The basic need of instrumentation in a process is to get the best and most amount of information so as to successfully complete the process. When referring to the completion of the project with reference to instrumentation, it basically means maximum efficiency with minimum production expense and desired output quality.
The information that is achieved from these processes may be very simple and may mostly involve a direct measurement method. But as the process becomes more complex, direct measurement may seem to be impracticable and so indirect methods must be used for measurements. These methods involve a derived relationship between the measured quantity and the result that is needed.
Most of the indirect methods involve electrical techniques as they have high speed and also simple processing methods. The output from such methods is easier to link to computers.
The obtained information may not necessarily be the direct value of a measured quantity. That is, the value obtained may be a variation of the value with respect to other parameters. It may also be a signal corresponding to the end limit. It could also be a specific value with an indicating hand over a suitable scale. Thus, one instrument may be needed to perform the required operations individually or a number of them at a time.
When it comes to industrial measurements, the measurands are all physical variables which is used to determine the flow of energy in these dynamical units. If so, they can be classified as
1. Flow through or per- variables
Flow trough variables can be measured from a single point in space. Some of the most measured variables using this method are force, momentum, flow, charge, current, volume and so on.
2. Across or trans-variables
Trans-variables need a referencing point and a measuring point. Some of the measured variables are displacement, velocity, pressure, temperature, level and voltage.
If we are mentioning instrumentation systems based on industrial applications it can be broadly classified into two. They are automatic type and manual type. The former works automatically without any help and the latter will need the assistance of an operator. If viewed from the system design view, the instruments will be classified into self-operated type and power operated type.
Whatever maybe the performance of an instrument, there will be some basic building blocks for its functioning. The correct combination of these blocks in a measurement system helps in converting a process condition into a suitable indication.
These blocks are also called as functional units and are present in all instrumentation systems.
All together, instrumentation systems can be classified into two. They are
1. Analog Instrumentation System
The block diagram is shown below.
An analog instrumentation system includes three functional units. They are
- The Primary Element/Transducer
The input receives the quantity whose value is to be measured and is converted into its proportional incremental electrical signal such as voltage, current, resistance change, inductance or even capacitance. Thus, the changed variable contains the information of the measured variable. Such a functional element or device is called a transducer.
- The Secondary Element/Signal Processing Unit
The output of the transducer is provided to the input of the signal processing unit. This unit amplifies the weak transducer output and is filtered and modified to a form that is acceptable by the output unit. Thus this unit may have devices like: amplifiers, filters, analog to digital converters, and so on.
- The Final Element/Output Unit
The output from the signal processing unit is fed to the input of the output unit. The output unit measures the signal and indicates the value to the reader. The indication may be either through: an indicating instrument, a CRO, digital computer, and so on.
2. Digital Instrumentation System
All the functional units that were used in an analog system will also be used here. He basic operation in a digital system includes the handling of analog signals, making the measurements, converting and handling digital data, programming and also control. The block diagram and functional units are given below.
All the physical input parameters like temperature, pressure, displacement, velocity, acceleration and so on will be converted into its proportionate electrical signal.
- Signal Conditioning Unit
This working of this unit is exactly the same as that of a signal processing unit in an analog instrumentation system. It includes all the balancing circuits ad calibrating elements along with it.
Multiple analog signals are received by this device and are sequentially provided on to a measuring instrument.
- Signal Converter
It is used to convert an analog signal to a form that is acceptable by the analog to digital converter.
- Analog to (A-D) Digital Converter
The analog signal is converted into its proportional digital signal. The output of an A-D converter is given to a digital display.
- Auxiliary Equipment
All the system programming and digital data processing functions are carried out by this unit. The auxiliary equipment may be a single computer or may be a collection of individual instruments. Some of its basic functions include linearizing and limit comparison.
- Digital Recorder
It is mostly a CRO or a computer.