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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: Indoor

Product 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: Black  

Package 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 Circuit board

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.

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  • \$\begingroup\$ Show the wire and the probe please. \$\endgroup\$ – Vladimir Cravero Jan 23 '17 at 10:38
  • \$\begingroup\$ @VladimirCravero - I had to remove the image because I cannot post more than 2 links. Use the first link above that takes you to the Aliexpress product listing which has the pic of the thermometer I'm talking about link \$\endgroup\$ – xs400 Jan 23 '17 at 11:02
  • \$\begingroup\$ It looks like a PTC. Do you have a multimeter? Can you measure the resistance of the probe? \$\endgroup\$ – Vladimir Cravero Jan 23 '17 at 11:06
  • \$\begingroup\$ That sensor could also be a DS18B20 digitial 1-Wire sensor, in which case the data is transmitted digitally over the power line as a series of pulses (shorts). I've got a few with 5 meter cords and fairly thin wires (think CAT5 network cable thickness) and that works fine. 20 meters may push the limit though, as resistance and capacitance of the parallel conductors of the extension cord could distort the signal too much. \$\endgroup\$ – JvO Jan 23 '17 at 14:49
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    \$\begingroup\$ xs400 I bet it was actually 9.9K. And dropping when you put it in your hand means NTC unless your hand is colder than room temperature. \$\endgroup\$ – Spehro Pefhany May 7 '17 at 12:33
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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.

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If current has to flow through the wires, you're definitely going to have an additional voltage drop. By how much? With this little information, I don't know. If the sensor's impedance is very high compared to that of 2x20 meters of wiring, then you don't need to worry about it. I'm leaning towards this possibility considering how the module is powered.

On the product info, it says "Operating voltage = 1.5V" but nothing about the max operating voltage. There is a way to find out, since we don't know much about the schematic nor the ICs. Try it yourself! Use a power supply which supports current limitation feature to limit the current in case the board tries to draw to much current and fry itself.

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