best method for temp sensing, 65c - >200c

[large_ghostman]

What would be the best type of sensor to measure temperatures from around 65c to well over 200c, I am not sure the max temperature. I made a solar collector from some old double glazed window panels, and a few metal grids from the backs of fridges, these are mounted on a copper sheet and everything painted black, I had a bit of a mishap, I soldered the joints of the copper tubes, and even at this time of year after a day or so with no water in the tubes, the inside of the sealed box has reached 90c on a cloudy day (the copper temperature of back plate). So now I am thinking in summer those joints are not going to hold!! I will have to get dad to braze them, I have tried to find out but cant find the information for max temperature, some of these home solar sites suggest that some parts inside the glass box can reach over 200c.
I sort of believe it seeing how hot its got on a dull winter day! Anyway I want to measure the temperature of the pipes, panel,glass and water, water isnt such a problem but not sure the best way to measure the collector panel. Anyone running home solar water heaters here?

 

[picbits]

I use a cheapo infrared handheld thermometer which cost me a tenner delivered from China. Handy for spot temperature measurements.

If you want a permanent fixed sensor then you might just scrape it with a thermistor but more than likely will need a thermocouple of you're going over 200 degrees C.

 

[large_ghostman]

What type of thermo couple? it will be permanent, I had no idea how hot you could get these things!!!! even on a dull day with rain. Its just part of my messing with eco projects, I like the technical side to it.

 

[picbits]

You need to choose one that most fits your temperature range. I've only used them a couple of times but they need some serious amplifying before you get a usable voltage out.

Have a flick through the wikipedia page on them as it will explain them better than I can

https://en.wikipedia.org/wiki/Thermocouple

 

[dr pepper]

How about a PT100, sort of like a K type thermocouple but cheaper, and you can buy them in sealed tubes which can be used with a pipe fitting.
You can get ds18b20 temp sensor in a tube like this too, but they only go to +127 degrees C, but they are only a quid.

I hope you took thos heat exchangers from frisges that were allready environmentally responisbly disharged!

 

[mab2]

umm... If you're planning on running water through the pipes (which I assume is the point of the panel) then the water should keep it below the solder melting point.

I did once make a extra cheap solar water heater - an old central heating rad painted black in a box with bubble wrap over the front - worked OK on days when it was continuously sunny but took too long to warm up for intermittent sunshine.

 

[Zerotolerance]

Just buy a K type Thermocouple and call it a day.

https://www.google.com/shopping/pro....3&ei=44EPU7D8D8r22QW234G4Bg&ved=0CPMBEKkrMAk

 

[large_ghostman]

Yeah I am running water through the pipes, and as long as the water stays water, then its fine. But some I have seen get so hot they turn the water into steam before it gets to the end stage. for winter it needs 4 or 5 stages and for summer just 1 or 2, so maybe a valve system, anyway K type looks the way to go , that way I can automate the valves, Valves are another problem. I have spent ages at the recycle center and got loads of washing machine valves, but I am not sure how much heat they will take.

I hope you took thos heat exchangers from frisges that were allready environmentally responisbly disharged!

Of course!! they were very carefully discharged with the angle grinder, and I responsibly swept up after!! .
Actually what we do is carefully take it all out, drop the compressor bit and collector thingy, in a old bath with iced water, then crimp the pipe and cut. As far as I can tell nothing escapes. Then we take it back to the dump with the rest of the fridge stuff.
Apparently if the water is cold enough the gas goes liquid and drops into the collector, so if you crimp and cut no gas escapes.
Probably not a official way, but at least we dont just cut and run

 

[JimB]

LG, beware of thermocouples.
If you want an accurate temperature measurement, you will need some kind of "cold junction compensation".

Thermocouples are simple and rugged, I have used them many times in a previous life, but there is more to them than just measuring the millivolts. Unless you have a meter with built in cold junction compensation.

Look here:

and here:
https://en.wikipedia.org/wiki/Thermocouple
for more information.

JimB

 

[4pyros]

Yeah I am running water through the pipes, and as long as the water stays water, then its fine. But some I have seen get so hot they turn the water into steam before it gets to the end stage. for winter it needs 4 or 5 stages and for summer just 1 or 2, so maybe a valve system, anyway K type looks the way to go , that way I can automate the valves, Valves are another problem. I have spent ages at the recycle center and got loads of washing machine valves, but I am not sure how much heat they will take.

I think the idea is to keep the water moving and the faster you move it the more heat it can carry away.
Some systems use the water flow to regulate the temp.
Most systems will use antifreeze or some other additive to keep it from boiling.
Are you planing a closed loop system with a heat exchanger?

 

[dr pepper]

Scrap central heating boiler is a good donor for a water to water heat exchanger.

Keep your system ventilated or use several safety valves, you dont want it to go kaboom.

Interesting tip on fridge discharging, I've never heard that one before.

 

[KeepItSimpleStupid]

For the temperature range you want, a type T thermocouple would be ideal.

Some readouts have subranges of the Thermocouple defined. I do have a "jack of all trades" wide range thermocouple thermometer. You just select the type.

Thermocouples should use extension wire to extend the type. In a previous life, I made the junctions for R,S, K and T. We did use a C type TC in one process.

RTD's, e.g. PT100 are good to about 250 C.

Some meters have a K type TC input.

If this is a one, time approximate deal, you can do thermocouples the hard way. Use two junctions in series with one of them in ice water. Use the right table and convert mV to temperature.

Junctions to extension wire are designed to be isothermal.

 

[tcmtech]

I have spent ages at the recycle center and got loads of washing machine valves, but I am not sure how much heat they will take.

Although not rated for it they will usually stand up to at least 120C at lower than stock pressures before they start having the plastic bodies start to warp.

If you don't mind doing a bit of fitting reconfiguration (and maybe a fit of drilling, threading, and or welding) the EGR solenoids from old car engines can take a huge amount of heat!

 

[large_ghostman]

Thanks guys! loads of tips!! I happen to know where to get hold of EGR's . Not sure which type of system we will try first, probably gravity with no pump to start off. The idea was a big hole insulated in the ground, then solar coils in it.From there a heat exchanger to the cylinder in the house tank.
But I think things are about to change!! Dad has tried getting hold of old boilers for the exchanger but he found them hard to get hold of!! seems everyone wants them. I will have a look at T type.
Wait till I post pics of our green house heater!!!! Its really cool, uses the compost heap as heat source and a exchanger, this goes to a car radiator in the green house and a solar fan blowing the warm air from it . Was great fun to build and keeps the green house toasty. Looking at the thermometer the center of the compost heap is currently 79C, looks like it needs turning soon as its starting to cool, top temperature so far for the heap is 88C. We have been told they can get alot hotter tho.
Maybe we are taking too much out?

 

[tcmtech]

As far as a DIY liquid to liquid heat exchanger goes a simple coaxial tube type can be made from a pair of copper pipes, two tees and two reducers with minimal work.

Just run the smaller pipe inside the larger one and use the tees and reducers at the ends. A 3/4" copper pipe inside a 1" pipe will transfer loads of heat. A 10 foot long one (or four 2.5 ' long ones in series or parallel) with a 100 F differential between the two fluids will easily transfer 100 - 120K BTU!

Some numbers for you. https://www.engineeringtoolbox.com/overall-heat-transfer-coefficients-d_284.html

The inner pipe flows straight through and the outer one flows in and out through the tees in the opposite direction of the flow of the inner pipe. You should be able to figure it out form there.

 

[KeepItSimpleStupid]

Since you don't know too much about thermocouples, see here: https://www.omega.com/prodinfo/thermocouples.html

K is the most common thermocouple type, but not all readouts will read the entire range.

Sensors come in sheathed or no-sheathed, grounded and ungrounded and naked. The R&S thermocouples are of a platinum/platinum-Rhodium alloy and a torch (I used Hydrogen Oxygen because that's what we had) to make the junctions. T is kinda messy to make because it's copper/constantan. J&K is Chromel/Alumel. These junctions were using a capacitive discharge spot welder and a carbon block. We used C when we were measuring copper melt temperatures. You have to try not to make double junctions. If we could, we used commercial probes. We used T for cryogenics and room temperature stuff. One system used an RTD that had a range of -80C to 200C which was perfect for that with much better temperature accuracy like 0.1 deg. C. Silicon diodes were used for cryogenics also.

If you can use K, use K.

Fluke has gone downhill:

This https://www.flukeonlinestore.com/736736.html clams -50 to 1000 C and this https://en-us.fluke.com/products/all-accessories/fluke-80ak-a.html#fbid=WVPYiXPEel- claims nothing.

 

[large_ghostman]

Ive just noticed my Tenma DMM uses one! its a good meter. I will try and get the ones they use (K type I think), then I can calibrate against my meter .
Heat exchangers are fascinating!! makes me wonder why this kind of thing isnt used every where

 

[tcmtech]

Heat exchangers are fascinating!! makes me wonder why this kind of thing isnt used every where

They are.

You just need to expand on your knowledge of everything so that you can identify them easier.

 

[large_ghostman]

They are.

You just need to expand on your knowledge of everything so that you can identify them easier.

I guess so

 

[KeepItSimpleStupid]

The TC readout has to know the temperature of the terminals at the meter. Then it can compensate. OR, as I said earlier, two junctions are subtracted. One at the reference temperature of 32 deg and the measured temperature. Then, you look up the temperature in the tables directly.

Otherwise, the TC wire get's attached to a big bloc where the temperature is known. Then you look up in the table the mV value of that temp (Ref = 0C). Then you add or subtract (forget which) from the mV value read on the voltmeter. Then look up the resulting mV value in the tables to get temperature.

The tables, usually read to the 0.001 mV level.