These coolers/chillers are probably hard to come by … and if you do find one, it is most likely VERY EXPENSIVE. We can fix these. The usual failure mode is that your 951A (LoTempCo version) or 951C starts misbehaving and it seems to misbehave based on room temperature. It could also be impossible to zero or maintain a good zero. What is probably happening is your PMT (655168) is no longer being maintained at 6°C and that is creating a significant ‘dark current’ in the PMT. Or equally bad is that it is close to 6°C but the temperature is not stable, so getting a good zero on the instrument is fruitless.

Another failure symptom is scorched or burned wires associated with the chiller circuit. The chiller is fed by a high current DC power supply (roughly 14 VDC); when the Peltier chips go bad, more current is required to achieve the cooling required and ultimately, there is so much current that the wire limitations are exceeded (especially when you factor in Rosemount’s use of 18awg high current wire). Look at the J16 plug and the J15 plug on the Power Supply Board. Check the terminations on the terminal strip on the side of the high current power supply too.

The cooler is built with a metal can liner that surrounds the PMT; the metal liner is thermally connected to two Peltier chips (or elements); the Peltier chips are thermally connected to the aluminum fin housing that you see when you open the instrument. These components are all potted in Stryofoam insulation.

Check the continuity of the red and black wires going to the chiller unit. New Peltier chips will read about 2 ohms in one direction and -2 ohms in the other direction (the negative reading come from the battery effect of the Peltier chips; essentially these are a million thermocouples amassed together … and thermocouples are little batteries in the presence of heat).

Our repair consists of extracting the old Peltier cells, installing new ones, repotting everything, checking the feedback thermistor, and testing the final assembly.

Call us for more information.

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