Errors in Measuring Instruments
When we measure any physical quantity like temperature, pressure, voltage, current etc we take it for granted that the instruments are accurate. Very few people can imagine that there can be errors in the instrument they are using, but there are possibilities of errors in the measuring instruments especially the analogue instruments that indicate the magnitude or value by the movement of the pointer. There are three types of errors in the measuring instruments: assembly errors, environmental errors, and random errors. All these have been discussed below.
1) Assembly Errors
The assembly errors are the errors in the measuring instrument due to improper manufacturing of the instruments. Various components of the instrument are made separately and then they are assembled together. There may be errors in the individual components or in the assembly of the components. If there are assembly errors in the instruments, the instrument will just not give the correct reading and the user can hardly do anything about it, but return it and get the new one. Here are some of the possible assembly errors:
a) Displaced scale: This is the incorrect fitting of the measuring scale. For instance the zero of pointer may not coincide with actual zero on the scale. Sometimes the scale gets cracked, thus showing the faulty readings.
b) Non-uniform scale: Sometimes the scale of the measuring instrument is not divided uniformly. In some part of the scale the markings may be too close and in other parts too far.
c) The pointer is bent: This happens in many cases. The pointer may get bent in either horizontal direction or the vertical direction, in either case, is shows erroneous reading.
d) Manufacturing errors in the components: The instruments are made up of a number of small components, which may be manufactured in different places. Sometimes there are manufacturing errors in some of these components like gear, lever, links, hinges etc.
2) Environmental Errors
The measuring instruments are assembled and calibrated in certain environmental conditions and are designed to be used in within certain restricted conditions, but when they are used in different conditions, there are errors in measurement, which are considered to be the environmental errors. Most of the instruments are designed to be used within certain limits of temperature, pressure, humidity, altitude etc and when the limits are extended there are errors in the measuring instruments.
It is quite easier to find the assembly errors in instruments, but the errors due to change in environmental conditions are highly unpredictable. These errors can be sometimes positive, sometimes negative and sometimes combination of the both.
Here are some precautions to be taken to reduce the environmental errors in the instruments:
a) Use in the instruments within the specified limits of temperature, pressure and humidity for which the instrument has been designed. These limits are mentioned in the instruments instructions manual.
b) If you have to use the instrument beyond the specified limits of environmental conditions, then apply suitable corrections to the recorded measurement.
c) One can also calibrate the instrument newly in the new conditions.
d) There are some devices that enable applying the compensation automatically.
3) Random Errors
Apart from the assembly and environmental errors there can be many other errors which may be very difficult to trace and predict, these are called as random errors. It is even not possible to list all these random errors, but some of the prominent ones have been described below:
a) Frictional errors: There are number of moving mechanical parts in the analogue measuring instruments. The friction between these components leads to errors. Due to friction some of the parts wear and tear, which further adds to the error of the instrument. Hence, one should not use the analogue measuring instruments for long periods of time and replace with the good quality ones from time-to-time.
b) Mechanical vibrations: When the instrument is used in vibrating place the parts of the instrument start vibrating giving faulty readings.
c) Backlash in the movement: This is the error due to time lag between the application of the parameter and the instrument actually showing reading. Even though some value of the parameter changes, there is no indication.
d) Hysteresis of the elastic members: Over the period of time the elastic members tend to loose some elasticity leading to errors in the indicated value of the instrument.
e) Finite scale divisions: The scale marking can be made only up to certain limits and they not be hundred percent accurate.
1) Book: Mechanical and Industrial Measurements by R. K. Jain, Khanna Publishers