Calibration of TMP36 sensors on Arduino?

Ok, I knew this already and just realized it - to get near one volt, all I have to do set up a voltage divider off the 3.3v with a 1meg and a 470k resistor. :)

Nico

My calculation is that you are somewhere in the (750mV) A0=232 at 25degC, which is less than quarter of the fullscale (assuming 3v3 supply)

Using a 3v3 supply I calculate each step is 0.0032258mV

Another method is to change the analogue reference to 1.1v

http://arduino.cc/en/Reference/AnalogReference

My calculations are that you could read up to 60deg C using the 1.1v internal reference available on a 328 and the adafruit formula.

Temp in °C = [(Vout in mV) - 500] / 10

Mark

I also answered your other post.

Hi Nico

Yes I have been quiet for a long time and no, sadly, I haven't completed my Greenhouse temperature control project yet. Most aspects of it are designed and in most case tested and proven. I haven't yet integrated the whole system! I too gave up on the idea of Gobetwino for this particular project and am attracted to the idea of of using a WiFi shield with the Arduino - maybe!

You asked a number of questions about error, accuracy and calibration.

Measurement error is normally divided into Systematic and Random:

1. Systematic error is like a constant offset, eg a 50g weight that has had achunk filed off it so it only weighs 49.5g
2. Random error is the fluctuation you get between readings that you hoped would be the same value.

Accuracy is around making measurements which are appropriate to the measuring device AND to the reason for taking the measurement in the first place.

So, if, at the ADC level, 1 point in 1024 represents 0.3 degrees, that suggests to me that quoting the temperature more accurately than the nearest degree is pretty optimistic.

Then we come to why are we measuring in the first place? To control my Greenhouse, I want to switch on a heater when the temperature falls below about 5 degrees and back off again when it rises to about 10 degrees. I also want to switch on a cooling fan when the temperature rises above about 25 degrees and back off again when the temperature falls below about 20 degrees. So an accuracy of + or - 1 degree is more than adequate (for my application).

Applying my crude approach to your readings, above, would give:

Temperatures = 25, 25, 24

Temperatures = 25, 24. 24

Temperatures = 25, 24, 24

In otherwords,  within + or - 1 degree for a device and even between devices.

As for calibration, my memory of School Physics (which was a very long time ago!) tells me that:            

• 0 degrees  C is pure water containing ice made from pure water, well stirred, of course.
• 100 degrees C is steam over boiling water.

Both are assumed to be at 1 standard atomsphere ie 760mm Hg. I guess anywhere close to sea level is near enough!

See here to become a temperature calibration expert:  http://en.wikipedia.org/wiki/Thermometer#Calibration

And finally, if I was happy that my old fashioned thermometer (double checked, as above) was accurate, or, if it had a systematic error then I knew what that error was and could correct for it ( I have one that consistently over reads by 2 degrees C), then I would compare my sensors with the thermometer at a series of temperatures in the range I was expecting them to work.

If I found that a sensor regularly disagreed with the thermometer by a significant amount, then I could correct for that in my software.........however, that would complicate the software and require different segments of code, which would need to line up with the correct Arduino pins and sensors.....could get a bit messy; might be easier to get hold of some more sensors and select a group which all have the same systematic error, which could be zero if you are lucky.

Oh dear, I have just read this through again and it sounds really pedantic and maybe a little patronisisng - it's not meant to! Again, many years ago, measurement error and appropriate accuracy was part of my life!

Nico, I hope you find some use in this

Best wishes

Neil

Neil/Nico

I did something you describe for 'The Shed' magazine as part of our 101 series.

The article and software is here (part 4 included the LCD in both normal and I2C version)

http://www.theshedmag.co.nz/online/arduino

I used Dallas temp chips to ovecome most of the calibration issues

There are some around that are already encapsulated and have a lead on them, to make it easier.

The other option is to solder leads and insulate the wires, then apply heat transfer compound that is used for transistors, and fit them into a piece of metal tubing.

The tube is sealed at one end and the other end fits into a compression gland commonly used in the electrical industry to hold a cable as it passes through an enclosure.

The tubing should be close to the size of the device, but as long as its making contact it will take on the temperature of the metal tube.

Mark

Mark

Thanks for the link to "The Shed" magazine - the projects are beautifully laid out and so clearly described.......why have I never come across these articles before?

Neil

Neil

They are well laid out.

The magazine is a NZ hobbyist magazine and the publisher has included the Arduino and now RaspberryPi into it.

The articles are put together to allow the reader to make them, rather than showcase what they built.

There are electronic subscriptions, but paper is always good.

I'll pass your comment back to the editor.

Cheers

Mark

Thanks Mark and Neil!

The reason I want the temperature sensors is just for data logging, and the accuracy is only important once we get near the freezing point - which hopefully won't be an issue for quite a while

And I agree, 0.1 degree accuracy doesn't mean much if the sensors fluctuate more than that, and in actual use I think just knowing that the temperature is close to 0 is enough.

Thanks, Mark, for pointing out the built-in 1.1v reference! I had actually looked at that documentation earlier and saw it, and then promptly forgot

I've been debating on whether to just use the 3.3v instead because of the other sensors running the full 0-1024 range, but I could probably just add a resistor to the other sensors to bring the range down to where 1.1v is fine as a reference.

For now, I think the 3.3volt reference will be the easiest and quickest way to go. I think my 5volt source was too noisy, so the 3.3v source immediately gives a much more stable voltage, plus hooking it to the ARef helps the sensors be a bit more stable.

Mark, I checked out those one-wire sensors you pointed out. I should have bought those to begin with, as they cost about the same as the TMP36's I bought, plus they are already waterproof and include something like 3 meters of wire! I think I'll buy one for the outside sensor.

So it looks like I need to do some more soldering to get things connected up

Thanks,

-Nico

Nico

There are varying prices for the DS18B20's and the encapsulated ones save a lot of time and effort.

One thing I have yet to see done well, is to differentiate between the various sensors.

Each has a unique number, and while you can target that, it doesn't translate to someone else using you sketch or replacing a sensor.

I did think of a calibrate mode where you apply heat to raise one and then mark that as 'inside1', 'outside3' or whatever.

You could automate it where it tells you to heat 'inside1', then 'inside2'  ..

In the meantime most people just code each sensor or put them on seperate inputs.

Mark