Getting Temperature Readings into EMC

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27 Dec 2010 13:20 #6262 by BigJohnT
Some recent discussion on the mailing list (started by me) on getting a couple of temperature readings into a PC (for a smoker) got me to thinking that having temperature monitoring of electrical cabinets or motors available to EMC would be a good thing to have. To be honest there were so many suggestions tossed at me it kinda overwhelmed me a bit. I wanted to get as much opinions as possible on how to do this and if lucky someone has done it all ready so I added this topic here.

My first thought is how can I get the raw temperature data into the PC via the USB port which all recent computer have.

This is one idea that seems to be easy maybe to do...

in my opinion the best device for digitizing a thermocouples milli-volt signal is the MAX6675 IC. This device is great because all you have to do is hook the thermocouple up to it and read the temperature using some SPI interface. I have used these devices with an Arduino (read cheap and easy IO) with EMC and other software. You can read several of these devices using an arduino and I think it is the cheapest way to get a thermocouple signal into a PC.


Anyone familiar with the Arduino and knows what all I might need?

Any thoughts are welcome.

Thanks
John

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27 Dec 2010 23:30 - 27 Dec 2010 23:34 #6275 by Mick
There are USB data loggers from several companies:

www.picotech.com/thermocouple.html

www.mccdaq.com/solutions/USB-Data-Acquisition.aspx

If you need high temperatures you would need to use a thermocouple but for temps below ~300 degF you can use a LM34 or LM35 that gives a direct reading, 1 millivolt = 1 degF or 1degC.

Linux drivers: www.picotech.com/linux.html

Using Arduino and a MAX6675 is more EE involved.
Last edit: 27 Dec 2010 23:34 by Mick. Reason: added last line

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28 Dec 2010 00:11 #6276 by PCW
www.chinwah-engineering.com/USBThermocoupleProject.html

Has a simple paring of a FT232 USB chip and the MAX6675, somewhat simpler than an Arduino and they have some example interface code.

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28 Dec 2010 14:42 #6286 by BigJohnT
Mick, That piotech and mccdaq stuff is way out of my league...

PCW, thanks for the link, now we are getting simpler.

John

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28 Dec 2010 14:44 #6287 by BigJohnT
I've found some thermistor probes that are cost effective and use a 2.5mm jack so that is just a resistor connection to the Uno.

John

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17 Jan 2011 13:20 #6746 by BigJohnT
Well nothing is as easy as it looks. Seems like the Arduino wants

The ADC is optimized for analog signals with an output impedance of approximately 10 k or less.


and my thermistor measures about 220k ohms at room temperature.

although I found this on the web using the same thermistor and he used a 22k resistor for the voltage divider circuit.

hruska.us/tempmon/

I also found this on using thermistors with the Arduino

web.cecs.pdx.edu/~gerry/class/EAS199B/ho...hermistorArduino.pdf

Now I'm really confused...

John

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17 Jan 2011 16:09 #6749 by PCW
Thermistors are very non-linear so a 22K divider resistor might make sense for a 220K Ohm (room temp) thermistor if the range of temperatures you are reading is towards the top end of the thermistors range.
The readout will have the highest resolution in the temperature range where the voltage divider resistor is close to the thermistor resistance value, so it makes sense to chose the divider resistor so you get the best performance in the temperature range of interest.

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17 Jan 2011 17:07 #6750 by BigJohnT
Peter, Thanks. I'll keep plugging along with this.

John

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19 Jan 2011 11:36 #6771 by Rick G

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19 Jan 2011 11:55 - 19 Jan 2011 11:57 #6773 by BigJohnT
Hi Rick,

Thanks for the links. I'm trying to use a thermistor that is easy to purchase and cheap like the replacement probes for the ET-73. I've looked at the AD595 and a MAXX one and both seem a good fit for a thermocouple.

My little breakout board has a place to solder on a SOIC chip so if I decide to go that route I think there are some chips that fit that.

John
Last edit: 19 Jan 2011 11:57 by BigJohnT.

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