DAQ USB 6008 Temperature measurement

[Marko1982]

Hi! I am starting a project for College, its relatively simple (for you guys not me!) It is a temperature control system, will have analog input of temperature which I want to view on Labview and also some digital outputs to switch some heaters and fans, I have already purchased the USB 6008 so this is what I need to use.

I have two questions...1 what type of thermo meter should I use? Would a pt100 work? The resoloution is not hugely important. 2. I will need to use a solid state relay for switching the heaters, will the D/O of the USB 6008 be able to switch a solid state relay directly?

Thanks!!!!

Message Edited by Marko1982 on 12-21-2009 06:15 AM

[memoryleak]

Marko,

  You've got to start somewhere 😉  I think I did this as part of a freshman year microcontrollers class.

  Here are the steps I'd go through if I were trying to decide on those two parts:

1) Thermometer:  I've never used a pt100, but from my googling it looks like a resistance that varies roughly linearly with temperature, described starting in page 2 here: EnGenius article by Bonnie Baker

The resistances looks like it changes by less than 4milliohms per degree, and the current needs to be limited to less than 1mA to prevent your excitation current from warming up your thermometer.  Using those numbers, you can calculate how much your thermometer voltage is going to swing for whatever accuracy you're looking to achieve (0.1C?, 1C? 10C?).

Next, you'll need to run to the datasheet of the 6008 to figure out if the analog input of the device is accurate enough to read your thermometer directly given your accuracy requirements above.  

What you'll likely find is that the accuracy needed to measure any reasonable temperature swing is very very accurate, and while we do make some products that can read this directly (for instance, a few microvolts is cake for our DMMs) the 6008 is not one of them.  If this is the case, Omega has a decent note on Bridge Circuits to help measuring small resistances (such as a single active element, pt100).  Following your bridge circuit with a differential amplifier as in Maxim's app note can bring your paltry several-microvolt voltage swing up to a range that can be easily read by any analog input, including the 6008.  A cheap rail-to-rail output instrumentation amplifier would probably do just fine.

2) Heater:  First, I would go back to the 6008's datasheet and figure out how much current it can supply and at what voltage.  Using that and the solid state relay datasheet you can figure out if you'll be able to drive it directly.  If you can't, there are some pretty simple ways of driving the likely small voltage/currents needed to turn on an SSR.

I hope that helps, and good luck with your project!

memoryleak 

PS:  I've made the assumption that you're an electrical engineering student that has been given a project geared at teaching sensing and control loops.  If you (and your professor) don't care about how you achieve your temperature measurement there are simpler ways to provide a temperature-related voltage to your 6008.

Message Edited by memoryleak on 12-21-2009 09:42 AM

Message Edited by memoryleak on 12-21-2009 09:44 AM

[Marko1982]

Hi Memory Leak,

Firstly wow and thank you very much for your detailed reply.

You are right in saying that I am an engineering student  (part time distance learning and working at the same time   ), This is my 2nd year project..

With regards the temperature sensor.....it does not have to be a PT100, this seems to be too difficult.... I can use what I like so whatever is easiest is best for me as I have a lot to do for this yet! If you had any suggestions that would be great? From reading forums I see K Type thermocouples mentioned a lot, but also I see with these and the 6008 I would have to amplify the signal???

Is this correct and how would I go about doing that!?

Thank you again for your reply, I dont have the benefit of face to face tutors so any help I can get is great.... 🙂

[GovBob]

Here is a simple one I've used in the past as suggested by someone else on these forums. Precon. I assume you would be using LabVIEW as this has drivers available. It uses a serial port to communicate. If you don't have a port available maybe a usb to serial converter will work. Of course check the specifications against your requirements before purchase.

Hope this helps.

[memoryleak]

Marko,

   You might be better off with a higher resistance that changes more over temperature.  Here's a college lab that is based around a USB-6008 and a 10k Thermistor.

   Also, if you decide to go the route of a more difficult resistive sensor, an easy way to measure it is to place the sensor in a Wheatstone bridge and amplify the heck out of it.  Thermocouples are another option, but like you noticed they will require lots of amplification to work with the 6008.  I've also just found the aptly named ni.com/temperature webpage that has a few devzone articles you might find interesting.

   Finally, if you find yourself out in the real world doing this sort of thing, NI does make instruments designed to do exactly what you want to do.  Let us worry about all the signal conditioning for you 😉

memoryleak 

[Marko1982]

Thanks again guys,

I will go and order some thermistors and resistors and see can I get that lab experimant working, if I can get that going then it will be a ood start!!

Thanks again all!

I will be in touch if there are more questions!!

[rockarmatte]

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