Resistance Bridges for Temperature Calibration

Resistance Bridges for Temperature Calibration

Several companies manufacture high quality resistance bridges in both the AC and DC formats. All of these bridges can take measurements at the 0.1 ppm level, and research has shown that all of them compensate well for any theoretical inaccuracies predicted in their design.

At Hart Scientific we have a state-of-the-art Primary Standards laboratory. In our lab we have three great bridges: a Guildline Bridge, an ASL Bridge and a Measurements International Bridge. We work with them on a daily basis and each one has its strengths and weaknesses across the wide variety of tasks for which we use them.

For Temperature Calibration work our favorite is the Measurements International Bridge. Not only does it take accurate and discriminating measurements, it is easy to use because of the sophistication of its digital control technology and automation features. This bridge was designed for automated operation. Other bridge companies simply retrofitted automation technology on top of their manual designs. Unfortunately, this didn’t produce the optimal system necessary to match the bridge to the way you work.

As an example, the Model 5581 Bridge does a true auto-balancing procedure. In the auto-self-check mode the bridge performs the function to nine significant digits. As the check proceeds, the bridge steps through an internal comparison of its transformers’ windings, the results of which are recorded as a standard of comparison to track its performance over time. This is helpful in establishing measurement uncertainties during lab audits.

Another function of this bridge is its real-time uncertainty analysis program. In this mode you enter the external uncertainty factors such as the uncertainty of your resistor, and the 5581 automatically combines your information with its own uncertainties to compute a system uncertainty for your measurement.

The optional Windows® control software offers history logging and regression analysis along with uncertainty analysis and the auto-self-check feature. The software can record daily Triple Point of Water readings for statistical prediction of your SPRTs’ uncertainty. The program calculates standard deviations if you need them. You can enter coefficients for your SPRT and read temperature rather than resistance.

Of course, if you prefer, you can operate the bridge manually. The choice is yours, but either way you’ll find this to be a great bridge to use.

We like the MI bridge because we feel confident about our measurements, and its software gives us more information than we can get from the other instruments. While it’s true we do use the other bridges for certain functions we undertake in our lab, including some experimental testing, the MI bridge is used every day for fixed-point calibrations of SPRTs. It’s extremely quiet. Our typical standard deviation is only 0.02 ppm!

In fact, we like this bridge so much we now sell it. When you buy an MI bridge from Hart Scientific, you get Hart’s well-known customer service and training programs. If you want to come to our lab and use all of the bridges just as you would in your own lab, you can do that. You’ll get a feel for the real differences between each one.