Resistance Bridge Calibrator
The RBC100M and RBC400M resistance bridge calibrators are designed for quick and simple, in-house calibrations of ac and dc thermometry bridges, with an accuracy of better than 0.1 ppm at 100 Ω. 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 application 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.
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.
The performance of the RBC is almost entirely determined by the temperature stability of the resistor network. The RBC100M and RBC400M calibrators have temperature coefficient of less than 0.3 ppm/°C and can be used to calibrate bridges to an accuracy of better than 0.1 ppm in a typical temperature-controlled laboratory.
Ordering information
The RBC is manufactured under licence by Isothermal Technology (UK) limited.
For all sales enquiries contact info@isotech.co.uk
Further technical information: brochure, specifications, and the operators handbook (pdf 1.36 MB) are available at the Isotech website