Fluke 917X Series Technical Manual page 114

917X Series Metrology Wells
Regular metrology well performance analysis
Accuracy drift
The display temperature of the Metrology Well will drift over time. This is due to
a variety of factors affecting the temperature control PRT. Any PRT is subject to
changes depending on how it is used and the environment it is used in. This is no dif-
ferent for any PRT in a calibration application. In addition, manufacturing variables in
the sensing element itself can result in greater or lesser impact from use and environ-
ment. Oxidation and contamination from the sensor's environment will create changes
requiring new calibration constants depending on the temperature range and normal
operation of the instrument. Oxidation and contamination are generally not factors
when Metrology Wells are used exclusively below 200°C. Oxidation can form in the
body of the PRT platinum sensor wire in the range of 300°C to 500°C. Contamination
is primarily a problem following prolonged use above 500°C. Additionally, vibration
from handling or transportation will strain the delicate PRT element, changing its re-
sistance. Some of this strain may come out by annealing at a slightly higher tempera-
ture than the instrument is typically used at. It is recommended to avoid unnecessary
temperature cycling. Cycling the temperature up and down between minimum and
maximum temperatures excessively may also cause strain on the PRT element.
Effects from control sensor drift may be avoided by using an external temperature
reference. In the case that the calibration of the display value is required, a program
of monitoring and recalibration must be implemented, just as with any calibration
standard. Regularly check the accuracy of the Metrology Well with an adequate tem-
perature reference and keep records as a part of your instrument maintenance routine.
When the accuracy drifts to a point where it is no longer acceptable, then have the
instrument recalibrated. Your records will provide data for determining a calibration
interval appropriate for your history of use and accuracy requirements.
Stability
The stability specification of the Metrology Well was determined under laboratory
conditions of steady ambient temperature and air flow. While this instrument has been
designed to minimize ambient effects, they will still have some effect. For the best
results, avoid quickly-changing ambient temperatures and drafty conditions.
Axial Uniformity
Metrology well axial uniformity should be checked periodically. Use the process out-
lined in EA 10/13 or a similar process. If, due to a drift in the differential thermocou-
ples, the axial uniformity has changed outside the limits set by the user's uncertainty
budget, adjust the axial gradient as outlined in the Metrology Well Calibration section
of this guide and recalibrate the Metrology Well.
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