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Message ID: 2590     Entry time: Fri Jun 18 10:15:14 2021
Author: Stephen 
Type: Update 
Category: Cryo vacuum chamber 
Subject: Temperature sensor considerations 

RTD thoughts - we have just been using the sensors that were provided, without noticing their constraints or deficiencies.

  • our RTD p/n is 615-1123-ND (Digikey), which is a Littlefuse PPG102A6 platinum RTD and has an apparent temperature range of -200 °C (73 K) to + 600 °C. The primary data sheet does not have a Resistance vs Temperature curve, simply presenting the slope of the temperature dependence in a parameter "Temperature Coefficient of Resistance" (TCR), but a table is available in an auxiliary document called "RT Chart". An image of this chart is attached below.
  • we have not violated that 73 K lower limit yet, but we are about to as we mount an RTD directly to the coldhead. Let's see how that RTD on the coldhead will withstand the lower temperatures.
  • the CTC100 manual indicates that any arbitrary calibration curve may be input, but there are a number of sensors' calibration curves built in - it might be a good idea to make our next sensor decision based on that list.
  • there is a Lakeshore guide and category page for temperature sensor selection - reconfiguring our 4 RTDs would cost over a kilobuck through Lakeshore, but perhaps we can learn general ideas from the guide as well.
    • it seems that switching to their Cernox line would be helpful in terms of packaging options, and would be the most accurate.
    • their line of silicon diodes would be suitable and has flexible packaging as well.
    • their platinum RTDs also have low temperature range and would be suitable. Packaging is cylindrical, so might be best to pursue the aluminum housing with a bolt hole.

Planning for next steps:

  • for now, it seems that we could get by with our generic and 73 K limited RTDs, and this option is tempting as it requires no additional effort.
  • if we decide we really want to have reliable sensing down to sub-50 K temperatures, we should move to one of the Lakeshore product lines (hopefully one which the CTC100 is configured with a calibration curve for) for about a kilobuck.
  • we should engage in more serious sensor design before Mariner, regardless of whether we take any action now.
    • as a starting point, the Lakeshore catalog and appendices (ref. product info page) and other resources should be absorbed, for considerations like thermal anchoring, lead length, benefits of 4-lead wiring, polyimide leads causing less conduction to the sensor than teflon, phosphor bronze having lower thermal conduction than copper, etc. Most of these topics are gathered from Appendix C.
Attachment 1: RTD_Resistance_vs_Temperature_for_PPG102A6.png  17 kB  Uploaded Fri Jun 18 11:27:22 2021  | Hide | Hide all
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