The performance of any measuring instrument is affected by several factors. There are two basic performance characteristics of measuring instruments. Static Characteristics and Dynamic Characteristics.
The following are the static characteristics.
Static Error: The difference between the true value of the measuring quantity to the value shown by the measuring instrument under not varying process conditions.
Static error = True value of a measured variable – Instrument reading.
+ Ve Static error means Instrument reads high,
– Ve Static error means Instrument reading low
Accuracy: may be defined as the degree of closeness with which the instrument reading approaching the true value of the quantity to be measured.
The measured quantity may be different from the true value due to the effects of temperature, humidity, etc.,
- Accuracy is expressed in the “ percentage of full-scale reading”. In the case of instruments having a uniform scale, the accuracy can be expressed as “ Percentage of Full-scale reading.
- The best way to develop the ideas of accuracy is to specify it in terms of the percentage of the true value of a quantity being measured.
Precision: is the degree of exactness for which the instrument is designed.
It composed of two characteristics: conformity and significant figures.
More significant figures, estimated precision is more. For example two resistors for values of 1792 ohms and 1710 ohms. A person even repeated measurement it indicates 1.7 K ohms. The reader can not read the true value from the scale.
He estimates from the scale reading consistently yield a value of 1.5 M ohms. This is as close to the true scale as he can read the scale by estimation although there are no deviations from the observed value, the error created by the limitation of the error is called precision error.
This example indicates that the conformity is necessary but not enough condition, because of the lack of significant figures obtained.
Sensitivity: Sensitivity can also be derived as for the smallest changes in the measured variable for which the instrument responds.
Sensitivity can be defined as the ratio of a change in output to change in input which causes it, in steady-state conditions.
The usage of this term is generally limited to linear devices, where the plot of output to input magnitude is straight.
Sensitivity = Change in output / Change in input
Sensitivity can also be derived as for smallest changes in the measured variable instrument responds.
The term sensitivity is some times used to describe the maximum change in an input signal that will not initiate on the output.
Note: The sensitivity of the instrument should be high.
Reproducibility: Under the different measurement conditions, if the successive measurements of the same variable produce agreed results are called Reproducibility.
Resolution: It is the smallest quantity being measured which can be detected with certainty by an instrument.
If a non zero input quantity is slowly increased, the output reading won’t increase until some minimum change in the input takes place. The minimum change which causes the change in output is termed resolution.
Dead Zone: for the largest range of values of a measured variable, to which the instrument does not respond.
- The dead zone occurs more often due to static friction in indicating an instrument.
- A practical example is: Due to static friction, a Control valve does not open even for a large opening signals from the controller.
Hysteresis: Hysteresis: Hysteresis is a phenomenon that illustrates the different output effects when loading and unloading.
Many times, for the increasing values of input an instrument, may indicate one set of output values. For the decreasing values of the input, the same instrument may indicate its different set of output values. When output values are plotted against input the following kind of graph is obtained.
From the above figure, it can be seen that for increasing inputs and decreasing inputs the maximum variation is seen at 50% of the full scale.
Drift is an undesired change in the output of a measured variable over a period of time that is unrelated to the changes in output, operating conditions, load.
Drift may be caused by environmental factors mechanical vibrations, changes in temperatures, stray electric fields, stray magnetic fields, thermal EMFs.
A drift in the calibration of the instrument occurs due to the aging of component parts. Drift occurs in flow measurement due to wear and tear of primary sensing elements such as orifice plates.
Drift occurs in temperature measurement due to scale formation on thermowell.
Drift occurs in Thermocouple or RTD elements due to the change of metallic properties.
Drift for a measuring device can be systematic or random or both some times. Due to wear and tear in the edge of an orifice plate the flow drift occurs systematic way.
Drift is further classified as :
- Zero Drift
- Span Drift
- Zonal Drift
Zero Drift: The zero drift is defined as the deviation in the measured variable starts right from zero in the output with time.
The whole instrument calibration may gradually shift by the same amount as shown in the above figure.
The mechanical bathroom weighing scale is a common example. It is quite casual to find that there is a reading perhaps 1kg with no one stood on the scale. If someone of known weight weighs 70 kgs were to get on the scale, the reading would be 71 kgs. If someone with a known weight of 100 kg the reading would be 101 kgs.
The Zero shift is normally removable by calibration.
Span Drift:: If there is a proportionate change in its indication right along the upward scale the drift is termed span drift or sensitivity drift.
Zonal Drift: In case if the drift occurs only a certain portion of the span of an instrument. It is called zonal drift.
The Following are the dynamic characteristics
Speed of Response.
Dynamic Error: The difference between the true value of the measured quantity to the value shown by the measuring instrument under varying conditions.
Speed of response: It is defined as the rapidity of the measurement system that responds to the changes in the measuring variable.
It indicates how active and fast the system is.
Fidelity: It is defined as the degree to which a measuring instrument is capable of faithfully reproducing the changes in input, without any dynamic error.
Lag: Every system takes at least some time to respond, whatever time it may be to the changes in the measured variable.
For Example Lag occurs in temperature measurement by temperature sensors such as Thermocouple or RTD or dial thermometer due to scale formation on thermowell due to process liquid.
Retardation lag: the response of the measurement begins immediately after the change in measured quantity has occurred.
Time delay lag: in this case after the application of input, the response of the measurement system begins with some dead times.
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