aromaticfanatic - 17-8-2020 at 14:22
I just wanted to share my experience with a homemade digital thermometer.
I used a Weber digital grill thermometer as it has a very large range of temperatures and was only $10. What is very very important for this to work
is to make sure the thermometer uses a thermistor and not a thermocouple. The way to check is to desolder the temp measuring probe and checking the
resistance with a multi meter. If the resistance is in the thousands of ohms and changes with temperature, you're good to proceed. If it's low ohms,
return your digital thermometer unless you have another use for it. (You can monitor oil bath temps with it quite well but I would not recommend using
it for chemicals. The probe is too short to reach the proper positioning of a thermometer in a 24/40 distillation adapter and is too thin for the
adapter. It is also too short to reach into a 500mL RBF.)
Solder on an extra length of wire as long as you want it. The wire should be similar in diameter to the existing wire and try to go as short as
possible. 1 foot of wiring is plenty for most applications.
Buy a 5mm OD x 12' glass tube or something that is 7mm OD or less and at least 200mm long. I got my tube from HomeScienceTools and the tube is 85
cents or so plus $8 shipping. I ordered some storage bottles to make it worth the shipping cost.
Use a propane blow torch and pliers or long tweezers and gently melt the glass. Slightly and slowly pull the molten glass apart until you are left
with a very thin tube. Let it solidify and then break off the excess glass at the thin connecting area. The resulting excess is usually thin enough to
be used as a capillary tube.
Using the blow torch, heat up the thin section of the longer glass tube that will be used on the thermometer and melt it as if you were closing an
ampule. *Slowly* let it cool down using warm air from the blow torch to let the temperature slowly drop to room temperature over the course of about 5
minutes. This may be overkill but will ensure a nice and temperature resistant finish.
Take off the metal cover of the probe of the Weber thermometer and slide the thermistor all the way to the bottom of the thermometer. You should use
some thermal paste as well to ensure no air gaps but in all honesty, I had no issues what so ever with the small amount of the paste that came over
from the probe.
Secure the wire so it won't fall out and add some heat shrink tubing or vinyl tubing to protect the wires from heat and fumes.
The way I support it is by wrapping a rubber band around the digital display and adding it to a ring stand but you can get creative with your ways.
You can also measure the wire you add so that the thermistor will reach the bottom of the probe and allow the digital probe to be glued to the glass.
In hindsight, for my applications that would've been a better approach but my thermometer still works just fine and isn't a hassle at all.
Voila, a surprisingly accurate digital thermometer. The only issue is that the thermometer turns off every 10 minutes but I find this useful instead
of a burden. The thermometer is extremely accurate for a grill thermometer and was able to measure an ethanol azeotrope at temperatures from 77-78.8C.
That is way more accurate than I would've expected.
Hope this helps someone!