I bought a cheap Chinese digital thermometer like this one
with this description Link to product
Item specifics
Power Type: Button Battery Model Number: Thermometer Max Measuring Temperature: 50°C - 69°C Style: Handheld Theory: Temperature Controller Display Size: 2.0 - 3.9 Inches Display Type: Digital Usage: IndoorProduct Description
Feature:
100% brand new and high quality. Quantity: 1 Measuring temperature range: -50Celsius~110Celsius Temperature accuracy: ±1 Celsius Temperature display resolution: 0.1 Celsius Operating voltage: 2 x 1.5V button LR44 batteries Dimension: 48×28.6×15.6mm(approx.) LCD dimension: 36×16mm(approx.) Operating Voltage: 1.5v, LR44/AG13(Include Cable length: 1m Color: BlackPackage Content:
1X Mini Thermometer Hygrometer Temperature Meter Digital LCD Display
It does not have a Hygrometer as far as I know.
I opened it up and found this
It has a probe wire length of about 1 meter. I wish to use it to find the temperature in the storage tank of a solar water heater about 20 meters away. My question is – can I just extend the wire (2-core wire) without affecting the temperature measurement? I looked up Amazon product reviews for similar products (not sure they are the same on the inside), but I didn't see any reviewers having extended the wire more than a few feet.
Also, what power supply can I use instead of LR44 cells. I cannot make out the chip inside, though some reviewers on Amazon have stated that anything upto 30 Volts would work.
Best Answer
Some of these devices use AC on the sensor (making the thermistor part of an RC circuit). In the case extending the probe will probably fail. The 1.5V operating voltage makes it much more likely it is such a chip. There also appears to be a 10K reference resistor beside the NTC thermistor connections, so it could be this type.
You can test it with a cable or by adding appropriate capacitors to the probe connections to simulate the cable capacitance. If your cable is 20pF-30pF per foot you might try 1nF.
If it is DC through a typical 10K @ 25 C thermistor you can extend it to any reasonable length- the wire resistance will have no significant effect.