COURSE OF
Active and passive instruments Null-type and deflection-type instruments Analogue and digital instruments
In active instruments, the external power source is usually required to produce an output Example: Strain gauges, resistance temperature detectors (RTDs), and thermistors, Slide-wire resistor, Differential transformer
Passive instruments don't require an external power to produce related output Example thermocouples, piezoelectric accelerometers, bourden gauge, Photovoltaic cell, Moving coil generator,
An analogue instrument gives an output that varies continuously as the quantity being measured changes. The output can have an infinite number of values within the range that the instrument is designed to measure.
A digital instrument has an output that varies in discrete steps and so can only have a finite number of values.
Transducers are defined as an element that converts a change in some physical variable into a related change in some other physical variable. It converts signal in one form to another form
Transducers are the physical element which is a part of a sensor. A sensor is merely a sophisticated transducer in the sense that it contains some signal conditioning circuit capable of amplifying and refining the weak and raw signal that is available at the output of the transducer. Example: A compass is a simple sensor of magnetic north, wherein the magnetic element in the compass is the transducer
The static characteristics of instruments are related with steady state response, it means the relationship between the output and the input when the input does not change, or the input is changing with a slow rate.
The range of a transducer defines the limits between which the input can vary. The span is the maximum value of the input minus the minimum value. For example, a load cell for the measurement of forces might have a range of 10 to 50 KN and a span of 40 KN
Error is the difference between the result of the measurement and the true value of the quantity being measured. Error = measured value true value Thus, if a measured system gives a temperature reading of 25C when the actual temperature is 24C, then the error is +1C. If the actual temperature had been 26C then the error would have been _1 C. A sensor might give a resistance change of 10.2KΩ when true change should have been 10.5KΩ. The error is 0.3 Ω.
Accuracy is the extent to which the value indicated by a measurement system might be wrong. A temperature measuring instrument might, for example, be specified as having an accuracy of +2C.This would mean that the reading given by the instrument can be expected to lie within + or -2C of the true value.
The sensitivity is the relationship indicating how much output you get per unit input. The sensitivity of measurement is a measure of the change in instrument s output that occurs when the quantity being measured changes by a given amount. Thus, sensitivity is the ratio: = output / input
Transducers can give different outputs from the same value of quantity being measured according to whether that the value has been reached by a continuously increasing change or a continuously decreasing change. This effect is called hysteresis.
For many transducers a linear relationship between the input and output is assumed over the working range, i.e. a graph of output plotted against input is assumed to give a straight line. The error is defined as the maximum difference from the straight line.
The terms repeatability and reproducibility of a transducer are used to described its ability to give the same output for repeated applications of the same input value. Repeatability is used for expressing the precision of an instrument
The stability of a system is its ability to give the same output when used to measure a constant input over a period of time.
The dead band or dead space of a transducer is the range of input values for which there is no output. For example, bearing friction in a flow meter using a rotor might mean that there is no output until the input has reached a particular flow rate threshold.
The term drift is often used to describe the change in output that occurs over time. The drift may be expressed as a percentage of the full range output. The term zero drift is used for the changes that occur in output when there is zero input.
The resolution is the smallest change in the input value that will produce an observable numerical change in the output.
Dynamic characteristics of an instrument (transducer) refer to the behaviour between the input value changes and the time that the value given by the transducer settles down to the steady-state value.
This is the time which elapses after a constant input, a step input, is applied to the transducer up to the point at which the transducer gives an output corresponding to some specified percentage, e.g. 95%, of the value of the input
It is related to the speed of the response and gives an indication as to how quickly the system response reaches its final value. This is the 63.2% response time (figure 1). Smaller time constant faster response
This is the time taken for the output to rise to some specified percentage of the steady-state output. Often the rise time refers to the time taken for the output to rise from 10% of the steady-state value to 90 or 95% of the steady-state value.
This is the time taken for the output to settle to within some percentage, e.g. 2%, of the steady state value.
To illustrate the above, consider the following data which indicates how an instrument reading changed with time, being obtained from a thermometer plunged into a liquid at time t = 0. The 95% response time is required. Time (s) 0 30 60 90 120 150 180 210 240 270 300 330 360 Temp. ( C) 20 28 34 39 43 46 49 51 53 54 55 55 55
Figure 2 shows the graph of how the temperature indicated by the thermometer varies with time. The steady-state value is 55 C and so, since 95% of 55 is 52.25 C, the 95% response time is about 228 s.