Look up Wheatstone Bridge……

Interesting one. I don't think running a long line to a thermistor is a good idea because it will pick up mains hum.

I'd suggest putting the Microbit in a box on the chimney and use the long line to power it, not to measure small voltages. I've not used one but the Microbit is fitted with Bluetooth. As Bluetooth is a serial radio link it should be possible to transmit the thermistor measurements to a computer in the house. I've done remote signalling with an Arduino and a Bluetooth module.

The thermistor needs a stabilised power source. The microbit has a 3V output that could be used via an external resistor to power the thermistor. The value of the resistor would depend on the thermistor used. I couldn't find a specification of the Microbit's ADC: measuring small voltages with it might be tricky, and you may need an op amp to beef things up.

Dave

0.25mm diameter (i.e. thin) copper wire is (according to the IEC) about 400 ohms per metre so even if you need a 50 metre there and back run the total resistance will only be 40 ohms i.e. less than 1% compared to your thermistor assuming it will be at least five thousand ohms. It's drift with temperature will be even tinier so don't expect it to significantly affect the accuracy compared to other factors (such as how accurately you place the thermistor in the plume)

Beware of using a low-resistance thermistors, these have a significant self-heating effect even when run off only a few volts, higher resistance ones ( a couple of tens of Ks) are significantly less prone to such errors. Something like the microbit will be quite happy taking readings from a high impedance source as, presumably, you are talking seconds between readings, not milliseconds.

Neil

<edit> seeing the comments above about hum etc. the simplest answer is  a twisted pair should stop any mains hum issues and you can also add a capacitor to smooth the signal, but use a low-leakage type. You can also filter the signal digitally by taking the average of any arbitary number of readings (make it a power of 2, for example add 256 readings then lose the lowest byte) – it may seem wasteful, but the 'cost' of multiple readings when you only need a results at a large time interval is effectively nil.

Edited By Neil Wyatt on 19/01/2017 20:27:10

The appropriate technology for this is 1-wire. There is probably some support software already written for it on the microbit.

The display. Understood. What I'm suggesting is that you ignore that and program the microbit to broadcast the thermistor readings over a Bluetooth radio link. If you have a Bluetooth capable laptop or tablet you can "pair" with the Microbit wirelessly. A bluetooth link appears on Windows as a com: port to which you can read with a Terminal emulator like putty or hyperterminal. In effect you transfer the display to another computer.

ADC Link. Yes it's there! The microbit ADC is in the range 0 to 1023, I guess across 3V. That is it reads from 0 to 3V in steps of about 3mV. That should be OK for a thermistor & resistor (I'm theorising!), for example this one from Maplins has a resistance of 7.73k at 100C, 150k at 20C, and 534.9k at 0C. The max temp is 125C

I've no idea how hot flue gas from a domestic chimney gets. For what it's worth, I read somewhere that smoke from the funnel of a Dreadnought Battleship emerged at about 400C. It could make working in the spotting tops very uncomfortable!

Dave

I agree with Paul above.

**LINK**

£14.99 from Maplin.

Amount of heat? You will also need to know the flow rate – ie the amount of gas – and salso some heat loss going out of the chimney will relate to the latent heat of water. Wet logs need a lot of drying and all that heat of converting water into steam adds up.

If I was starting from scratch and building the whole lot myself I would use a 4 wire Platinum resistance thermometer with a local interface chip to either Bluetooth or wired serial comms with the wired connection out of preference because that way I would not have to change batteries.

I'm staring at a project I have on the bench at the moment which uses just such a set up. RTD to MAXIM INTEGRATED PRODUCTS MAX31865ATP RTD interface chip with serial output to PIC18F45J10 driving a DDM4 4 digit display.

If you were doing it at home without all of the kit I have available at work you should look around for stuff that already does what you need functionally and hack it about.

Our lab building does cleaver things with inlet and outlet air (the building is essentially sealed) We have 4 towers each containing 3 pairs of ducts carrying inlet air and exhaust. There is a chamber containing a very large thick disc constructed like a honeycomb. Think of fitting a large number of drinking straws into a section of pipe. The disk is arranged to have part of it's flat surface in the air path of the inlet air and another part in the exhaust. A motor driven spindle causes the disc to rotate slowly. So hot exhaust air heats part of the disc and is cooled before exiting the building and the now hot part transfers the heat to in inlet air as it rotate into that air stream. We get around 8 to 10 degrees temperature change in the inlet air on a good day which amounts to a big energy save.

regards Martin

Most folks don't realise that the funnels were generally double skinned and vented so that air goes in at the bottom and out the top to cool the outer layer.

Even with this it could be unbearable to work near the funnel on some ships when they were driving the boilers hard.

Neil

I am curious as to how much heat goes up the flue

And I thoght flow and temperature would be required to find the amount of heat energy.

Ahhh, so all the OP really needs is to ascertain the lowest practical flue temperature just beyond the heat exchanger and either modify the length of said exchanger or the rate of air flow around it, shirley? Shirley, no need to keep recording the stack outlet temp on the roof? Unless, of course, there are other considerable heat losses between exchanger section of flue and the roof outlet.

If running on flue outlet temperature, the most practical control would be to modulate the air flow, as continually changing the exchanger length is the lesser practical solution? To change the draught through the log burner may well upset the optimum temperatures required to completely combust the wood volatiles

One bit of advice, make sure the cold air enters at the TOP of the heat exchanger and you take hot air from the bottom, nearest the fire. This is significantly more efficient (and a shortcoming of most superheater designs that isn't easily overcome)

Neil