I just completed assembling a thermometer built from an LM34DZ, a Harbor Freight DMM, and a 9 volt battery. Attached is a photo of the completed assembly.
The external battery provides power to the LM34DZ, and the output from the LM34DZ is fed back into the DMM. I compared the measurement of my assembly to a thermometer of known accuracy and found a significant discrepancy. Turns out, there is a trimpot inside the DMM, so a whim, I decided to try to calibrate the assembly. I placed the LM34DZ (in a plastic bag) in a styrofoam cup of ice and water and put the cup in the food freezer. In about 5 minutes, the reading on the DMM had stabilized and I then adjusted the trimpot in the DMM for a 320 millivolt reading, which represents a 32 degree F. temperature.
Thus with this adjustment, I matched the DMM to the LM34DZ and nullified any built in errors in both. Now, my assembly is within .15 degrees F. of my reference thermometer (which itself is only claimed to be +/- .1 degrees F.)
And best of all, I used one of several Harbor Freight 90899 DMM's that I have, most of which cost $1.99 or less. All told, my thermometer cost less than $10 including two new Duracell batteries.
The external battery provides power to the LM34DZ, and the output from the LM34DZ is fed back into the DMM. I compared the measurement of my assembly to a thermometer of known accuracy and found a significant discrepancy. Turns out, there is a trimpot inside the DMM, so a whim, I decided to try to calibrate the assembly. I placed the LM34DZ (in a plastic bag) in a styrofoam cup of ice and water and put the cup in the food freezer. In about 5 minutes, the reading on the DMM had stabilized and I then adjusted the trimpot in the DMM for a 320 millivolt reading, which represents a 32 degree F. temperature.
Thus with this adjustment, I matched the DMM to the LM34DZ and nullified any built in errors in both. Now, my assembly is within .15 degrees F. of my reference thermometer (which itself is only claimed to be +/- .1 degrees F.)
And best of all, I used one of several Harbor Freight 90899 DMM's that I have, most of which cost $1.99 or less. All told, my thermometer cost less than $10 including two new Duracell batteries.
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