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High Speed Thermocouple Reading


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We got a process which has very rapid heat up rates (~10,000 C/sec).  We are experimenting with small type R TC wires to measure this process.  Ideally we are looking for 1000 Hz readings on the TCs.  This is well outside the rate of standard TC DAQ hardware.  To add to the fun, we'll probably have very long TC wire runs (100-200 feet).  We are limited to TCs instead of non-contact measurement due to physical constraints.

Right now I'm using a cRio system with an NI-9205 for measurement.  I've got the TC wired in differential mode with 50 kOhm bias resistors.  I've also got a small thermistor IC on one of the channels for cold junction compensation.  This works OK, but it's not a great solution.  The TC is really susceptible to noise and other environmental conditions that affect the reading.

Has anybody ever done anything like this and has some tips that might help?  Ideally, I'd like to find a signal conditioner that can be placed near the process to convert to voltage to re-transmit to the DAQ.  This company has something that works with type K with 1ms response rate:

http://thesensorconnection.com/signal-conditioners/signal-conditioners/type-k-thermocouple-amplifier-signal-conditioner-0-5-vdc-out

But I need type R.  Anybody seen anything like that?

Thanks!

 

 

 

 

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Someone is working on cold fusion :D

If you insist on long distance, then you should be looking at 4-20mA current loops and I highly recommend not doing CJC yourself - it's a black art. However. I personally convert close into digital then transmit via ethernet, rs485, profibus, smoke signals, whatever) since you can string many off of one link.

Your FPGA should be plenty for the high speed bit.

Edited by ShaunR
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3 hours ago, ShaunR said:

Someone is working on cold fusion :D

 

Something like that ;).....

 

3 hours ago, ShaunR said:

If you insist on long distance, then you should be looking at 4-20mA current loops and I highly recommend not doing CJC yourself - it's a black art. However. I personally convert close into digital then transmit via ethernet, rs485, profibus, smoke signals, whatever) since you can string many off of one link.

 

Can you recommend a device that can do that?  None of the thermocouple signal conditioners I can find can come to anywhere near that speed.  They typically operate around 150-400ms.  

 

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Just spitballing here: maybe you could measure directly the electrical resistance with the 4-wire method of the part and correlate that to temperature using the temperature coefficient of resistance. Using a sine wave current at a particular frequency you can use a lock in amplifier which can detect small signals in where there is a lot of noise. I am assuming the part is metallic and conductive. You are putting TCs on it so you can get leads on it I assume.

The other thing about using that technique is you can extract other information: the specific heat capacity of the part as a function of temperature. That all falls under the technique known as "modulation calorimetry" .

Using TCs simultaneously would allow you to know the start and end temperatures accurately, with the high speed resistance measurement filling in the gaps in data.

 

Edited by MarkCG
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You may have more luck with an RTD and something like this: http://sine.ni.com/nips/cds/view/p/lang/en/nid/201609

I agree the c-series options do not look like you'll get the rate you need.  With a transmitter like ShaunR suggested, take note it's response rate and that some/most are not scaled to temperature, just the non-linear thermocouple signal is output.

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I thought I remembered seeing fast response thermocouple conditioners long ago, but don't see any now, with exception of that Omega unit you found. Most industrial transmitters and conditioners integrate over at least a couple of line cycles to get rid of noise, since most temperature loops simply can't change that fast. The benefits of 4-20mA transmitters are a bit overblown and you won't find any fast ones. They also introduce a secondary calibration hassle, since the current is almost always measured as a voltage across a shunt resistor with a pretty crappy tolerance.

If you want to stick with off the shelf technology, you could use a fast DAQ card with one of the SCXI modules, like the SCXI-1102, which can go way faster than 1000hz. At that speed, noise is going to be a problem, and you'll have to use twisted pair extension cables with differential inputs.

To get away from long runs of TC extension wire ($$ and hard to find in twisted pairs), you could maybe make or buy a remote reference junction device. I've done that before. It's just a matter of placing a sensor like an RTD in a position to monitor the temp of the terminal blocks or connector where the transition from TC alloy wire to copper occurs, with some consideration to keeping the temp uniform (isothermal block). There is, or used to be, example code for doing the reference junction compensation in LAbview. I use the term reference junction, because it doesn't have to be cold. It used to be common practice to place the TC junction terminal blocks in an oven enclosure that was held above ambient temp, like ~120 degF. I might be able to dig out the code I used, but it was back in Labview v3 days.

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  • 1 year later...

for such propose I use mccdaq product esp 2408  it support up to 3k sample rate 24 bit resolution
but before start  but  regard two thing  thermocouples are like antennas and pick up noise from the power grid and other equipment nearby you should keep your thermocouple a way from such noise

also mcc labview toolkit is based on dot net and is not so nice and have some  bugs but work 

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