24-03-2012, 12:41 PM
Measurement Systems Specifications
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Introduction
One of the most frequent tasks that an Engineer involved in the design, commissioning, testing, purchasing, operation or maintenance related to industrial processes, is to interpret manufacturer’s specifications for their own purpose. It is therefore of paramount importance that one understands the basic form of an instrument specification and at least the generic elements in it that appear in almost all instrument specifications.
Different blocks of a measurement system have been discussed in lesson-2. The combined performance of all the blocks is described in the specifications. Specifications of an instrument are provided by different manufacturers in different wrap and quoting different terms, which sometimes may cause confusion. Moreover, there are several application specific issues. Still, broadly speaking, these specifications can be classified into three categories: (i) static characteristics, (b) dynamic characteristics and (iii) random characteristics.
1. Static Characteristics
Static characteristics refer to the characteristics of the system when the input is either held constant or varying very slowly. The items that can be classified under the heading static characteristics are mainly:
Range (or span)
It defines the maximum and minimum values of the inputs or the outputs for which the instrument is recommended to use. For example, for a temperature measuring instrument the input range may be 100-500 oC and the output range may be 4-20 mA.
Sensitivity
It can be defined as the ratio of the incremental output and the incremental input. While defining the sensitivity, we assume that the input-output characteristic of the instrument is approximately linear in that range. Thus if the sensitivity of a thermocouple is denoted as 100/VCμ, it indicates the sensitivity in the linear range of the thermocouple voltage vs. temperature characteristics. Similarly sensitivity of a spring balance can be expressed as 25 mm/kg (say), indicating additional load of 1 kg will cause additional displacement of the spring by 25mm.
Accuracy
Accuracy indicates the closeness of the measured value with the actual or true value, and is expressed in the form of the maximum error (= measured value – true value) as a percentage of full scale reading. Thus, if the accuracy of a temperature indicator, with a full scale range of 0-500 oC is specified as ±0.5%, it indicates that the measured value will always be within ±2.5 oC of the true value, if measured through a standard instrument during the process of calibration. But if it indicates a reading of 250 oC, the error will also be ±2.5 oC, i.e. ±1% of the reading. Thus it is always better to choose a scale of measurement where the input is near full-scale value. But the true value is always difficult to get. We use standard calibrated instruments in the laboratory for measuring true value if the variable.
Dynamic Characteristics
Dynamic characteristics refer to the performance of the instrument when the input variable is changing rapidly with time. For example, human eye cannot detect any event whose duration is more than one-tenth of a second; thus the dynamic performance of human eye cannot be said to be very satisfactory. The dynamic performance of an instrument is normally expressed by a differential equation relating the input and output quantities. It is always convenient to express the input-output dynamic characteristics in form of a linear differential equation. So, often a nonlinear mathematical model is linearised and expressed in the form:
Step response performance
The normalized step response of a measurement system normally encountered is shown in Fig. 7. Two important parameters for classifying the dynamic response are:
Peak Overshoot (Mp): It is the maximum value minus the steady state value, normally expressed in terms of percentage.
Settling Time (ts): It is the time taken to attain the response within ±2% of the steady state value.
Rise time (tr): It is the time required for the response to rise from 10% to 90% of its final value.
Frequency Response Performance
The frequency response performance refers to the performance of the system subject to sinusoidal input of varying frequency.