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Max 6675. Taking Precise Temperature Measurements?
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Max 6675. Taking Precise Temperature Measurements?

by dir on Fri Sep 03, 2010 1:23 am

I need to find the heat added by a water heater by measuring the temperature at the input and output pipes. It needs to be fairly precise (within 1 °C). I just need ΔT between the two pipes, not the actual temperature readings.

Can I accomplish this with two Pipe Clamp Temperature Probes/ Type K thermocouples?
Could I use two thermocouples and input them into a difference op amp and then take the output of that into the max6675? What kind of problems am I going to run into? I realize at 55 µV/°C there's not a lot of room for precision.

Any ideas or direction would be greatly appreciated.
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Re: Max 6675. Taking Precise Temperature Measurements?

by adafruit_support_bill on Fri Sep 03, 2010 5:51 am

Can I accomplish this with two Pipe Clamp Temperature Probes/ Type K thermocouples?

Assuming copper pipe, this is a good approach.

Could I use two thermocouples and input them into a difference op amp and then take the output of that into the max6675?

This adds another component in the analog signal path and potentially invalidates the cold-junction compensation built into the max6675. I think you are better off to use 2 max6675's and calculate the difference in code.

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Re: Max 6675. Taking Precise Temperature Measurements?

by adafruit on Fri Sep 03, 2010 12:00 pm

thermocouples themselves are not precise to greater than 2 degrees
you need a Class A PT100 or PT1000 sensor

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Re: Max 6675. Taking Precise Temperature Measurements?

by adafruit_support_bill on Fri Sep 03, 2010 12:25 pm

Good point. An RTD will do 0.05C or better.

Here's another option that might be easier to interface with: http://www.national.com/ds/LM/LM35.pdf

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Re: Max 6675. Taking Precise Temperature Measurements?

by Volkemon on Fri Sep 03, 2010 11:06 pm

I got two of the 6675 kits and two thermocouples, and put them both onto one arduino. The readings are converted to *F, and converted to integer value. Both thermocouples returned 32* , sometimes flicking to 33* when sandwiched between two ice blocks.

Boiling water they went from 207* to 213*. I am suspecting the bubbles were the fault.

In a household oven, they were within 2*F of each other up to 874* on clean cycle.

Using the thermocouples built into my buddies kiln, both tracked within 3*F of each other up to 1874*F. one was *always* lower than the other, maybe a dirty connection...

After using the TMP36 's and having three 'side by side' bundled with heat shrink disagree by +/- 4*F , I wanted to test the 6675 before spending a lot of time on them. GREAT product.

Now to go see wtH a RTD/ classA PT100/PT1000 is :) I <3 the forums.......
Livin' , learnin' , lovin' and laffin'. Thanks for the help!
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Re: Max 6675. Taking Precise Temperature Measurements?

by linuxworks on Tue Sep 14, 2010 9:09 pm

I have tried 3 things, so far, on my espresso machine (my app for a therm sensor).

lm35 via an analog-in. works ok, easy to connect but its 'analog' and that is not fun for longer cable (remote) use, which is my use-case.

lm75a via i2c. liked it! but it was not really meant for 150C use and the so8 pkg was not really friendly to mount (lol).

now the max6675 chip (behind an i2c PE chip, actually) and that is working out well for me.

not very cheap but I think its the best solution for my application. its robust and accurate enough for me (I only need repeatability to a degree or two, anyway).

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Re: Max 6675. Taking Precise Temperature Measurements?

by samroesch on Mon Sep 20, 2010 1:35 pm

If I have a long(ish) run from my microcontroller to my sensor, is it better to have a long thermocouple wire to the MAX6675, or a long serial connection from the MAX6675 to an arduino?
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Re: Max 6675. Taking Precise Temperature Measurements?

by adafruit_support_bill on Mon Sep 20, 2010 2:14 pm

How long is "long(ish)"? If you want to extend the TC wire, you should use a twisted pair TC extension to minimize noise pickup.

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Re: Max 6675. Taking Precise Temperature Measurements?

by samroesch on Wed Sep 22, 2010 8:10 pm

Thanks. The run would be 2-3 m. If using a twisted pair, and TC is one of the lines, what is the other?
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Re: Max 6675. Taking Precise Temperature Measurements?

by adafruit_support_bill on Wed Sep 22, 2010 8:29 pm

"TC" is thermocouple. Due to the way thermocouples operate, you need a thermocouple-specific extension to extend the thermocouple to the Max 6675 amplifier. A twisted-pair extension will minimize any noise pickup.

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