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Thread ID: 140649	2015-11-18 20:43:00	Question for the electrical gurus	Tony (4941)	PC World Chat

Post ID	Timestamp	Content	User
1411601	2015-11-18 20:43:00	I have a sensor that measures the temperature in my hot water cylinder and sends it ( via wire ) to a display on the wall. It was put in when the house was built so the wire goes all through the walls. At one point the display was replaced and ever since it has read about 15° low by my estimate. My guess from my experience with temperature probes in my pottery kiln is that the wire is no longer the correct length for the sensor/display combo. Questions: Is my hypothesis likely to be correct? Would I correct it by lengthening or shortening the wire? If I wanted to replace the whole thing, is there a wireless option available these days and where would I get one? TIA	Tony (4941)
1411602	2015-11-18 20:52:00	I'm no guru, and my electronics theory is 25+ years back in the fog of my memory so bear that in mind BUT, The wire length should have little to no effect on the readings, it will add a very small amount of resistance into the circuit but it should be low enough to be unimportant. Rather than changing the length changing to a heavier (thicker) conductor would be the biggest change, but at the current levels this circuit probably uses it really shouldn't make a difference. Maybe the sensor and display are not meant to work with each other, or maybe the sensor is worn out. How does the reading compare to the thermostat setting ?	dugimodo (138)
1411603	2015-11-18 21:05:00	Just adding a bit to dugimodo, if the replacement display unit has a much lower input impedance than the original unit, then it is possible for it to be giving a low reading. Best to let the tank temperature stabilise, then run off hot water and measure its temperature with a thermocouple meter, otherwise it is guess work. There may be some calibration adjustment in the display unit. Edit: also need to verify that the new display unit is the correct type for the fitted temperature sensor, as there obviously several different types of temperature sensors/readouts.	Terry Porritt (14)
1411604	2015-11-18 21:21:00	Thanks for the swift replies. Maybe the sensor and display are not meant to work with each other, or maybe the sensor is worn out. How does the reading compare to the thermostat setting ?I have measured the water temperature at the tap with a conventional mercury thermometer and as far as I can tell, allowing for heat losses between the tap and the cylinder the difference is about 15°, as I said. It is entirely possible that the sensor and display are incompatible, although in theory it was replaced like for like. I don't think it is actually worn out as it started misreading as soon as the display was replaced and has been consistently low ever since. The reason I mentioned the wire length is that I know on my pottery kiln the relationship of the probe to the meter to the connecting wire was important. Mind you that was measuring up to 1300° so it could be a different kettle of fish altogether. This has been an issue for some years; I've just finally got fed up with it. I haven't seen any obvious adjusting mechanism at the display end - mind you I wouldn't know what to look for.	Tony (4941)
1411605	2015-11-18 21:49:00	Well if the wire was conducting heat to the display rather than electricity the length would be very important, which makes me realise I have no Idea how this would work. I'd need a circuit diagram to really have clue, the rest is guesswork. Does this setup have a power source and what kind of display is it? I was thinking along the lines of a small DC current passing through a sensor to a moving coil meter kind of thing, in which case the wire wouldn't matter much and reading too high would be easier to fix than too low. If it has a power supply and is reading too low you'd need a higher supply voltage would be my guess.	dugimodo (138)
1411606	2015-11-18 23:54:00	Why do you need to measure it?	pctek (84)
1411607	2015-11-19 00:19:00	Why do you need to measure it?It went in originally as part of the solar water heating installation and is supposed to give you an indication of when to maybe override the automatic switch on/off. In practice I just leave it switched on all the time in winter and off all the time in summer. It is during summer when we have had a series of dull days and the water temp has fallen significantly that having the display helps in reminding me to turn the electric immersion heater on, if nothing else to make sure the cylinder temperature is at the minimum 55° that is recommended. Also in theory the water could get to boiling, which would not be a good thing. I've had it up to about 80° (i.e. 65° on the display) just on solar heating. So I don't "need" to measure it, but it is certainly useful having the display there. And if it is there I am anal enough to want it to be accurate.	Tony (4941)
1411608	2015-11-19 03:01:00	The new sensor simply may not be a direct swap & the sensor circuit may need to be re-calibrated to suit . Or it could just be a cheapy & wildly in-accurate . So unless its the same part no from the same manufacturer ....	1101 (13337)
1411609	2015-11-19 03:36:00	The new sensor simply may not be a direct swap & the sensor circuit may need to be re-calibrated to suit . Or it could just be a cheapy & wildly in-accurate . So unless its the same part no from the same manufacturer ....All of those may be true (although I don't think it is cheap and nasty). I'm actually wondering whether I can replace the whole system with some sort of wireless solution.	Tony (4941)
1411610	2015-11-19 04:34:00	Tony...What does the tank sensor look like? Is there no instruction booklet with the display unit? Is the display unit mains powered? What, roughly is the distance between the sensor & the display unit? PJ	Poppa John (284)
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