Verifying the accuracy of
Thermocouples and RTDs is a difficult but exact science. It requires a
system that has a stable temperature source, an accurate reference
thermometer, repeatable measurement and control and finally a data
processor. Each component of the system must be in concert with the
other components in order to minimize system uncertainty. The
components must have corresponding supportive characteristics for
resolution, accuracy, linearity, traceability, stability and
repeatability. Examples of how these specifications can affect system
Resolution and Accuracy
If desired accuracy is .01 degrees C then the resolution or ability to
read this accuracy must be at least .001 degree C.
It is tempting to state linear accuracy at one temperature (usually 0
degrees C), while this is helpful (all thermocouples have zero output
at this temperature) it is important to know the measurement accuracy
over the entire range of the readout. If the readout were perfectly
linear, its accuracy specification would be the same across its entire
range. However, all readout devices have some non-linearity component
and are not perfectly linear
Readout stability is important, since most measurements are made in a
wide variety of ambient conditions and over varying lengths of time.
Consequently the temperature coefficient and long-term stability
specifications are extremely important.
Beware of “no calibration” claims. Latest ISO specifications require
calibration at least once annually.
A manufacturers’ calibration tolerance is at best at a secondary
standard level. It is important for the reference thermometer to have
NIST traceable accuracy.
lets’ put this all together. It all starts with the ability to maintain
the desired calibration temperature. In order to provide the optimum
stability two sources are necessary. This is due to the broad range of
temperatures involved. In general if a RTD is being tested a
temperature bath would be used. If the sensor is a thermocouple with a
higher temperature range a furnace is used. Depending upon the source
employed the stability ranges from .0001 degrees C to .5 degrees C.
Achieving this type of stability requires a highly stable control
sensing element, fast maximum stability, and a source design that
minimizes and controls heat loss.
our calibration procedure employs the comparison method, the need for a
highly accurate reference thermometer is essential. Our system uses a
Standard Platinum Resistance Thermometer (SPRT) with accuracies of
better than + or - .006 degrees C at 0 degrees C. This accuracy is achieved by abiding
by the International Temperature Scale – ITS-90. The SPRT has
accuracies traceable to NIST.
final element, the processor allows the information to be formatted
into a user defined report and can analytically address the tolerance
and accuracy of the sensor. A good example is the Callendar Van Duesen
(CVD) equation. The system uses CVD equations and applies associated
uncertainties of a Platinum Resistance Detector over any point within
its operating temperature range. The result is a report that provides a
resistance limit of error function.The practical uses of this report
are many, but one of the most useful is determining sensor resistance
interchangeability as a function of temperature. Simply stated this
allows the user to determine uncertainty within a predetermined range
and correct for the error in the instrument.
ability to put together the high tech components that have the
characteristics necessary to assure uncertainty data is an important
element in our success. The system is fully integrated into our quality
assurance program and is a testament to our motto…Temperature
measurement…the right way!