Weatherproof extra long puncture thermometer

A quickly hacked together thermometer I built for a farmer to check the temp. in >1m depth in a huge pile of compost.

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A farmer friend of mine needed to know the temperature in a huge pile of compost to determine when the bacteria involved work as they should and when he has to shift the whole pile.

Units on the market are really expensive (in the >300€ range), especially if you want the temperature lance to be over 1m long, made from stainless steel and the whole unit weatherproof...

Features needed

  • weatherproof
  • precise to about +-1°C
  • easy to handle
  • temperature probe mounted to a > 1m lance

What I built

I didn't have time to create a PCB, solder everything etc. so I bought a off the shelf thermometer with a 3m cable DS18B20 probe. The probe is glued into a Ø10mm stainless tube, so it can be pushed right into the compost and stay there. The glue itself creates a nice chamfer. The stainless steel probe has no metal to metal contact to the outer tube. Thats important, as the huge mass of the tube would need much longer to get to the same temperature as the compost, thus delaying measurements a lot.

The case is a IP65 case with a clear top part. I glued the display right into the clear top, added a switch to the bottom part (with a waterproof cap).

The switch does simply connect the unit to a 9V battery (mounted with velcro glued to the case), no fancy circuitry. Mounted to the lance it is easily operated by your right index finger, while grabbing the case. The sensor cable goes out through a threaded waterproof connection.

In total cost was about 45€ for all parts. I could've bought a cheaper thermometer unit, but this one had a long enough cable and the right precision. Anyway, altogether this unit is fully waterproof for a price about 200-250€ below a commercially available one!

  • Finished device

    Jan03/21/2016 at 18:30 0 comments

    I have completed the device by now. Mounted the case to the temperature lance by a aluminum bracket and stainless hose clamps. It may not be the prettiest setup but it's sturdy and it works. I did leave it sticked into the soil outsie for a few days now. Night temperatures below 0°C and some nice test-rain: no signs of moisture inside.

    I left a bit of extra cable because who knows....

    Status: finished

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Jan wrote 03/12/2016 at 15:04 point

Thanks for your skull! Yeah, I should've written that more clearly:
The whole grey unit incl. display, PCB and sensor is 30€ incl. shipping.

  Are you sure? yes | no

Roman wrote 03/12/2016 at 14:59 point

Hi Jan, may be I missed it somehow. Did you buy the processor board as well?

  Are you sure? yes | no

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