Experimental insights: Low temperature thermometry

Low temperature Physicists like to know the temperature of their experimental apparatus, usually to the point of obsession. On the current dilution refrigerator (F.R.O.S.T.Y – FRidge Of Substantially Temperamental Yield) there are 7 thermometers. One on the 1K pot, one on the SORB, one on the STILL, and 4 on the mixing chamber (slight overkill perhaps?), consisting of two Ruthenium Oxide resistors, one calibrated and one uncalibrated, a nuclear orientation thermometer and a generic diode thermometer. The latest addition is the calibrated Ruthenium Oxide resistance sensor, provided by the nice guys at Lakeshore Cryotronics. I could do with 1 more thermometer actually, for the 100mK plate (discrete heat exchanger stage). Perhaps I’ll put that on my Christmas list.

Here is a picture of two of the thermometer readouts:

shebridges

Here the bridge apparatus is measuring the conductances of the 2 little RuOx resistors, which go down with temperature, and change quite noticably below 1K. The top readout is the accurately calibrated sensor, which measures from 40K down to 20mK. The bottom readout uses a generic calibration for an ‘average’ RuOx thermometer, and so it gives a rough idea of the temperature but is not extremely accurate.

With a quick bit of software coding you can hook up these readout boxes to the PC via a GPIB bus and calculate the temperature of the experiment from the data.

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