Resistance Bridge Calibrator
The resistance bridge calibrators are designed for quick and simple calibrations of ac and dc thermometry bridges. The products are supported by software for analysis of the results, and an extensive user manual with a complete description of the operating principles and an uncertainty analysis.
Temperature measurement is one of the most demanding applications of resistance measurement. It requires the measurement of resistance ratios with accuracies of 1 part in 107 or better. While dc resistance standards are sometimes available at this level, ac resistance standards are generally not. So how can we show that our resistance bridges are accurate at this level, and that our temperature measurements are traceable?
The linearity check: One simple method for checking a bridge is to measure a pair of resistors separately, and then measure the two connected in series. Ideally the series measurement should equal the sum of the two individual measurements. If not, then the measurements give us a little bit of information about the errors in the bridge readings. This test will expose non-linearities in the bridge readings. Note that we do not need to know the values of the resistors to make this test useful.
The complement check: Another check is to measure the ratio of two resistances, say R1/R2, then swap the resistors and measure the reciprocal (or complement) ratio, R2/R1. Ideally the product of the two measurements should equal 1.0 exactly; if not, the measurements give us more information on the errors in the bridge readings. Unlike the linearity check, this test will expose errors in the scale of the readings. Again, we do not need to know the values of the resistors to make the test work.
Manual Resistance Bridge Calibrator
The Resistance Bridge Calibrator: The Resistance Bridge Calibrator is based on a network of four very stable 4-terminal resistors all connected to a single point. By connecting the resistors in various series and parallel combinations the network will generate a total of 35 different resistances. Since all 35 resistances are related in a calculable way to the four base resistances, 35 measurements of the network will effectively provide 31 measurements of the bridge behaviour. This is sufficient to characterise the non-linearity in the bridge, and sufficient for most thermometry applications.
With bridges that use an external standard resistor the connections to the RBC and the standard resistor can be exchanged to provide an additional 35 resistance ratios all inter-related to the same four base resistors. By including some of these complement or reciprocal ratios amongst the measurements, it becomes possible to determine the absolute accuracy of the bridge. With a typical seven-digit ac bridge all 70 possible measurements can be made in about one hour.
We have developed two versions of the RBC. The low-cost manually-operated RBC has a nominal accuracy of 0.1 ppm and is suitable for small laboratories who want to calibrate their own bridges in-house. The automatic RBC has a nominal accuracy of 0.01 ppm, is USB controlled, and is manufactured for immersion in temperature controlled standard resistor baths. It is suitable for larger laboratories with several resistance bridges, or laboratories offering calibration services for thermometry bridges.
The RBCs are manufactured under licence by Isothermal Technology (UK) limited.
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Further technical information: brochure, specifications, and the operators handbook (pdf 1.36 MB) are available at the Isotech website