Although often overlooked, resistance bridges are crucial to measuring resistance thermometers with very low uncertainties and must have their performance checked. Resistance bridges generally perform a measurement of the ratio of the resistance of a thermometer and a reference standard. This process assumes that the linearity of the electrical response is constant over the full range of resistances which may be used. This is not always the case, and must be monitored, if the measurement uncertainty is to be correctly accounted for
Using a resistance bridge calibrator (RBC), the ratio indicated by the bridge can be validated at 35 distinct ratios to yield a check on the linearity of the bridge. By reversing the terminals, validation at a further 35 complementary ratios can be provided, giving a total of 70 distinct ratios.
NPL is now able to offer a linearity check and full calibration of resistance bridges of all types. The full calibration can be performed with an uncertainty of the order of tens of parts per billion of bridge ratio. Direct reading bridges can also be calibrated.
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