an instrument will usually be better than its accuracy.
Resolution is used to indicate the smallest change of the input variable to which the instrument
(sensibly, through its readout) can respond.
Just because the original specifications for a given instrument state a given accuracy doesn’t mean
that the instrument is guaranteed to retain that accuracy indefinitely. Measuring instruments, like all
manufactured items, are subject to “wear and tear”. An instrument which is being used within its
ratings i.e. hasn’t been abused electrically or mechanically (like being dropped on a workbench or
onto the floor) still requires periodic maintenance and recalibration. The procedure for recalibration
is usually specified in the instruction manual for the instrument, provided by the manufacturer. It is
the usual (good) practice for the person performing the recalibration to place a “sticker” on the outside
of the instrument giving the date of servicing and the name (or initials) of the person responsible. In
industrial plants, where accurate measurements are important, the calibration procedures are usually
performed in an instrument shop which is equipped with “secondary standards” of voltage, current,
resistance, etc. traceable to the National Institute of Standards and Technology (NIST formerly known
as NBS, the National Bureau of Standards).
When using an instrument, refer to the instruction manual for correct usage and for specifications on
the accuracy and resolution of the instrument. Also check to see what the last date of calibration was.
The longer the interval between calibration and your date of using the instrument, the more suspect is
the accuracy of your measurements. It is good practice to have a “known” voltage source (such as a
“fresh” 1.5V battery) to quickly check a voltmeter or current source (such as a 1.5V battery in series
with a 1.5 k resistor to provide 1 mA) to obtain a quick check of an ammeter. If several instruments
are available, they can be used with the same source to obtain a comparison of readings. The best rule
of measurement is “check it out before you believe the readings”.
How Good Are Their Measurements Compared To Mine?
Often the measurements made by two engineers, or two different groups of engineers, under ostensibly
similar conditions, using ostensibly similar sets of instruments and equipment will result in different
values, sometimes what appear to be significantly different value. The consequences may range from a
“friendly dispute” to lawsuits.
Various engineering organizations, such as the American Society for Testing Materials (ASTM),
go to great lengths to standardize measurement procedures and then to compare the results obtained
by various people in different organizations using these procedures. One method of doing this is the
so-called “round-robin” method.
In the “round-robin” method, a set of objects to be measured for example, a set of standard
voltage cells will be sent from laboratory to laboratory for measurements to be made. Then each group
making the measurements will report them to a central group, the referee, for comparison. Following a
statistical analysis of the individual and pooled measurements, the results are reported back to all of the
participants. This method helps to establish the practical limits on repeatability of the measurements
made using the procedure.
In general, there are many types of errors that can interfere with an accurate measurement of a
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