Instrumentation Basics: Measurement Terminology

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Instrumentation is the science of automated measurement and control. Applications of this science abound in modern research, industry, and everyday living. From automobile engine control systems to home thermostats to aircraft autopilots to the manufacture of pharmaceutical drugs, automation is everywhere around us. This piece will focus on the fundamental principle of measurement terminology.

The objective of any measurement endeavour is to be able to measure a given process variable in order to possibly control it. Hence what you cannot measure, you cannot control.

Instruments are used to measure and control the condition of process streams as they pass through a Plant. Instruments are used to measure and control process variables such as: Temperature; Flow; Level; Pressure; Quality. Automatic instrument control systems are most commonly used to continually monitor these process conditions and correct them, without operator intervention, if there is a deviation from the process value required. The main reason for using automatic controls is that production is achieved more economically and safely. In fact, some of our processes could not be controlled in a stable condition without automatic control systems.

Although I have laid out the basic foundation for measurement and control  here, you might check out: Measurement and Control Basics This book is very good for learning the basics of measurement and control.

Common Measurement Terminology:

Measured Variable:
Is the physical quantity or condition, which is to be measured. Common measured

variables are: Temperature, pressure, rate of flow, level, speed, etc

Measured Signal:
Is the electrical, mechanical, pneumatic, or other variable applied to the input of a device. In a thermocouple, the measured signal is an E.M.F, which is the electrical analogue of the temperature applied to the thermocouple. A measured signal is normally produced by the primary element (sensing element) of an instrument.

Input Signal:
Is a signal applied to a device element, or system. The pressure applied to the input connection of a pressure transmitter is an input signal.

Output Signal:
Is a signal delivered by a device, element, or system. The signal (3 to 15 psig, 4 to 20 mA dc, etc) produced at the output connections of a transmitter is an output signal.

Range

Is the region between the limits within which a quantity is measured, received or

transmitted, expressed by stating the lower and upper range-value.

-20 to + 2000C ; 20 to 1500C ; 4mA to 20mA

Span

Is the algebraic difference between the upper and lower range-values.

Range: -20 to 2000C, Span is 2200C; Range: 20 to 1500C, Span is 1300C.

Suppressed zero, is used when lower range-values is greater than zero

Range = 20 to 1500C

Elevated zero, is used when lower range-values is less than zero

Range = -20 to 2000C

Rangeability or turndown
Ratio of the maximum adjustable span / the minimum adjustable span for a given

instrument, R = 100 bars / 10 bars = 10

Repeatability
Ability of an instrument to give identical indications or responses for repeated

applications of the same value of the quantity measured under the same conditions of used. Good repeatability does not guarantee accuracy.

Uncertainty

Range of values within which the true value lies with a specified probability

Uncertainty of +/-1 % at 95 % confidence means the instrument will give the user a

range of +/-1 % for 95 readings out of 100.

Sensitivity

Is the ratio of the change in transducer output to the corresponding change in the

measured value, i.e. sensitivity = (change of output signal) / (change of input signal). For example: A pressure-to-current converter could have a sensitivity of 0.1 mA / mbar.

Accuracy

Accuracy is the conformity of an indicated value to an accepted standard value, or

true value. It is usually measured in terms of inaccuracy and expressed as accuracy.

It is a number or quantity, which defines the limit that errors will not exceed, when the device is used under reference operating conditions. The units to be used must be stated explicitly. It is preferred that a + and - sign precede the number or quantity.

The absence of a sign infers both signs (±).

Accuracy can be expressed in a number of forms:

Accuracy expressed in terms of the measured variable

Accuracy = ± 1Degree F.

Accuracy expressed in percent of span.

Accuracy = ± 1/2 %

Accuracy expressed in percent of the upper range-value

Accuracy = ±1/2 % of URV.

Accuracy expressed in percent of actual reading

Accuracy = ±1% of actual output reading

Accuracy for an instrument loop:

For an instrument loop including three

elements:

A sensor with an accuracy of 1 %

A transmitter with an accuracy of 0.5 %

An indicator with an accuracy of 1 %

Absolute accuracy: +or-1+ or -0.5+ or -1 = + or -2.5 %

Most likely accuracy (Root Mean Square) = SQRT[(±1)2 ±(0.5)2±(1)2] = 1.5 %

Absolute Error

Algebraic difference between the indication and the true value of a quantity to be

measured. Absolute Error = indication - true value. ΔX = X’ – X

Relative Error

Ratio between the absolute error and the true value of the quantity to be measured.

Expressed in percent: x = (ΔX/X) x 100

Hysteresis
Hysteresis is the difference in the output for given input when the input is increasing and output for same input when input is decreasing. When input of any instrument is slowly varied from zero to full scale and then back to zero, its output varies as shown in the diagram below

This is where the accuracy of the device is dependent on the previous value and the direction of variation. Hysteresis causes a device to show an inaccuracy from the correct value, as it is affected by the previous measurement.

Linearity

Linearity expresses the deviation of the actual reading from a straight line. If all outputs are in the same proportion to corresponding inputs over a span of values, then input output plot is straight line else it will be non linear (see diagram below) For continuous control applications, the problems arise due to the changes in the rate the output differs from the instrument. The gain of a non-linear device changes as the change in output over input

varies. In a closed loop system changes in gain affect the loop dynamics. In such an

application, the linearity needs to be assessed. If a problem does exist, then the signal needs to be linearised.

Response

When the output of a device is expressed as a function of time (due to an applied input) the time taken to respond can provide critical information about the suitability of the device. A slow responding device may not be suitable for an application. This typically applies to continuous control applications where the response of the device becomes a dynamic response characteristic of the overall control loop. However in critical alarming applications where devices are used for point measurement, the response may be just as important. The diagram below shows response of the system to a step input.

Ambient

The surrounding or environment in reference to a particular point or object.

Attenuation
A decrease in signal magnitude over a period of time.

Calibration

The procedure of comparing and determining the performance accuracy is called calibration. To configure a device so that the required output represents (to a defined degree of accuracy) the respective input.

Closed loop

Relates to a control loop where the process variable is used to calculate the controller output. In a closed loop system the control action is independent on desired output.

Controller

A device, which operates automatically to regulate the control of a process with a control variable.

Gain

This is the ratio of the change of the output to the change in the applied input. Gain is a special case of sensitivity, where the units for the input and output are identical and the gain

is unitless.

Hunting

Generally an undesirable oscillation at or near the required setpoint is called hunting.

Hunting typically occurs when the demands on the system performance are high and possibly exceed the system capabilities. The output of the controller can be over controlled due to the resolution of accuracy limitations.

Ramp

Defines the delayed and accumulated response of the output for a sudden change in the input.

Reliability

The probability that a device will perform within its specifications for the number of operations or time period specified.

Reproducibility

The similarity of one measurement to another over time, where the operating conditions have varied within the time span, but the input is restored.

Resolution

The smallest interval that can be identified as a measurement varies.

Resonance

The frequency of oscillation is maintained due to the natural dynamics of the system.

Sensitivity

This defines how much the output changes, for a specified change in the input to the device.

Setpoint

Used in closed loop control, the setpoint is the ideal process variable. It is represented in the units of the process variable and is used by the controller to determine the output to the process.

Span Adjustment

The difference between the maximum and minimum range values. When provided in an instrument, this changes the slope of the input-output curve.

Steady state

Used in closed loop control where the process no longer oscillates or changes and settles at some defined value.

Time constant

The time constant of a first order system is defined as the time taken for the output to reach 63.2% of the total change, when subjected to a step input change.

Transducer

An element or device that converts information from one form (usually physical, such as temperature or pressure) and converts it to another ( (usually electrical, such as volts or millivolts or resistance change). A transducer can be considered to comprise a sensor at the front end (at the process) and a transmitter.

Transient

A sudden change in a variable, which is neither a controlled response, nor long lasting.

Transmitter

A device that converts one form of energy to another. Usually from mechanical to electrical for the purpose of signal integrity for transmission over longer distances and for suitability with control equipment.

Variable

Generally, this is some quantity of the system or process. The two main types of variables that exist in the system are the measured variable and the controlled variable. The measured variable is the measured quantity and is also referred to as the process variable as it measures process information. The controlled variable is the controller output which controls the process.

Vibration

This is the periodic motion (mechanical) or oscillation of an object.

Zero adjustment

The zero in an instrument is the output provided when no, or zero input is applied. The zero adjustment produces a parallel shift in the input-output curve.