2
Operation
26
Keysight 478A Operating and Service Manual
Thermoelectric Effect
Mount calibration uncertainties given in
Chapter 3, "Specifications and
include inaccuracies caused by thermoelectric effect error.
Calibration Factor uncertainty of
±
1.5% and Effective Efficiency uncertainty of
±
2.5% can be maintained on the three lowest power ranges of the model 431
series power meters by correcting for the measurement error introduced by
thermoelectric effect. An error connection procedure is given in
Effect Error Correction for 478A / 431 Combination”
A mild thermocouple exists at each point of contact where the connecting wires to
join the thermistor elements. Each thermocouple creates a DC voltage. Thus, two
thermocouple voltages of opposite relative polarity are formed, one at each
junction to each thermistor element.
Ideally, each thermocouple voltage would be equal in magnitude so that they
cancel with no resultant effect on the accuracy of power measurement. In practice
however, each point of contact does not have identical thermocouple
characteristics, and in addition, the temperatures at each junction may not be the
same. These differences cause an incomplete cancellation of the thermoelectric
voltages, resulting in a voltage that causes a thermoelectric effect error. The
magnitude of the error is important when making DC substitution measurements
on the 0.1 mW, 0.03 mW and 0.01 mW ranges with one of the model 431 series
power meters. On other ranges, the effect is negligible. Maximum error introduced
by thermoelectric effect is about 0.3
μ
w and is typically 0.1
μ
w on the 0.01 mW
range.
Содержание 478A
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Страница 45: ...Keysight 478A Thermistor Mount Operating and Service Manual 45 A Appendix A Introduction 46 Manual Changes 47...