CHOOSING TEST POINTS (TEST TEMPERATURES) FOR
If your thermometer is an ASTM thermometer (it
will have the inscription 'ASTM 1C' or similar), the standard test temperatures
are specified by ASTM specification E-1 (and appear in our ASTM thermometer
listings). These test temperatures have been specified considering the intended
application as well as the behaviors of the particular instrument, and should be
used in order to assure that the calibration has been performed in
accordance with ASTM requirements.
If your thermometer has been previously calibrated, the test points have already
been established and appear on the test report. Generally, those test points
should be repeated in future calibrations, which will allow the user to see the
magnitude and the direction of any changes with each new calibration.
If your thermometer has never been calibrated (or you don't know, or don't have
a test report), you can either let us choose the most suitable (default) test
points, or you may wish to specify those test points to us.
To assist you in choosing test points, we present the following
considerations, which are drawn from ASTM and NIST recommendations. One should
consider all three suggestions:
1. A minimum of three temperatures should be calibrated, generally low,
medium and high on the scale of the instrument. This is the old "10% - 50% -
90%" (of scale) rule. Example: you have a thermometer with a range of -10 to 110oC
in 1o divisions. Calibrating this thermometer at 0o (low
on the scale), 50o (mid scale), and 100oC (high on the
scale) is sufficient, and will allow you to use the thermometer at virtually any
temperature it measures by making a straight-line interpolation. See ASTM-E-77
and NBS Monograph 150 for more information on interpolating. Unfortunately,
this is not adequate for all thermometers. See suggestion #2.
2. There should be no more than 100 graduations between any two calibrated
temperatures; for the ultimate precision, calibrate every 50 divisions. For
example, if your thermometer has a range of -1 to 51oC in 0.1o
divisions, suggestion #1 above, to calibrate three temperatures, does not
provide an adequate calibration. You must calibrate every 100 divisions: 0, 10,
20, 30, 40 & 50oC to have an adequate calibration.
3. If the temperatures used by the
manufacturer for scale placement are known (or can be easily determined
visually), the temperatures used for calibration should correspond. This will
assure linearity of spacing between the calibrated points, therefore allowing
the user to interpolate intermediate values. Example: your thermometer has a
scale of 25 to 60oC in 0.1 degree divisions, and a careful
examination of the thermometer reveals that "scale placement marks' (usually a
scratch in the glass, under a major graduation line, visible with a magnifying
glass) were made at 25o, 30o, 40o, 50o
& 60oC, then those temperatures should be used for the calibration to
afford the best linearity.
FREQUENTLY ASKED QUESTIONS REGARDING TEST
Can I just have one temperature calibrated?
A. Certainly. You are the customer, and the one
who best knows your needs. Many times a single point calibration is all that is
needed, for example when a thermometer is dedicated to the measurement of a
single temperature and will not be used for other work. Under ANSI/NCSL Z-540-1
we are required to identify the test report of a single point calibration as a
"limited calibration", or "not a full scale calibration". The intent is logical
and desirable: if your thermometer is calibrated only at 37oC, for a
dedicated test, you want to know that, and not to use it for a critical
application at 50oC.
I want to use the thermometer only across a defined range within its scale.
Do I need to do a full scale calibration?
A. No, we can choose test temperatures which "bracket" the range within which
you are going to work. As above, the test report will specify that this
calibration is NOT a full scale calibration, and that the thermometer can be
used with full confidence only within the range bracketed.