High Speed Thermocouple Readings

[Draffut]

Hi!

I need to acquire 12 thermocouples inputs at a very high speed. Typically, the thermocouples will be very small, i.e. less than 0.0020 inch and should then react very fast (10-100 ms). The bottleneck of the system is the acquisition speed and not the temperature accuracy.

The 5B Modules for thermocouples have a lowpass filter at 4Hz. Does that mean I need to use a 5B40
(±50mV -> 5V) and use a conversion chart to obtain the temperature from the voltage? If so, how can I be sure that the temperature value will be accurate? (±5 Celsius is accurate enough for this system)

How do I connect the thermocouple (Type K) to the 5B40? (I imagine there are no CJC or anything like that with that module).

Thanks for helping for I am very confused 🙂

François Tremblay

[Buechsenschuetz]

Most thermocouple acquisition boards have low-pass filters, since the voltage delivered by the thermocouples is very low - limiting the bandwidth limits the noise, too. This is possible in most cases since thermocouples typically react rather slow (which seems not to be the case in your application).

If you want to use a 'standard' DAQ board (i.e. with no dedicated thermocouple connection) you will have to use a thermocouple amplifier which has to include cold junction compensation. I do not know whether someone makes ready-made modules for this application. Analog Devices has integrated circuits providing cold junction compensation.

[Henrik_Volkers]

-use any DAQ device that has sufficient resolution, accuracy and speed to measure your TC voltages (if 5B40 is fine, OK)
-build your own CJC: basically a copper, aliminium or ceramic (thermal) block with isolatet terminals to switch from TC to standard copper wires and a RTD or thermistor to measure the thermal block temerature
-use the 'Convert Thermocouple Reading.vi' to convert your voltages to (hopefully) correct values (I once wrote my own ones, but that was LV3.0 and I needed the 'new' ITS-90 convertions)

I used a al-block with standard screw terminals glued on it and a PT100 all in a styrofoam-box. All temperature drifts and errors made here can directly be added to your measurement of intrest 😮

A good source for TC knowledge:
Manual on the use of thermocouples in temperature measurement,
ASTM PCN: 28-012093-40,
ISBN 0-8031-1466-4
(Page1)'Regardless of how many facts are presented herein and regardless of the percentage retained, all will be for naught unless one simple important fact is kept firmly in mind. The thermocouple reports only what it "feels." This may or may not the temperature of interest"

[AnalogKid2DigitalMan]

Why not use a 5B37 instead (it is made specifically to interface to thermocouples).

But a word of caution- with the 4HZ filter on either the 5B37 or 5B40, your fast thermocuple response will be lost due to the filter's slow time response.