Signal conditioning for heat flux gauge

[oilyfingers]

I am trying to find suitable input signal conditioning for a thin film heat flux gauge (eg Omega type HFS, http://www.omega.co.uk/ppt/pptsc.asp?ref=HFS-3_HFS-4&flag=1). This gives a signal  output of about 2mV maximum, so we need something with microvolt resolution in order to get an accurate reading (e.g. we expect around 0.84mV typically).

I can't see anything obvious in the NI range of compact DAQ modules, but perhaps a thermocouple input module would be okay? (eg 9211) or the general purpose input, 9219?  The problem is thermocouples have higher output levels (about 0.5mV/degC) than this heat flux gauge. Also I'm not sure all the thermocouple stuff (CJC, linearisation etc) can be 'turned off' properly.

Any thoughts?

Dave

[Ben_L]

Hi Dave, 

This is Ben from National Instruments Applications Engineering, I hope you are well.

I believe the 9211 would be the best suited C-Series module for your application as your sensor will display a K Type Thermocouple output. The module has an high resolution ADC so you should not have any problems with your signal.

Please let me know if you have more questions.

Best wishes,

[oilyfingers]

Hi Ben, yes that's what I think. Just to clarify, although the heat flux sensor uses K-type thermocouples, they are connected as a thermopile which gives a direct (linear as I understand) voltage output in proportion to temperature difference (i.e. heat flux) across the device. I think you are assuming the sensor is a K-type thermocouple. It isn't. Yes it's made of 50-100 K-type thermocouples, but it behaves differently to a K-type thermocouple. It produces an output voltage of about 6 microvolts per BTU/hour/ft^2 which in proper units is about 2microvolts per W/m2 I believe.

My questions are (1) what is the minimum voltage that the 9211 can read? (I know it has 24 bit resolution, but there must be a lower noise limit)

(2) can I 'turn off' the CJC etc that is normally applied by the 9211?

Thanks

Dave

[Ben_L]

Hi Dave,

In response to your first question I would recommend having a look through the manual for the 9211 specifically the specifications section on page 22.

Secondly the 9211 is designed for thermocouples or ±80 mV analog inputs so you can just use the module as a normal analog input therefore bypassing the TC specific signal conditioning.

I will leave it up to you to decide if this module is appropriate for your application. In theory the 24 bit res is more than adequate but the manual will allow you to determine the low end noise the module could pick up.

Best wishes,