Fluke 914 Series Manual page 42

914X Field Metrology Wells
Field Metrology Well Performance Analysis
Accuracy Drift
The display temperature of the instrument 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 different for any PRT in a calibration appli-
cation. In addition, manufacturing variables in the sensing element itself can result in greater
or lesser impact from use and environment. Oxidation and contamination from the sensor's
environment will create changes requiring new calibration constants depending on the tempera-
ture range and normal operation of the instrument. Oxidation and contamination are generally
not factors when Field 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 resistance. Some
of this strain may come out by annealing at a slightly higher temperature 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 recali-
bration must be implemented, just as with any calibration standard. Regularly check the accu-
racy of the Field Metrology Well with an adequate temperature 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 de-
termining a calibration interval appropriate for your history of use and accuracy requirements.
Stability
The stability specification of the Field Metrology Well was determined under laboratory condi-
tions 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
Field Metrology Well axial uniformity should be checked periodically. Use the process outlined
in EA 10/13 or a similar process. If the axial uniformity has changed outside the limits set by
the user's uncertainty budget, adjust the axial gradient as outlined in the Field Metrology Well
Calibration section of the Field Metrology Well Technical Guide and recalibrate the Field Me-
trology Well.
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