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I am trying to repair several reasonably expensive pieces of equipment (ABI Veriti thermocyclers) in our lab which report the same error. They were all bought at the same time and must have a bad batch of components (they failed immediately after the warranty period).

I believe I have tracked the issue down to a failed thermistor. ABI (now ThermoFisher) will not sell me the part or tell me the specs.

The specs I have ascertained (on a good component) with a lab thermometer and multimeter are:

  • Radial PTC
  • 1.1kohm @25°C
  • 1.293 @ 76°C
  • Beta(k) ??
  • Length: 5mm
  • Diameter: 2.1mm
  • Shape: cylindrical

A slightly smaller diameter is probably OK but larger will not fit in the orifice machined into the heat sink.

I can not find anything even vaguely close to this on Mouser or a couple of other sites I have tried.

Any suggestions would be appreciated. We're a cancer research lab if that info motivates you.

Mystery Thermistor

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  • \$\begingroup\$ Welcome to EE.SE. That is a bad sensor. It should have a tremendous difference in resistance from 25 C to 100 C. \$\endgroup\$ – Sparky256 Aug 8 '18 at 20:07
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    \$\begingroup\$ @Sparky256: It seems very close to the correct values for a Pt1000. \$\endgroup\$ – Transistor Aug 8 '18 at 20:22
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    \$\begingroup\$ @Thomas: Be careful with the edits. You've changed the temperature readings referenced in my answer. It's best to leave the error but add the correction as a comment or update. You can leave it as it is as this comment will notify any readers. \$\endgroup\$ – Transistor Aug 8 '18 at 20:43
  • \$\begingroup\$ Sorry, I specified that the reason for the edit was to correct the value and expected that to be featured prominently but don't actually see it. \$\endgroup\$ – Thomas Leete Aug 8 '18 at 21:09
  • \$\begingroup\$ A non technical fix is that your institution probably has a lawyer, and getting them to give the recalcitrant supplier a polite phone call to blow smoke about "legal obligation to supply parts" and "cancer ...publicity..." may find the correct part supplied promptly, with no real effort on your part. No downside if it doesn't work. Lawyers love being able to do something easy and constructive for a change. \$\endgroup\$ – Henry Crun Aug 8 '18 at 23:45
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That looks like a Pt1000 resistor. These are platinum resistors with a resistance of 1000 Ω at 0°C and a postive temperature coefficient of resistance with temperature.

Have a look at the resistance ratio chart. This one by Kongsberg shows the resistances for a Pt100. Scale by 10 for a Pt1000.

Temperature    Your reading   Table
 25°C          1100 Ω         1097.3 Ω
100°C          1340 Ω         1385.1 Ω

Pop it back in the kettle (at sea level) and measure the resistance again to check.

Pure platinum has α = 0.003925 Ω/(Ω·°C) in the 0 to 100 °C range and is used in the construction of laboratory-grade RTDs. Conversely, two widely recognized standards for industrial RTDs IEC 60751 and ASTM E-1137 specify α = 0.00385 Ω/(Ω·°C).

Before these standards were widely adopted, several different α values were used. It is still possible to find older probes that are made with platinum that have α = 0.003916 Ω/(Ω·°C) and 0.003902 Ω/(Ω·°C). Source: Wikipedia's Resistance thermometer.

There are very many manufacturers and several can make to order with case and lead lengths to suit.

I note that yours has only two wires. For industrial applications where the sensor may be remote from the amplifier a three or four-wire connection is made to allow correction for voltage drop along the current-carrying wires. Since yours does not have these additional wires it is probably located close enough to the measuring circuit that the error produced is not significant.

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  • \$\begingroup\$ How do you get the degree symbol? Is it part of Latex? \$\endgroup\$ – Sparky256 Aug 8 '18 at 20:09
  • \$\begingroup\$ Alt-0172 on Windows with number keypad. Charmap when on the laptop. Right now I have °²Ω on the "Characters to copy" field as these are the most useful on htis side. We should probably have a character/symbol tool on the EE.SE editor. \$\endgroup\$ – Transistor Aug 8 '18 at 20:14
  • \$\begingroup\$ Thank you! You've figured it out despite me giving you some erroneous data. My "boiling" water was from the microwave and not evenly 100c. I repeated the point with it a thermometer instead of assumed 100c and it matches the pt100 precisely. Now, is "pt1000" a general description of this part or that specific item from that specific vendor. The dimensions i found so far were close but not exactly what I specified. \$\endgroup\$ – Thomas Leete Aug 8 '18 at 20:35
  • \$\begingroup\$ Pt1000, not Pt100 which is 100 Ω at 0°C so don't get them confused. Note also the capital 'P' as in 'Pt', the chemical symbol for platinum. Don't forget to accept the answer if nothing better comes along. \$\endgroup\$ – Transistor Aug 8 '18 at 20:38
  • \$\begingroup\$ Transistor, you are correct that the leads are <6cm from the amplifier board. \$\endgroup\$ – Thomas Leete Aug 8 '18 at 21:06
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In point of contributing to the answer to the question, I want to add:

  • Not all variable-resistance temperature sensors are called "thermistors". Part of my problem was that the component was not classified to the space I was searching was (thermistors).
  • This particular form factor of Pt1000 is called a "wound wire".
  • The error (in case/so that someone finds this by Google) is "0x8006" -thermal runaway
  • There is no legal obligation to supply parts. Massachusetts, where I am from has tried to pass a "right to repair" law which would compel at least technical documentation but said law does not exist either.

Yesterday, after after talking to some well-meaning but less helpful people and begging to speak to an actual repair technician, I got a return call from an older gentleman who confirmed my diagnosis and indicated that it was a known problem. Apparently our failures a year ago (they have been collecting dust for a year) were the first of several. He called me back this morning having received permission to take the units back for repair free of charge. Previously, the written repair estimate for each was 95% of the cost of the machine when new. Our group owns 6 of these machines and my impression was that this was a one-time offer because of this volume but maybe somebody else will have luck with this approach.

Thanks Transistor for the help.

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