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MCP9600 Bottoming out at -151C
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Please be positive and constructive with your questions and comments.

MCP9600 Bottoming out at -151C

by ElectroLaser on Fri Oct 22, 2021 2:32 pm

Hi guys,

I am using the MCP9600 breakout board from adafruit, along with the adafruit_mcp9600 library. We are trying to measure our cryogenic chiller with a type K thermocouple and unfortunately the temperature range seems to bottom out at about -150C.

Anyone else get this? Could it be a library issue, or (i hope not) a hardware issue with the MCP9600?

Thanks!

ElectroLaser
 
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Re: MCP9600 Bottoming out at -151C

by mikeysklar on Mon Oct 25, 2021 5:23 pm

@ElectroLaser,

What are you reading for the ambient_temperature?

Are you able to confirm with another temperature device the temperature of your chrygenic chiller where the thermocouple is placed?

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Re: MCP9600 Bottoming out at -151C

by blakebr on Mon Oct 25, 2021 7:06 pm

Could you have made the mistake I have made? Got the thermocouple leads reversed.

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Re: MCP9600 Bottoming out at -151C

by ElectroLaser on Tue Oct 26, 2021 10:54 am

Thanks for the replies.

The cooler's thermocouple is being read correctly on a different device (omega controller), and the MCP9600 works just fine reading temperatures greater than -151C. For ambient I'm reading 23.46C.

It's not that it's just stuck reading the temperature at -151C. It reads correctly through the entire range, except for when it approaches <-151C, it does not read any colder than that. Otherwise it works fine.

I'm pretty sure I've narrowed it down to a couple potential issues. It's possible that the Adafruit circuitpython library has a bug. Or what I'm more afraid of, is the MCP9600 chip itself has an issue reading temperatures (or the coresponding negative voltages) lower than -151C. According to a table I looked up, that voltage would be almost right at -5mV.

Also, the temperature where it won't go any lower, might actually be -153C, not -151C. I'm recalling from memory (or attempting to heh)

Link to table: https://www.thermocoupleinfo.com/type-k-thermocouple.htm

ElectroLaser
 
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Re: MCP9600 Bottoming out at -151C

by mikeysklar on Tue Oct 26, 2021 3:15 pm

What is the Omega reading the temperature at?

I ask because the type-k thermocouples come with a side note of:

Resolution of ±0.0625 °C - note that K thermocouples have about ±2°C to ±6°C accuracy


https://www.adafruit.com/product/4101

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Re: MCP9600 Bottoming out at -151C

by ElectroLaser on Wed Oct 27, 2021 9:38 am

mikeysklar wrote:What is the Omega reading the temperature at?

I ask because the type-k thermocouples come with a side note of:

Resolution of ±0.0625 °C - note that K thermocouples have about ±2°C to ±6°C accuracy


https://www.adafruit.com/product/4101


It's reading at about -250C.

ElectroLaser
 
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Re: MCP9600 Bottoming out at -151C

by mikeysklar on Wed Oct 27, 2021 5:03 pm

If the Omega is reading at -250F you are outside the usable range of the type-k thermocouple. They are not meant to be used in conditions of more than -200C.

It would be interesting if you have the ability to test the thermocouple just above it's limit of -200C to see if it is still responsive or if it really is bottoming out at -151C.

Is the temperature you wish to monitor going to be a consistent -250C?

Are you using the glass tipped or steel tipped type-k thermocouple from Adafruit?

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Re: MCP9600 Bottoming out at -151C

by ElectroLaser on Thu Oct 28, 2021 9:32 am

Update:

I'm not certain of the type of thermocouple we are using other than knowing it's a type K. It's a thermocouple that is embedded inside of a cryogenic cooler, I think maybe Omega. I did also just read on the MCP960x datasheet that Type K's bottom range is -200C

We had this set up with a logger monitoring the temperature upon starting up the cryogenic cooler. This showed the entire temperature curve starting at room temperature and progressing to cryogenic temperatures. The curve was well defined and smooth, but as soon as it hit -151.25C -- the temperature was "stuck" there. The curve returned as expected after turning off the cooler and letting it slowly creep back up to room temperature.

Yesterday a software engineer colleague of mine wrote his own C# drivers for the chip. He tested it on both the cryogenic cooler, and a thermos filled with liquid nitrogen. He saw the exact same results. Everything was performing as expected, except for 'bottoming out' at -151.25C. These results are seen on both the MCP9600, and MCP9601 chips.

That being said, It appears it's not an issue with adafruit's python driver nor breakout board. Instead it's likely both an error in our part on specifying an incorrect chip, and potentially an issue with the MCP960x chip on it's inability to get to the specified -200C rating.

Thanks for your help on this, I believe we will need to go a different route on measuring such low temperatures.

ElectroLaser
 
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Re: MCP9600 Bottoming out at -151C

by mikeysklar on Thu Oct 28, 2021 12:58 pm

Thank you for the summary and further custom code testing with C#.

There multiple different tips for type-K thermocouples which might make a difference (glass vs. metal).

The MCP9600/01 datasheet is pretty clear about the range the chip is expected to produce. While it might be incorrect I suspect another factor is at play.

https://cdn-learn.adafruit.com/assets/a ... 1628886942

42ADEB70-9499-44FB-84B9-0615F49BCDEC.jpeg
42ADEB70-9499-44FB-84B9-0615F49BCDEC.jpeg (830.67 KiB) Viewed 65 times


I agree with you assessment thought that this hardware configuration is not ideal for such low temperatures especially if you are going to be dipping below -200C.

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