I'm trying to wire four thermal probes in a way that I can get an average temperature between them all, but I really can't figure out how I'd do that, exactly.
For example, I stick all four probes into a big block of ice, and leave them in the freezer, until each probe's readout is stable within 0.01. (doesn't fluctuate more than that) If I wire them in parallel, and keep adding thermal probes, the value readout goes down, but it doesn't go down in a pattern and I can't derive any function to fit it. I would much prefer a factual way to average them as accuracy is important to me.
If there is some alternative way I could wire the probes, or if you know the math required to get an average, please please let me know.
Diode probe instruments, I think, provide a fixed current and measure the junction voltage that results. The derivative of the solved diode equation gives something like (8.617 uV/K * ln(1+I/Is)). So with an operating current (I) of, say 10uA, and using a guess of Is=1e-15, the slope would be something like 1.98 mV/Kelvin -- call it 2mV/K. That's what the instrument will be looking at, for changes. It will also need at least one calibration point.
Paralleling junctions would firstly operate each junction at a current lower than the instrument expects, so the conversion of voltage to temperature would be misguided. A new table may be needed or, at least, a two-point calibration step. But placing junctions at different places, meaning that they are potentially at different temperatures, and paralleling them would force all their junction voltages to be the same and thus, to simply adjust the balance of the current branching through them. And what that would look like would require some analysis. I think it would roughly mean that: T1*ln(I1/Is1+1) = T2*ln(I2/Is2+1) = T3*ln(I3/Is2+1) = ... And that all the probes would adjust their relative I1, I2, etc., so as to balance that equation at their respective T1, T2, and so on, with their somewhat varying Is1, Is2, etc. But that probably wouldn't behave they way you want.
If you placed them in series, you might get something closer to an average (a sum of the voltages, anyway), but then your instrument probably wouldn't be able to support the voltage compliances required and wouldn't really drive them properly. And in any case, its tables or math probably wouldn't apply, and again you'd be in trouble.
I don't have any experience with this, so I'm just thinking off the top of my hobbyist head. Perhaps someone can be specific for you.
Since you mention that these probes are diodes, I assume that the readout is a current source; the resistance of a diode is a *very* linear function of temperature (negative slope), so by applying a constant current the readout just has to measure the voltage across a diode and adjust for a particular slope and offset. But the actual slope constant differs among diodes, so if you put them in parallel they won't all be seeing the same current. Putting them in series would be better, but then the readout would see the sum of the 4 voltage drops and would definitely not read right.
So you either need to make your own current source and measuring circuit, or tweak with the one you have and hope it can handle the larger voltage range. Alternatively, you can always use 4 separate circuits and sum them together afterward, with a divide-by-4 scale factor thrown in.
Best regards,
Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis
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Diode probes are not passive components like resistors. Most of them require a well defined constant current through it so the voltage accross it can be measured to find out the temperature. That's what the electronics in the instrument do for you. If you wire some probes in series or parallel, the measured voltage will be out of its normal operating range and the result - if any - makes no sense. So to obtain your objective, the average temperature, you'll have to adapt some other measurement technics. As I have no idea of you application (and your budget), I can only shoot at random:
- Find a resistor based instrument. Four NTC (or PTC) probes can be wired to look like one probe.
- Use four independent instruments. You'll have to find a way to read them out and do the calculation. If you can read out the voltage for instance you can "calculate" by some opamps. If the instruments provide some serial output, you can use a PC to do the calculations for you.
- Build a system that connects the probes to the instrument one at a time. Reed relays may do a good job for you. You'll still need a PC or a micro to control the relays and do the calculations.
Next time provide some more information about the real problem and the equipment available.
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