Types of Instrument Errors


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Different Types of Instrument errors are

Any given instrument is prone to errors either due to aging or due to manufacturing tolerances. Here are some of the common terms used when describing the performance of an instrument.

RANGE
The range of an instrument is usually regarded as the difference between the maximum and minimum
reading. For example a thermometer that has a scale from 20 to 100oC has a range of 80oC. This is also called the FULL SCALE DEFLECTION

ACCURACY
The accuracy of an instrument is often stated as a % of the range or full scale deflection. For example a pressure gauge with a range 0 to 500 kPa and an accuracy of plus or minus 2% f.s.d. could have an error of plus or minus 10 kPa. When the gauge is indicating 10 kPa the correct reading could be anywhere between 0 and 20 kPa and the actual error in the reading could be 100%. When the gauge indicates 500 kPa the error could be 2% of the indicated reading.

REPEATABILITY
If an accurate signal is applied and removed repeatedly to the system and it is found that the indicated reading is different each time, the instrument has poor repeatability. This is often caused by friction or some other erratic fault in the system.

STABILITY
Instability is most likely to occur in instruments involving electronic processing with a high degree of amplification. A common cause of this is adverse environment factors such as temperature and vibration. For example, a rise in temperature may cause a transistor to increase the flow of current which in turn makes it hotter and so the effect grows and the displayed reading DRIFTS. In extreme cases the displayed value may jump about. This, for example, may be caused by a poor electrical connection affected by vibration

TIME LAG ERROR
In any instrument system, it must take time for a change in the input to show up on the indicated output. This time may be very small or very lar ge depending upon the system. This is known as the response time of the system. If the indicated output is incorrect because it has not yet responded to the change, then we have time lag error.

RELIABILITY
Most forms of equipment have a predicted life span. The more reliable it is, the less chance it has of going wrong during its expected life span. The reliability is hence a probability ranging from zero (it will definitely fail) to 1.0 (it will definitely not fail).

DRIFT
This occurs when the input to the system is constant but the output tends to change slowly. For example when switched on, the system may drift due to the temperature change as it warms up.