what digital i/o to use to read out temperatures from a pt100?

[Peluche]

Hi all,

So this is not a question about programming but about hardware and I need advice!

For my experiment, I have a usb-6008. The problem is I have 3 pt100 that I need to read out and of course the usb-6008 does the job very poorly, because it doesn't read out the resistance but the voltage and so I end up with ridiculous fluctuations in my temperature readings, making it completely useless.

So here is my question, what would you guys use? Honestly the usb-6008 is way enough for my needs and works well with everything else I do except this. I also doubt that my boss would accept for me to buy a 1500$ labview digital i/o, so I kinda would need a cheaper solution. Or maybe there is a way to tweak the usb-6008 to make it do what I want? 🙂 

Any suggestion would be very very welcome!

Thanks a lot!

[Norbert_B]

A digital I/O device is incapable of measuring PT100 temperature values.

I think you have a severe mixup of terminology here. PT100 are resistors which have a quite static temperature coefficient. That means: If temperature changes, the resistance of the element changes. So you have to measure the resistance value of the PT100 in order to know what the temperature is.

See wikipedia for further information on RTDs (like PT100) and how to measure them.

The USB-6008 is a very low cost measurement device with very limited sampling speed and resolution. OK, speed is usually not an issue for temperatures, but resolution definetly is.

Also, the USB-6008 does not supply you with bridge-completion, so you have to build one yourself....which will induce many sources for measurement errors.

Therefore, you either have to live with very limited accuracy and additional external handycraft-things (bridge) or you purchase a module capable of configurable RTD measurement (included bridge completion in the module).

thanks,
Norbert

[Peluche]

Hi,

thanks for your answer but you completely misunderstood me. I am very well aware of how a RTD works and even more aware of how one should measure its temperature! With a digital i/o like the usb-6008 you can do this by measuring a voltage and then converting it back to resistance (provided you have a stable current source), however you can't do that precisely, which I need, and no I can't live with it.

So while I agree with you (I did say that in my question) that I want to buy something else to measure an RTD, my question was "what device should I buy?". 

[Norbert_B]

Sorry if i misunderstood you, but you are (still) using a very wrong term in the context of temperature measurement: Digital IO.

I *think* you are referring to "Analog to Digital Converter", which is a basic element of analog measurement. So, essentially, you are talking about ANALOG INPUT rather than any digital whatever.

I expect this page to be a good starting point for you for collecting further information.

hope this helps,

Norbert

[Peluche]

Aaah I see what you mean! By digital i/o i meant nothing more than that usb6008 device that I indeed use as analog for reading the voltage! Sorry my mistake!

Anyway thanks for the link, that's exactly what I was looking for!

Thanks again!

[IanW · Accepted solution]

For NI hardware you could use NI 9217 and the cDAQ 9171 carrier:

4 Ch, 100 Ohm RTD Module for USB

Alternatively you could use pt100 to DC or possibly pt100 to 4-20 mA These type of modules can then be fed to your USB9008. I haven't used these particular devices, but if you do a web search you will find many similar products.

[Peluche]

That helped tons! Thanks alot!!