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I'm trying to heat a fluid to a certain temperature (under 100C) using nichrome wire, with a thermistor measuring the temperature of the fluid to allow controlling the nichrome wire using PWM and a PID controller. However, it is absolutely necessary that the nichrome wire itself remain below 150C or so. I could just test different PWM values and cap the PID output to a value that never reaches the maximum temperature, but then initial heating from room temperature will be slower. Is there any other way I can achieve this?

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  • \$\begingroup\$ Nichrome wire, by design, has little resistance variation with temperature so not ideal as a temperature sensing resistance. If using nichrome is essential you may need to monitor the heater temperature directly. Also note that if your fluid is conductive and your wire exposed you may have electrolysis taking place due to the terminal voltage or cause unintended dangerous or interfering leakage currents to flow through the liquid path. \$\endgroup\$
    – KalleMP
    Sep 13, 2015 at 12:30
  • \$\begingroup\$ There are some thermistor type heating devices that self regulate, selecting one such that cannot go over 150degC and using a PID loop to maintain your <100degC setpoint might be an option. If you fluid is aqueous and boils below 150degC then good thermal contact with the fluid will prevent it from exceeding the boiling point. \$\endgroup\$
    – KalleMP
    Sep 13, 2015 at 12:33

2 Answers 2

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What you are trying to do is usually achieved with two (2) temperature sensors.

  1. One sensor is in the fluid. It has a good thermal contact with the fluid, but no direct contact with the heater.
    It's used by the control loop to hold the setpoint.

  2. The second sensor is on the heater itself. It has a direct contact with the heater. Ideally, it would be buried inside of the heater.
    This sensor is used to monitor for overheat.

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  • \$\begingroup\$ Thanks, I was hoping that I could do it with just one temperature sensor but I guess Ill probably have to use two. I have a followup question, though. Could I have two control loops in this case, with the first loop taking the temperature of the fluid as input and outputting a temperature (capped at the maximum wire temperature i.e. 150C), which is then used as the input of the second loop whose output controls the heater itself? Or would it make more sense to just cut power when the heater temperature is at maximum? \$\endgroup\$ Sep 13, 2015 at 0:25
  • \$\begingroup\$ The second sensor could be the varying resistance of the heater wire itself, if you independently measure the voltage across it and the current through it. \$\endgroup\$
    – user16324
    Sep 13, 2015 at 10:32
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You could do what you want by using the thermistor and the wire resistance as two sensors. That would allow you to limit the average temperature of the wire to some value (however if the physical situation does not result in fairly uniform heat loss along the wire then it might not be acceptable).

If you are using a PID algorithm you should inhibit integration of error (I term) when the temperature of the wire hits the limit).

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