DIY

Wood stove thermometer

Thermometer for wood-burning stove In winter and on New Year's holidays, thinking about something unusual, wonderful and completely useless)). I focused on measuring the temperature of the wood stove. A digital thermometer, which is located at a distance in the home heating control unit, does not create a festive mood. We need a furnace proper, mechanical or at least electromechanical. A measuring head and a disassembled rheostat were right at hand. They made a thermometer on a thermocouple.
For the thermocouple, twisted wires of steel and constantan were suitable. Steel wire with a diameter of 1.0 mm was taken from the conductor of the corrugated pipeline. A constantan wire with a diameter of 1.2 mm is wound from a low-resistance rheostat (photo 1).
Thermometer for wood-burning stove
The thermoEMF of iron is + 1.6mV, and the constantan is –3.4mV. This means that when the temperature difference between the two contacts (twists) of these metals is 100 ° C, the voltage across them will be 1.6 + 3.4 = 5mV. (I do not recommend using copper and nichrome for thermocouples. There Thermoelectric power is only +0.7 for copper and -1.6MV
0.7 + 1.6 = 2.3mV, that is, half as much.)
Photo 2 shows a prototype of the structure and the sequence of wire connections is clearly visible.
Thermometer for wood-burning stove The final design in photo 3 and 4.
Thermometer for wood-burning stove Thermometer for wood-burning stove Seven pairs of twists are located as close to each other as possible (the length of the wire segments is 12 cm). The piece of plastic not only fixes the wires, but also protects the “cold” strands from the heat radiation of the oven. Thermocouples will provide a voltage of 5×7 = 35mV at a temperature difference across the contacts of 100 ° C. The pipe temperature is around two hundred degrees. “Cold” twists are not cooled, so their temperature is fifty degrees. The scale of the measuring head is designed up to 100 mV. Therefore, at a voltage of 35×1.5 = 52mV, one can expect a deviation of the instrument arrow by half the scale. Surprisingly, after production it happened. In the temperature range of several hundred degrees, the thermoEMF changes little, and it can be assumed that the deviation of the instrument arrow will be proportional to the furnace temperature – the temperature scale will be linear. This is not entirely true, but it will suit for approximate measurements.
The thermocouple mount is shown in photos 5,6 and 7.
Thermometer for wood stove Thermometer for wood burning stove Thermometer for wood-burning stove There is no hole for the sensor in the furnace, therefore the“ hot ”contacts of the thermocouple assembly are pressed with a clamp to the chimney outlet directly at the furnace body , and cold ones are freely located between the furnace body and the water heating tank. The air temperature there is about 50 degrees.
Photo 8 shows the measuring head.
Thermometer for wood-burning stove Until a decent place has been found for it, and it is attached to the water pipe coming from the circulation pump.
All materials used in the construction must be heat resistant.
I used glass tape, fluoroplastic from a coaxial cable (it can probably be replaced with a thick FUM tape), a piece of heat-resistant plastic from a car heating radiator. The wires to the device must also have fluoroplastic insulation or at least silicone. Better to be safe than to poison your loved ones with caustic smoke.
More interesting than manufacturing was measuring the internal resistance of the thermocouple voltage source. It turned out to be very small – about one Ohm. In this case, the ohmic resistance of the thermocouple assembly was in the region of 2 ohms. I, of course, cannot explain this, because I know about thermoEMF only that it was discovered 200 years ago by the German physicist Seebeck. Probably, if the thermocouples are short-circuited, then the cold contacts will be heated by the flowing current, thermal equilibrium will be established and the potential difference will disappear? The current, after all, cannot be infinite.
The measuring head has a resistance of about a kilohm, so the thermocouple can be loaded with something else. Unfortunately, the microcircuit voltage converter starts working at only 0.2V, and my thermocouple assembly produces 0.05V when the oven is moderately heated. This means that the next useful design will be ready by 2022 New Year – a boost converter based on a blocking generator. He just needs a low-resistance power supply. Several multi-colored LEDs will blink merrily on the chimney (blinking mode will save the meager thermocouple energy). The question/topic is automatically published in the social. site network – follow the answers there too:

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