Units and Standards

In order to avoid confusion and to obtain a consistent result, a set of units and standards have been commonly followed by all countries. Each instrument used is given a separate symbol which makes it easier for its identification and also for process control drawings. All the lists have been developed by The Instrument Society of America (ISA) and is being used worldwide.

The units that are used for the measurement f different variables fall mainly under two categories. One is the International system, SI (Systéme International D’Unités) and the other is the English system. The problem is that the latter is followed by very few countries including USA, but the former is followed by most of the other countries.

Parameters

There are some parameters that are to be checked during a process. They are all explained below.

• Accuracy – It is defined as the difference between the indicated value and the actual value. The actual value may be a known standard and accuracy is obtained by comparing it with the obtained value. If the difference is small accuracy is high and vice versa.  Accuracy depends on several other parameters like hysteresis, linearity, sensitivity, offset, drift and so on. It is usually expressed as a percentage of span, percentage of reading or even absolute value. The standard value is set by the government so as to maintain the standard.
• Reading accuracy is the deviation from true at the point the reading is being taken and is expressed as a percentage. Absolute accuracy of an instrument is the deviation from true as a number not as a percentage.
• Span – It can be defined as the range of an instrument from the minimum to maximum scale value. In the case of a thermometer, its scale goes from −40°C to 100°C. Thus its span is 140°C. As said before accuracy is defined as a percentage of span. It is actually a deviation from true expressed as a percentage of the span.
• Precision – It may be defined as the limits within which a signal can be read. For example if you consider an analog scale, which is set to graduate in divisions of 0.2 psi, the position of the needle of the instrument could be estimated to be within 0.02 psi. Thus the precision of the instrument is 0.02 psi.
• Range – It can be defined as the measure of the instrument between the lowest and highest readings it can measure. A thermometer has a scale from −40°C to 100°C. Thus the range varies from −40°C to 100°C.
• Reproducibility – It can be defined as the ability of an instrument to produce the same output repeatedly after reading the same input repeatedly, under the same conditions.
• Sensitivity – It can also be called as the transfer function of a process. It is the ratio between the change in the output of an instrument to the corresponding change in the measured variable. For a good instrument or process, the sensitivity should always be high, thus producing higher output amplitudes.
• Offset – Offset is the reading of an instrument with zero input.
• Drift – Drift is the change in the reading of an instrument of a fixed variable with time.
• Hysteresis – It can be defined as the different readings taken down when an instrument approaches a signal from opposite directions. That is the corresponding value taken down as the instrument moves from zero to midscale will be different from that between the midscale and full scale reading. The reason is the appearance of stresses inside the instrument material due to the change of its original shape between the zero reading and the full scale reading.

Hysteresis

• Resolution – It is the smallest difference in a variable to which the instrument will respond.
• Repeatability – It is a measure of the closeness of agreement between a number of readings (10 to 12) taken consecutively of a variable, before the variable has time to change. The average reading is calculated and the spread in the value of the readings taken.
• Linearity – It can b defined as a measure of the proportionality between the actual values of a variable being measured to the output of the instrument over its operating range.