel-Chromium) and Alumel (Nickel Aluminium).
• Thermocouple Type T is a combination of Copper and Constantan.
• Thermocouple Type S is a combination of Platinum 10% rhodium and Platinum.
• Thermocouple Type R    is a combination of Platinum 13% rhodium and Platinum.

Extension Wires

Thermocouple wires are expensive because they are fabricated to very stringent quality control requirements.  Therefore, it is customary to change to so-called “thermocouple extension” wires at the closest (to the point of measurement or hot junction) convenient connection point.  These connection points must be isothermal to each other.  These thermocouple extension wires are less expensive because they are fabricated to less stringent quality requirements.

 Color Coding

In order to distinguish among different types of thermocouple wires their insulation is color-coded.as per the standards of ISA MC 96.1.The different thermocouple conductor combinations, their operating ranges, and color coding as per the international standards are shown below.

Thermocouple Junctions

A thermocouple used for industrial applications has three junctions. They are

1. Exposed Junction – This junction is directly exposed to the environment. As a result its life is very less. Though it has a very high response time, it is not frequently used.

2. Ungrounded Junction – This junction has very minimum response time, but is known for its incredible electro-magnetic shielding properties. It is used mostly for the measurements on electrical equipment, but is usually suitable for many process applications as well.

3. Grounded Junction – The response time of this is more than ungrounded junctions and also provides good shielding properties for most process applications.  It is preferred for most of the oil and gas industries and process industries for control applications because of its speed of response.

The figures of grounded junction, ungrounded junction and exposed junction are shown under the heading ‘thermocouple grounding’

Thermocouple Probes

Sometimes, a thermocouple has to be installed inside a Thermowell for providing protection. This is applicable only in cases where the temperature of a fluid flowing inside a pipe is to be measured. To do so, a probe is to be sheathed and enclosed in a corrosion resistant tube. Typically, the sheath should be a quarter inch in diameter and the probe should be spring-loaded so as to make a strong contact with the bottom of the Thermowell.  A screw-cover head is used for electrical connections.  In the probe the thermocouple wires are separated from each other and the sheath by means of ceramic insulation material such as Magnesium Oxide (MgO) or Aluminum Oxide (Al2O3).

Some variations of thermocouple probe are explained below.

Clamp-on Thermocouples – As the name suggests they are clamped to a pipe, and thus presses the measuring junction against the pipe. This method is applicable only in places where a temperature measurement is desired for which normal provision was not previously made.  This thermocouple may be useful in trouble-shooting a process. The output may not be so accurate unless the point of measurement and the surrounding area are well insulated from the outer environment. Another type of the clamp-on thermocouple is the skin couple that is used mostly to measure furnace tube and reactor surface temperatures. Skin couples can either be tack welded to the surface being measured (furnace tubes) or clamped on (reactor walls).

The figure below shows a washer type thermocouple that is to be installed on the pipeline and temperature transmitter for this application.

Many types of clamp-on and surface thermocouples are present in the market and they are selected according to the requirement. Pad type thermocouples are used for surface temperature measurement. Its application can be seen in temperature measurement of heater tubes. These are installed on pipe lines and heater tubes by means of welding.

Duplex Thermocouples – Duplex thermocouples are very similar to the conventional thermocouple except that this device has two pairs of thermocouple wires in the measuring junction. There may be two Alumel and two Chromel wires, joined together and the wires are brought out in two separate circuits to provide two individual measurements.  The two temperature readings should then be identical, assuming that both circuits are otherwise similar.  This may be useful for checking one instrument against the other, especially by using a test voltmeter locally to determine whether a remote reading is inaccurate because of the instrument or because of circuit problems.

As the whole device can be made in one assembly, the cost of production is very less. But, if either one of the two thermocouple circuits fail due to measuring junction problems or short circuit, the whole device will have to be replaced. Thus, it is less reliable than two single thermocouples. Furthermore, if one pair of a duplex thermocouple is used for checking, then there is a risk of shorting or grounding the other pair of wire.

Thermocouple Grounding

Thermocouple circuits can be either grounded or ungrounded (floating or insulated). Generally grounded thermocouple circuits are recommended for personnel safety, to reduce the effects of electrical noise and provide good thermal response characteristics. Ungrounded thermocouples should be considered for applications where equipments are likely to get damaged due to ground faults or lightening strikes particularly in tank farm areas.

Thermocouple grounding should be done to the low- or negative-potential side of the circuit, and should be done at the source rather than at the secondary instrument in order to achieve maximum rejection of common-mode noise.  There is a normal practice that any electrical or electronic circuit should be grounded at only one point s as to avoid a ground current in the circuit. With this rule in mind, thermocouple grounding is carried out in one of the following ways – It is always good practice to ground the thermocouple measuring junction or ground the thermocouple elsewhere than at the measuring junction.

Thus, thermocouples can be classified according to the manner in which they are grounded.

Thermocouples may be either:

• Intentionally grounded
• Intentionally ungrounded
• Unintentionally grounded – This is used in places where a bad contact or no contact of the measuring junction with a well occurs or because of formation of a high-resistance chemical film at the measuring junction.

The different combinations of thermocouple type, grounded intermittently /unintentionally grounded, or ungrounded; of secondary instrument type, isolated input/output or not; and of output grounding are shown in the figures below.

Points

• PT:1    Interconnection of shield and thermocouple is not applicable if shield is not required.
• PT:2    The ground may be located  anywhere on the line.
• PT:3   Grounding the thermocouple may be done by connecting to the thermocouple head or junction box screw, assuming that these are grounded. Otherwise, connect the thermocouple to any other point that is grounded.
• PT:4   Ground the thermocouple through a resistor if circuit operation shows a need for improvement in repeatability AND/OR noise rejection. If used, the resistor may be approximately 100,000  ohms, ½ watt, carbon type.
• PT:5   If the thermocouple, proper, is grounded, then the remainder of the circuit, from thermocouple to receiver, if any, shall not be grounded. Alternatively, if another ground exists, then the thermocouple, proper shall not be grounded.
• PT:6   If the secondary instrument is not a transmitter, but of a type, such as a recorder, that does not have a measurement output, then the output lines and associated  details on the diagram should  be ignored.
• PT:7   Ground the shield ,if any at that point that is closest to the signal ground.

Thermocouple systems used with DCS are shown below.

Shielding

The device could be interfered by external noises from different sources like electrostatic fields, magnetic fields and common mode interference. Electrostatic fields originate from voltage sources that are capacitively coupled to the thermocouple extension wire. Varying electrostatic fields, usually originating from AC conductors, produce a capacitive current flowing through the coupling path to the signal conductors.

The best way to minimize the interference of such electrostatic fields is to cover the thermocouple wires with a grounded metal shield. The capacitive current flow will flow through this shield to the ground.  The purpose of the shield is to remain at or near ground potential, and thereby couple no signal to the signal wires contained within the shield, since there is no difference in voltage.  Note that a shield that is not grounded provides no protection.

A varying magnetic field (such as the one produced by AC current in a power cable) may cause interference to thermocouple signals by generating small currents in the signal wire by induction.  The magnitude of the induced current is a function of the field strength and the dimensions of the conductive loop into which the current is being induced. Twisted conductors are effective at reducing the induced currents by alternating the polarity of the induced current with each half twist to cancel out most of the induction. Note this effect occurs whether the thermocouple wire is shielded or not.

Common mode interference generates noise that is identical in both conductors of a twisted pair with respect to ground.

Thermocouple Head and Connectors

The most frequently used thermocouple assemblies/probes are provided with a thermocouple head that is the screw-cover type; is weather-proof, high-temperature provided with gaskets; has terminal block for single or duplex thermocouple, as appropriate. The cable entry shall be as per specific project requirement. The terminal shall be ceramic based spring loaded type.

Advantage

• Fast Response
• Suitability for remote measurement
• Wide Range
• Freedom of effect by wire length and diameter provided that a high-impedance secondary instrument is used.

Disadvantage

• Need for cold-junction compensation
• Susceptibility to error from extension wire termination temperature gradient
• Possible sensitivity to signal noise
• Need for secondary instrument
• Need for thermocouple transmitter on long runs
• Need to avoid intermediate junctions of dissimilar metals
• Inability to accurately measure temperature over a narrow span

Thermopile

Like a thermocouple, a thermopile is also a device that is used to measure temperature in terms of electrical energy. The device is actually a combination of a number of thermocouples in series or parallel connection. Series connection is commonly used for most applications.

The device is capable of generating an output voltage, that will be a measure of the temperature difference or temperature gradient. Its response to absolute temperature is minimum. The output is in the range of millivolts.

The device finds its application as a part of temperature sensors, like the infrared thermometer, a device used to calculate body temperature. They are also used as safety controls in heat burners and heat flux sensors. They can also be used to produce electrical energy, that is, by dissipating the heat from electronic devices. It is also used for spatial temperature averaging.

Thermistor is a resistance thermometer, similar to a Resistance Temperature Detector (RTD) and is used for temperature measurement. It consists of a non-metallic resistor that is used as the temperature sensing element.

Thermistor is the short form for ‘Thermal Resistor’. The device consists of a bulk semiconductor device that acts as a resistor with a high and negative temperature co-efficient of resistance, sometimes as high as -6% per degree Celsius rise in temperature. Due to this property of high sensitivity (that is, huge resistance change for a small change in temperature), the thermistor is mainly applicable in precision temperature measurement, temperature control, and temperature compensation, especially in a lower temperature range of -100 degree Celsius to +300 degree Celsius.

Difference between Thermistor and Resistance Temperature Detectors (RTD)

Both devices work on the same principle that a constant current flowing through the resistor, and its changing resistance due to change in temperature, is measured as a voltage drop across it. The main difference is that the electrical resistance of the resistor used in a thermistor varies in a non-linear manner with respect to temperature. The sensing element used in the thermistor is made up of either a ceramic or polymer, while RTD uses pure metals as its sensing element. Another major difference is in its operating range. Due to its high sensitivity, thermistors are used in narrow span measurements and low temperature ranges from -20 degree Celsius to +120 degree Celsius. But RTD’s are used over wide and larger temperature ranges.

Construction

The device is manufactured from materials like sintered mixtures of oxides of metals such as manganese, nickel, cobalt, and iron. Their resistances range from 0.4 ohms to 75 mega-ohms and they may be fabricated in wide variety of shapes and sizes. Smaller thermistors are in the form of beads of diameter from 0.15 millimeters to 1.5 millimeters.  Such a bead may be sealed in the tip of solid glass rod to form probe which is easier to mount than bead. Alternatively thermistor may be in the form of disks and washers made by pressing thermistor material under high pressure into flat cylindrical shapes with diameter from 3 millimeters to 25 millimeters. Washers may be stacked and placed in series or parallel to increase power disciplining capability.

Characteristic Curve

The resistance versus temperature curve is one of the main characteristics that is used in measurement, control and compensation applications using a thermistor. The characteristics graph is shown below.

From the characteristics graph of a typical thermistor, we can see that the resistivity changes from 107 to 1 ohm-cm as the temperature changes from -100 degree Celsius to +400 degree Celsius. This high negative temperature coefficient of resistance makes thermistor an ideal temperature transducer.

Thermistor as Temperature Sensor

A thermistor used for the measurement of temperature is shown in the figure below. The thermistor is designed to have a resistance of 2 kilo-ohms at 25 degree Celsius and temperature coefficient of -4% per degree Celsius will bring a reduction of 80 ohms per degree Celsius change in temperature.

The device is connected in series to a battery and a micrometer. A change in temperature causes a change in the resistance if the thermistor and the corresponding micrometer current reading is noted. Usually, the meter is calibrated in terms of temperature with 0.1 degree Celsius resolution. As shown in the figure, a bridge circuit is also used so as to increase the thermistors sensitivity.

Thermistor Types

For studying about the different types of thermistors, it is important to understand the formula which shows the linear relationship between resistance and temperature.

As a 1^st order approximation, the change in resistance is equal to the 1^st order temperature co-efficient of resistance times the change in temperature.

dR = k.dT

where, dR – Change in Resistance

k – 1^st Order Temperature Coefficient of Resistance

dT – Change in Temperature

If the value of temperature coefficient of resistance (k) is positive, an increase in temperature increases the resistance. Such a device can be called a Posistor or Positive Temperature Coefficient Thermistor (PTC). If the value of k is negative, an increase in temperature will decrease the resistance value. Such a device is called a Negative Temperature Coefficient Thermistor (NTC).

Posistor/Positive Temperature Coefficient (PTC) Thermistors

PTC Thermistors that are used in industries are broadly classified into two. The first one is called by the name ‘Silistors’, as to Sensitive Silicon Resistors. Silistors are known to have a positive temperature coefficient of 08% per degree Celsius. If the temperature goes higher than 175 degree Celsius, the device jumps to a negative temperature coefficient region. The other classification of PTC Thermistors is called Switching Type PTC Thermistors. It is made from ceramic type materials and are known to have a very high resistance from a small change in temperature. Dopant’s are also added to the material so that they show a semi-conductive behavior as well. The device is known to have a transition or “Curie” temperature. Until the device reaches that particular point, it shows a negative temperature co-efficient pattern in its resistance-temperature characteristics. After this point, it starts to show an increasing positive temperature coefficient of resistance. At this point, the resistance also begins to develop. The main difference in the temperature-resistance curve between a silistor and switching PTC Thermistor is shown below.

Applications

1. The device is famous for its application as a circuit protecting device, such as a fuse. The flow of current through the device causes a heat to build up due to its resistive property. Thus, if excessive current flows through the device, the device begins to heat up accordingly and thus increases its resistance. This increase in resistance again builds up more heat. This creates such an effect that develops more resistance in the device, and limits the amount of voltage and current in the device.
2. Another major application is as a timer in degaussing coil circuit of CRT monitors. When a CRT monitor is turned on, an initial current reaches the PTC thermistor and degaussing coil. The PTC thermistor will be of large size and thus, the resistance of the device increases as the current flows in. This causes the heat to build up and thus the degaussing coil shuts off very fast. The degaussing coil is necessary to decrease the continuous magnetic field in a smooth manner. This help can be provided only by the PTC thermistor.

Negative Temperature Coefficient (NTC) Thermistors

NTC Thermistors that are used in industries are broadly classified into two. Thus classification is based on the method by which the electrodes are placed on the ceramic body. This main category could be further divided depending on the different types of geometries, shapes and processing methods. One of the main categories that is most commonly used in the industries is the bead type thermistors.  According to the shape and manufacturing methods, bead thermistors can be again classified into Bare Beads, Glass Coated Beads, Ruggedized Beads, and Bead in glass Enclosures and many more.

Another group of NTC Thermistors is the ones with metalized surface contacts. These thermistors can be mounted using spring contacts or by surface mounting.

Applications

1. NTC thermistors are used for temperature measurements (usually in a narrow span and low temperature ranges).
2. The device can be used to limit the sudden over current that flows in supply circuits. The device is known to have a very high value of resistance in the beginning. The resistance gradually decreases by the heating up of the device. As the resistance decreases, the usual operation of the circuit is restored and the high current flows through it without damaging other parts of the circuit.
3. This device is used to measure the temperature of incubators.
4. NTC thermistors are used to measure and monitor batteries while they are kept for charging.
5. They are used to know the temperature of oil and coolant used inside automotive engines. This information is sent back to the driver through indirect ways.