How to Interpret Standard Ratings for Meter Accuracy
May 1934 Radio News and the Short-Wave
with the ubiquitous (and inexpensive) presence of digital multimeters,
there are still times when a meter sporting an analog movement is
more useful than a numerical display. This is especially true is
when a reading is varying about a mean value rather than being fixed.
Sampling and display update times of digital meters can be too slow
to realistically reflect what a time-varying signal is doing. An
analog meter's pointer can more readily be followed than needing
to read and mentally comprehend a rapidly changing numerical value.
Digital meters are great for reading a fixed value or for precisely
setting a fixed value, but tracing signals through a circuit with
a digital meter can be very misleading because unless you are mainly
interested in DC bias levels, the information presented can be misleading.
In order to use an analog meter meaningfully, you really need to
understand how the range chosen - be it voltage, current, power,
or resistance - affects accuracy. This article does a great job
of explaining why the scale selector should always be chosen such
that the reading being indicated lie as close to full scale as possible
- understanding this is extremely important!
May 1934 Radio News and Short-Wave
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio &
Television News, published 1919 - 1959. All copyrights hereby acknowledged.
How to Interpret
Standard Ratings for Meter Accuracy
How many readers know the meaning of manufacturers' accuracy ratings,
as applied to measuring instruments?
Figure 1 - DC volts meter face (0 - 100 V)
Figure 2 - AC volts meter face (0 - 100 V)
Figure 3 - Unmarked meter movement face
Figure 4 - Ohmmeter movement face (0 - ∞ Ω)
Figure 5 - Power meter movement face (0 - 6 mW)
Figure 6 - DC volt meter movement face (30-100 V)
Harold L. Olesen
The accuracy of electrical indicating instruments and certain
other devices as used in the radio service field, seems to be a
subject that is generally misunderstood. Even though these units
are rated by their manufacturers as being correct within a specified
accuracy most users do not know how to apply this information advantageously.
The instruments used in radio service practically all of
the "indicating" class, as distinguished from instruments of the
other classes, "integrating," "recording," etc. As a class, "indicating"
instruments are treated as one group; the underlying principle regarding
accuracy is the same for all.
The length of the meter scale
is the distance from the zero position to its other or full scale
end, as measured along the arc traced by the tip of the pointer.
This assumes that the zero position is at one end of the scale,
which is generally the case. When the zero point is located at some
mid-position on the scale, the pointer can move either to the right
or left of it; the scale is then considered as having two end scale
positions and the full scale length then becomes the sum of the
distances from the zero point to each end scale position.
The recognized standard for electrical measuring instruments
in this country is that issued by the American Institute of Electrical
Engineers. In this group of standards, Standard No. 33-33 covers
the accuracy of indicating instruments. This standard reads as follows:
"Accuracy of Indicating Instruments. - In specifying the
accuracy of an indicating instrument; the limits of error at any
point on the scale shall be expressed as a percentage of the full
Instruments whose scales are uniform, or
reasonably so, and whose scale markings increase from zero at the
point of zero indication to a maximum value at the other end of
the scale fall in this group and are covered by the standard directly.
This group includes all standard voltmeters, ammeters, and wattmeters
for both a.c. and d.c. Figures 1 and 2 show typical scales of this
In Figure 1 the scale is marked off into 50 uniform
divisions which bear the identifying numerals 0-100. An accuracy
rating of 2% on an instrument using this scale would mean that the
pointer should indicate with an error not to exceed ±2 volts (2%
of 100) at any point on the scale. Should the reading be made at
the 100 mark, the maximum allowable error, ±2 volts would be 2%
of the indicated value. However, if the reading should be made at
10 on the scale, the maximum allowable error, ±2 volts, would become
20% of the indicated value. For this reason greatest accuracy is
obtained by properly choosing the ranges on indicating instruments
of this sort so that large deflections of the pointer may be obtained.
In Figure 2 the scale is marked off in units, so that the
instrument to which this scale is attached may read directly in
a.c. volts. Instrument practice considers this scale as having 50
divisions because each of the ten cardinal divisions contains 5
smaller divisions. The application of an accuracy rating to this
instrument is made in exactly the same manner as in the case of
Figure 1, except that the rating does not apply for the first 1/5th
of the scale. The usable part of a scale of this nature is considered
to be the upper 4/5ths and no attempt is made either to use the
first 1/5th at the left-hand end of the scale, or to cover this
portion of the scale with the accuracy rating. A reading made at
the 100 point might be in error ±2 volts or 2% of the indication,
but a reading at the 50 point might be in error ± volts, or 4% of
the indication. The cramping present on this scale does not affect
the application of the standard for accuracy. On instruments of
this type it is obvious that deflections which carry the pointer
into the open part of the scale are required for best accuracy.
Special scale instruments in the group are those that:
(a) are without divisions as such, (Figure 3);
are calibrated in secondary values, (Figure 4);
their markings so distributed that zero and maximum readings do
not coincide with the zero and maximum deflection points, (Figure
(d) have a suppressed zero reading point, (Figure 6).
This group requires a somewhat special interpretation of
the accuracy rating standard.
An examination of Figure 1
will show that as far as an evenly divided scale is concerned, there
is no difference between an error expressed as a percent of full
scale reading, and one expressed as the same percent of scale length.
In either case the result is the same.
Figure 3 shows a
scale which has no divisions as such. The instrument on which this
scale is used is one of many that are made to indicate when the
circuit in question is properly adjusted to some definite value
of voltage, current, or power. The length of this scale is twice
the distance along the arc traced by the pointer tip from one end
to the line at center scale.
The instrument is calibrated
at the mark shown on the scale, and hence the 2% tolerance applies
at the mark and not as a percent of scale length or the reading
that might be obtained at the full scale or end scale position.
Figure 4 shows the scale of a typical series type ohmmeter.
A true series type ohmmeter scale has but one arc, marked off in
ohms. The scale shown in Figure 4 is that of a volt-ohmmeter and
is taken from the Weston Model 663 volt-ohmmeter. This scale was
chosen in order to have available a uniformly divided arc below
the ohm arc, thus facilitating the explanation made below.
It is obvious that, since the ohmmeter scale can be added to
the face of an instrument already bearing a uniformly divided arc,
the method of figuring the accuracy must be the same for both. In
other words, the accuracy of the instrument is a function of pointer
movement and is, therefore, independent of the type of scale used.
Like the uniformly divided scale the ohmmeter accuracy can
be expressed as a percent of scale length. In Figure 4 2% of the
full scale range of the voltmeter becomes one division of the scale
length. While this one division is always a fixed number of volts
on the voltage scale, it is not a fixed number of ohms on the ohms
scale, but depends on the location of the division along the arc.
Figure 4, 2% at the zero ohms position is equal to approximately
.5 ohm. At center scale the maximum error may be equal to 2 ohms
in 25, or 8%; at 1/5th scale 12 ohms in 100, or 12%. Here it is
again apparent that the greatest accuracy is available when ranges
are selected such that large deflections are obtained.
instrument having a 2% rating and using a scale as shown in Figure
5 would be considered as being within its guaranteed accuracy if
the pointer indicated within plus or minus a distance equal to 2%
of the scale length of the true value at any point on the scale.
Figure 6 shows a suppressed zero scale which is also special
as far as applying an accuracy rating is concerned. The zero deflection
point of the instrument using this scale is suppressed by winding
up the pointer spring equivalent to 30 volts deflection and permitting
the left-hand pointer bumper to hold the pointer slightly to the
left of the 30 volt mark when no current is flowing. Current through
the instrument produced by the first 30 volts in the circuit under
test is not indicated on the scale. The deflection of the pointer
produced by the additional current through the instrument due to
each additional volt between 30 and 100 volts is greater than would
be the case if the scale and instrument were adjusted to read from
0-100 in the regular way. The accuracy of an instrument bearing
a scale of this sort is taken as a percent of the top mark value
indicated on the scale.
Posted February 18, 2014
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