Interfacing Thermocouple with BS2

Jack L.

Hello -

I have the BS2/BofEd and love it. Automated data collection has made my life in lab so much easier. Usually monitoring various voltages with the LTC1298 and logging them on the datalogger is plenty.

However, my project now involves monitoring the output from both a k-type and c-type thermocouple. (Working on the development of a high-temperature oxygen sensor). I was hoping to use the Parallax DS2760 as an interface for each thermocouple, but now realize that the component is not compatible with the BS2.

Now I'm thinking of interfacing thermocouple modules (http://www.dghcorp.com/d1000/d1300.htm) that measure thermocouples and report temperatures back as ASCII. I am hoping the ASCII data will not be too difficult to record using the datalogger. Any thumbs up/thumbs down reactions at this point would be appreciated.

My main question: the modules come in RS-232C or RS-485 output versions. Which would be easier to interface with the Stamp? My BofEd kit has the mini-USB connector, not RS-232. Does the module ASCII output need to come through the USB connection (using the proper adapters) or can I feed this output into the Stamp and out to the datalogger through the I/O pins?

Alternate routes: If there is an easier way to record thermocouple data with the BS2, I'm open to suggestions. I really only need, with high-precision, the mV thermocouple output (with cold-junction compensation) - I have the lookup tables and equations I need to convert that into temperature for k and c-type thermocouples.

Or I could just buy the BS2pe, which is compatible with the DS2760, since that'd be cheaper than buying 2 of the $300 modules. Is that the way to go? Will it plug right into the space on my BofEd where my BS2 currently is?

Any suggestions? Thanks!

???O? ????

I have been pursing a thermocouple for my application as well. I will be interfacing a max6675. Maybe this will work for you as well?

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DP
(ΜΟΛΩΝ ΛΑΒ&#917[noparse];)[/noparse]

$WMc%

JL
I read Your post and You said
"I was hoping to use the Parallax DS2760 as an interface for each thermocouple, but now realize that the component is not compatible with the BS2."

I have used this with the BS2.

I used the 2.4 editor and not the 2.2

This might help. ·A type "C" T/C isn't supported



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The Truth is out there············································ BoogerWoods, FL. USA

Post Edited ($WMc%) : 6/20/2009 3:19:32 AM GMT

Tracy Allen

Jack,
The BS2pe is a good alternative in order to use the DS2760. By the way, cold junction compensation is not built in. It does have a temperature sensor, which provides a cold junction temperature, and a sensitive voltmeter (about 15 microvolts per bit I think). Those two readings are then used in a table lookup/down computation to arrive that the compensated temperature. The type K table is provided but you will probably have to type in the table for the type C (tungsten - tungsten/rhenium) couple and understand how it works.

You could also roll your own if you either amplify the thermocouple output up to the level of a standard ADC, or buy a sensitive ADC such as the 16 bit ADS1100 with its own x8 programmable gain amplifier, to get a sensitivity of about 10 microvolts per bit. A separate temperature sensor has to measure the cold junction where the thermocouple attaches to the ADC. Then the table lookup/down computation is pretty much the same as with the DS2760.

The MAX6675 is a good option for the type K couple. It has built in cold junction compensation and its output is a breeze to use. Its digital output is directly in degrees Celsius without any fancy computation. SPI interface for any Stamp. However the built in cold junction compensation is only for the type K. Type T is close enough too. I think with an additional temperature sensor and a bit of computationto un- and re- compensate, is should be possible to use it for other types too.

An advantage the DGH devices might have is that they are industrial strength, and there is input protection and noise reduction filters and other features. Either the RS232 or the RS485 could work with the Stamp, and you use the normal i/o pins for this, perhaps with a few resistors for isolation. The data comes out in an easy to use format, but you'd have to master the command set.

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Tracy Allen
www.emesystems.com

Jack L.

Thank you all for the replies - they are very helpful! I plan on trying the DS2760 route first and then going to the DGH modules if needed. I'm confident I can get accurate information from the C-type T/C with the DS2760 by logging the voltage, skipping the lookup table, and converting the voltages later, including cold-junction info. If the data looks good, maybe I will program a c-type look-up table by hand. That's great to know that both the RS232 and RS485 could be interfaced with the Stamp fairly easily if needed. I'll keep that as plan B.