Faster temperature measurement when temperature compensating?

How can I speed up the procedure when I temperature calibrate unit X? Must I wait for the temp to stabilize at a given temperature before measuring or is it possible to measure "on the fly" as the temp increses and not waiting for it to stabilize?

The temp sensor is internal to the device (in the same IC package).

Reply to
HJ
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If the temperature is rising, and your reference thermometer reacts quicker or slower to this change than the device you're calibrating, you will introduce an error. Whether this will be acceptable depends on the test setup, the rate of change, and the accuracy you want. For modest accuracy requirements it may work.

If you want to get an idea of the error, first do a proper calibration where you let the temperature stabilize, and then do an "on the fly" comparison.

Reply to
Arlet

It depends on what accuracy do you need.

Must I

The temperature gradients of several degrees C are typical. It is very difficult to get the gradients to 0.1C or below.

Vladimir Vassilevsky

DSP and Mixed Signal Design Consultant

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Reply to
Vladimir Vassilevsky

This will provide for the accuracy of no better then several degrees C. It is very difficult to have a consistency and a control for all kinds of the temperature gradients at the board level.

Vladimir Vassilevsky

DSP and Mixed Signal Design Consultant

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Reply to
Vladimir Vassilevsky

For a single lag T = To + (Tf - To) * (1 - exp((t-t0)/tau))

Solve for Tf* and if you know tau (which should be fairly consistent if the assembly is consistent and cleaned well) and you can predict Tf given arbitrary t and To. As Robert says, you want t-t0 to be at least tau to have any hope of getting a good extrapolation, and of course the temperature difference has to be large in comparison to the resolution and stability of your measurement.

  • Tf = To * (T - T0)/( 1 - exp((t-t0)/tau))

If it's much more than a single lag, which depends on the physical configuration, you'll probably have to do some kind of numerical fit and I have more doubt as to the ability to improve things enough to be worth the trouble.

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward"
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Reply to
Spehro Pefhany

Not necessarily. It is possible that the sensor might be taken from a storage area that is already not too far from the temperature of the calibration chamber. In that case you would not have to deal with "all kinds of temperature gradients". Instead you would have a fairly repeatable starting condition. The on-the-fly extrapolation would only be used to make a relatively small correction. The accuracy of that extrapolation will be nearly proportional to the difference between the storage area temperature and the calibration chamber temperature. Without knowing this difference you cannot conclude that the extrapolation can never be more than "several degrees C".

Of course if the starting condition is totally unknown, then accuracy will be poor. But if this calibration is part of a manufacturing process that is going to be done over and over again, then several improvements can be made. One is to make the starting condition as uniform as possible as described above. Another method was suggested by an earlier responder. That is to subject a few units to the extra-long stabilization time in order to develop a generic model for the extrapolation. There is no reason to treat each unit as a brand new problem. Knowledge gained from a few extended calibration trials can be used to refine the lag model.

Robert Scott Ypsilanti, Michigan

Reply to
Robert Scott

You see, Robert, I had to deal with this problem before.

In that case you would not have to deal with "all kinds of temperature

It all depends on what place on the shelf in the calibration chamber is the particular unit, how many units do you have in the chamber, and even what is the weather today (since the efficiency of the cooling/heating depends on the conditions outside). As for the board level problems, a single LED creates ~50mWt of heat, which is enough to create a gradient of 1C in the area of 1 cm.

The

Speculation.

to

Usually is not worth it, since it is very dependent on everything. Just select a reasonable heating/cooling speed and hold time.

Vladimir Vassilevsky

DSP and Mixed Signal Design Consultant

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Reply to
Vladimir Vassilevsky

That depends entirely on how precise you want the measurement to be. The longer you wait, the better the chance that you won't have any systematic error in the final result.

Now, if you know your device and its environment really well, it can be possible to do a "ballistic" measurement, i.e. observe the "flight path" of temperature over time, to extract data like thermal capacitance and resistance between the thermometer and the point where you really want to know the temperature, so you can predict arithmetically where this path will eventually end. But given that you're trying to "calibrate" something, it's rather unlikely that this can work.

Reply to
Hans-Bernhard Bröker

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