Ultra low temperature (-80ºC) monitoring probes

  • Hardware Contributor

    Did any of you had experience in monitoring Ultra Low Temperature freezers?
    I wanted to stick a probe into a -80ºC fridge but it seems that DS18b20, SI7021, DHT11 and others are pretty much limited below -40 to -55ºC.

    Still many freezer monitoring solutions seems to exist, including some with temperature probes, so I was wondering if any of you had some insights on what probes to use for this purpose.


  • Mod

    @emc2 some thermocouples can go way below -100 degrees Celsius.

  • Hardware Contributor

    Yes I saw some, or some sensors such as http://www.mouser.com/ds/2/619/Pt100_KN1510_ceramic_wirewound_RTD_element-270979.pdf but I have no idea how to use these guys so far (but googling into it).

    I was hoping for an obscure 1Wire or I2C sensor that someone may know about, but it will probably won't be that easy 🙂

  • @emc2
    According to some datasheets a K-type thermocouple would be my first choice. Look here for a table on K-types

    a MAX31855 can be used to interface to the arduino. See some explanations here and here.

    I haven't read all of the information above, but the datasheet for the MAX31855 tells us that it can be used from -270º up to 700º.

    Could be a starting point I think.

    Good luck,


  • Hardware Contributor

    @boozz Thanks.
    I actually ordered a bunch of thermistors and a K-type thermocouple + MAX31855 amplifier from adafruit yesterday.
    I will see how it goes.

  • Hardware Contributor

    Digging a little more in the docs, it seems that most of these freezer have a "4-20mA" output port, and this one does. (for 250 Ohm loads)

    0_1467342250918_Capture d’écran 2016-06-30 à 8.03.49 PM.png

    I'm starting to find explanations on how this work, mostly for 24V non powered sensors and 5V arduino.
    This freezer seems to use a 12V version, already powered. I now have ~12V between Pins 1 and 2, and there is a 7mA current going through while it's -82ºC.

    So my guess is that a simple voltage divider should do it, right?
    0_1467341541122_Untitled Sketch 2_bb.png

    • Resistors are 182 and 68 ohms (250 Ohm total)
    • I should have ~3.2V across R2 that can then be measured with A0 (forgot it on the schematic, Analog - and Arduino GND will be linked)
    • Freezer also have a "normally open" contact, supposed to close when in alarm mode, so could use it as an Interrupt too.

    I already have the temperature map: 4mA is -100ºC and 20mA is +50ºC so a

    temp = map (AnalogRead(0), 0, 1023, -100, 50);

    should be pretty much it it code wise.

    If anyone have experience on these things I'm listening (and googling) while waiting for the parts to come. At least then the MySensors part is easy to add!

  • @emc2
    I would connect a 250 Ohm (standard 249 Ohm..) resistor from Analog Output (+) to GND (pro-mini). Connect the Analog (-) to GND (pro-mini) as well and a wire from Analog Output (+) to an A0.

    The 4-20 mA is 'pushed' through the 250 Ohm resistor which creates a voltage of 1VDC at 4 mA and 5 VDC at 20 mA (and of course any value in between). U=I*R

    Under '4-20mA current loop' with your preferred search engine you will find lots of background information.

    Change the temp map as follows:

    temp = map (AnalogRead(0), 205, 1023, -100, 50); 

  • Hardware Contributor

    @boozz Yes that's what I found used for 5V arduino, that's why I broke my 250 ohm into a 182/68 divider.
    The fact that at some point A0 may be connected to >3.3V is not a problem?

    Worst case scenario I can add a 3.3V regulator for the radio and use a 5V arduino.

  • @emc2
    I was not aware of the fact that a 3.3V pro-mini was used. Anyway, replace the 250 Ohm resistor in my post by a 165 Ohm resistor and the value on A0 will change between 205 and 1023.....

    165 = (3.3/5)*250

    No need for a voltage divider in a 4-20mA current loop.



  • Hardware Contributor

    @boozz everything seems to work.

    I also realized that as the node will be battery powered, it may be better to use the 1.1V internal ref.
    If I'm right a 47-55 Ohm resistor should be all what I need, as long as I'm also updating the map() upper scale (ex: use 930 instead of 1023 as I use a 50 Ohm then 20mA voltage will be 1V)

  • Hardware Contributor

    In case someone ends up on this topic using the search function, PCB and code for using a 4-20mA sensor is available on https://forum.mysensors.org/topic/5508/mysfreezer

  • @emc2, do you sell these fully created. how can these be adapted to send email/text/ tweet. I would like more direct information on connection than the current instruction. Post your reply.

  • Hardware Contributor

    No I do not sell these assembled.
    I think the only option on openhardware is DIY kit, but with the included Gerber files and BOM you should be able to have it assembled at a local fab house.

    For notifications, mine is actually linked to IFTTT using a script in domoticz, but basically you can use any notification functions used in your controller.

  • Thank you very much. I will ask the local tech to look at it.

  • @emc2 does every component comes with the DIY kit or I have to buy some of the items.

  • Hardware Contributor

    @sbpurohit I assume it comes with everything (else it would defeat the point of the kit), but I would suggest you to email PCBway to have direct confirmation with them.

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