control the power of a microwave.

[Le0b95]

Okay, as I understand it, the microwave is probably controlled by a DAQ assistant, which outputs a volt signal, if that makes sense.

[Blokk]

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@Le0b95 wrote:

Okay, as I understand it, the microwave is probably controlled by a DAQ assistant, which outputs a volt signal, if that makes sense.

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DAQ Assistant is an Express VI in LabVIEW, not a hardware component. You need first of all a piece of hardware, which has analogue output option, like the USB-6009. When you have this hardware, you can program it to create a voltage signal regarding to the output of the PID VI algorithm to control the heater's power. Besides, for performance reasons, you should not use the DAQ Assistant, better to program using the DAQmx functions (one problem with the DAQ Assistant express vi is that, it always reinitializes the HW at every iterations, like the mistake in the while loop in your VI).

[NIquist]

Keep in mind that most microwaves do not have a true power adjustment.  The magnetron is either ON (oscillating) or OFF (not oscillating).  The "power level" setttings just make the duty cycle change so set at  HIGH (100%) they stay on constantly and set at DEFROST (10%) the cavity is oscillating for 1 second, then turns off for 9 seconds, then repeats until the time is up.  Some new models use a faster PWM system but it's still the same for you.  You will need to control the duty cycle digitally, not with an analog voltage output like you would use for a simple thermal heating element.

Is there some reason you must use microwaves for your heat source?  There are much easier to control (and SAFER) ways to heat things

[arteitle]

Also be aware that having thermocouples inside of an operating microwave oven might not be good for the thermocouples or the microwave. Thermocouples basically look like a pair of wires connected at the tip, so I would be concerned about the microwaves inducing current in the wires that could damage your DAQ hardware or the wires themselves. Also be aware that microwave ovens don't heat very evenly, especially when the food or container inside is stationary (which I imagine it will be if you have thermocouples in it). Here's an interesting demonstration of the phenomenon: http://www.evilmadscientist.com/2011/microwave-oven-diagnostics-with-indian-snack-food/