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
of Contents]These articles are scanned and OCRed from old editions of the Radio & Television
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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.
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
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 scale reading."
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 group.
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.
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,
(b) are calibrated in secondary values, (Figure
(c) have their markings so distributed that zero and maximum
readings do not coincide with the zero and maximum deflection points,
(d) have a suppressed zero reading point, (Figure
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.
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
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.
An 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.