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[on:]

Hi, I'm trying to design a renewable energy heating/hot water system, Arduino controlled.

The immersion temperature sensors required for thermal stores all seem to be PT1000 type, and I'm a little confused by what I have read on the subject so far - lots of talk of opamps, 4-20ma converters, lookup tables and lots of ways to wire them up.

I am armed with an Arduino, data logging shield, various components and a PT1000 sensor to play with. I have managed to cobble together various sketches to get things going, but I could really do with a nice simple place to start with the PT1000s - any suggestions?

Many thanks

[Reply #1 on:]

Hi, I'm trying to design a renewable energy heating/hot water system, Arduino controlled.

The immersion temperature sensors required for thermal stores all seem to be PT1000 type, and I'm a little confused by what I have read on the subject so far - lots of talk of opamps, 4-20ma converters, lookup tables and lots of ways to wire them up.

I am armed with an Arduino, data logging shield, various components and a PT1000 sensor to play with. I have managed to cobble together various sketches to get things going, but I could really do with a nice simple place to start with the PT1000s - any suggestions?

Many thanks

This app note might help you out:

http://www.national.com/an/AN/AN-1559.pdf

From what I can tell from that, though - using these won't be easy...

[Reply #2 on:]

Here's another app-note that might be useful:

http://www.analogmicro.de/products/info/english/analogmicro.en.an1017.pdf

[Reply #3 on:]

You'll need to post what you have so far including schematics of anything you've wired up and links to any data sheets that might apply. Be sure and use the # tag when you post your code.

You posted a link while I'm writing this. Do you already have your RTDs? To get the accuracy from RTDs that one normally uses them for requires a bit of work. If you are happy with +- a few degrees accuracy then not so much work but then you might as well use thermistors or thermocouples, which are usually cheaper.

What is your temperature range and what sort of accuracy do you require?

[Reply #4 on:]

Here's this - from the playground:

http://openenergymonitor.org/emon/node/75

[Reply #5 on:]

Thanks for all of those.....I have just one sensor so far for development, I'd like to use alternatives but all the immersion type sensors on the market for insertion into hot water tanks seem to be RTDs. Range needed is only from 0 to +90 deg C, and +/- 1 deg accuracy would be fine.

I'll post whatever I end up with here...

[Reply #6 on:]

What kind of material does the sensor have to be made of to immerse in hot water? Thermocouples come in stainless steel and Inconel for example.

A lot of the issues regarding signal conditioning RTDs go away with PT1000s because of the higher resistance. For your application, I'd be tempted to use a precision resistor with a good temperature coefficient of, say 3K3, in series with the sensor and monitor the sensor voltage with the Arduino. Then, I'd do a three point calibration with a good thermometer, say ice slurry, boiling water, and something in the middle and call that good.

Definitely let us know how it goes.

[Reply #7 on:]

Sorry, I didn't describe it well - they are not actually immersed themselves, but slide into a close fitting immersion sleeve, like this

http://www.alternativeenergystore.co.uk/item/item/102201/1/0/100mm-Stainless-Steel-Immersion-Sleeve.htm

which are screwed into fittings in the water tank and the solar collector.

Thanks for the advice on the resistor, I'll certainly try that and report back when it's all together.

[Reply #8 on:]

Sorry, I didn't describe it well - they are not actually immersed themselves, but slide into a close fitting immersion sleeve, like this

http://www.alternativeenergystore.co.uk/item/item/102201/1/0/100mm-Stainless-Steel-Immersion-Sleeve.htm

which are screwed into fittings in the water tank and the solar collector.

Thanks for the advice on the resistor, I'll certainly try that and report back when it's all together.

Those are called 'thermowells' in the process control industry. They allow one to remove/test/replace the sensor without having to drain the tank. However one has to insure the sensor tip 'bottoms out' inside the well or you can end up with slow responding or even inaccurate measurement values.

http://www.temperatures.com/twells.html

Lefty

[Reply #9 on:]

Thanks Lefty - is some kind of thermal grease used to ensure good readings? I was toying with the idea of using a sensor easier (to me at least) to hook up rather than a PT1000, and packing it in with something, as a plan B really if else fails.

[Reply #10 on:]

Thanks Lefty - is some kind of thermal grease used to ensure good readings? I was toying with the idea of using a sensor easier (to me at least) to hook up rather than a PT1000, and packing it in with something, as a plan B really if else fails.

I never heard of anyone trying to pack a thermowell with grease. I would worry that the heat of the process might cause the grease to flow and not maintain consistance contact with the sensor. Some thermowells have small internal clips at the end of the well inside, so that when the sensor is shoved in far enough help maintain a mechanical contact between the sensor and the inside wall of the well.

Lefty

[Reply #11 on:]

A lot of the issues regarding signal conditioning RTDs go away with PT1000s because of the higher resistance.

I agree EmilyJane having to deal with PT100 and PT1000
In my case and my resolution needs economical current source build around LM334 does the job for PT1000 driving.  (R1 = 330,   R2= 10*R1 = 3K3)

[Reply #12 on:]

What resolution are you looking for? By my calculation, if you make a potential divider using a PT1000 and a 1K resistor, and connect that directly to an Arduino analog input pin, you can measure the temperature with a resolution of 1 deg C. If you want higher resolution, you'll need an op-amp to amplify the voltage change.

Using a constant current source is all very well, but then both the current source and the ADC reference have to be stable (or the current source has to be controlled by the ADC reference). The potential divider (whether amplified or not) driven from the same voltage as is used for the ADC reference has the advantage that it is not very sensitive to changes in the ADC reference.

[Reply #13 on:]

If you decide to not use a constant current source to power the sensor, remember that the critical component is the series resistor. For the best accuracy, it needs to be of high precision and have a low temperature coefficient with respect to temperature. Since your excitation current will be a function of the RTD resistance, accuracy will depend on knowing that the voltage drop across the series resistor varies only with that current.