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Weather Station Hardware => Other Weather Station Hardware => Topic started by: hmderek on August 28, 2021, 02:19:22 PM

Title: PT100 calibration?
Post by: hmderek on August 28, 2021, 02:19:22 PM
I set up a PT100 temperature sensor in a Barani MeteoShield Pro today. I'm using an Adafruit MAX31865 amplifier (https://learn.adafruit.com/adafruit-max31865-rtd-pt100-amplifier) to hook it up to my micro controller. The actual PT100 is this one: https://www.amazon.nl/gp/product/B088V6K53R/ref=ppx_yo_dt_b_asin_image_o00_s00?ie=UTF8&psc=1

The PT100 is supposed to be an 'accurate' sensor. But the readings I'm getting are way off compared to my other sensors. After looking at the graphs for a while, it seems like it is pretty much always 1 degree C from my other identical Barani shield.

I haven't really done anything other than hook it up yet. The PT100 is supposed to have a resistance of 100 ohm at 0 C. Could it be that this sensor is just off and needs calibration? Or are there other reasons I could be getting this consistent 1 degree difference? If it's calibration, does anyone have a suggestion what would be the best way to go about it?
Title: Re: PT100 calibration?
Post by: zoomx on August 29, 2021, 03:45:37 AM
You bought an uncalibrated Pt100 maybe C class accuracy, the worst. IT is cheap.

https://www.epicsensors.com/en/faq/what-are-the-pt100-accuracy-classes/ (https://www.epicsensors.com/en/faq/what-are-the-pt100-accuracy-classes/)

Calibrated Pt100 AA or A clas is an accurate and precise sensor. It is expensive.

Edit:
you can do a cheap calibration using melting ice, better to use deionized water or millipore water if you can have it, commonly used in chemical laboratory.

Boiling water can be another point but I don't believe that the cable can work at this temperature.

Then there are some others substances for others calibration point, maybe Gallium is the common one. Take care in using it.
Title: Re: PT100 calibration?
Post by: hmderek on August 29, 2021, 09:44:17 AM
You bought an uncalibrated Pt100 maybe C class accuracy, the worst. IT is cheap.

Makes sense. :-)

Calibrated Pt100 AA or A clas is an accurate and precise sensor. It is expensive.

I understand that there are a lot of applications that require super high accuracy. Not looking for that. Anything better than 0,1 C would be great. Do you have a suggestion for a calibrated sensor? I saw that Barani has a PT100 at 175,-. That's doable, but a bit expensive compared to the prices of my other sensors. ^_^

you can do a cheap calibration using melting ice, better to use deionized water or millipore water if you can have it, commonly used in chemical laboratory.

Read about that somewhere. Sounds like a fun. I might try that.
Title: Re: PT100 calibration?
Post by: zoomx on August 29, 2021, 11:47:17 AM
Do you have a suggestion for a calibrated sensor? I saw that Barani has a PT100 at 175,-. That's doable, but a bit expensive compared to the prices of my other sensors. ^_^

As I wrote before, they are expensive...
For example this one
https://store.vaisala.com/en/products/hmp9 (https://store.vaisala.com/en/products/hmp9)
890.00 (+ taxes) and shipping cost.

Maybe an SHT85
https://www.sensirion.com/en/environmental-sensors/humidity-sensors/sht85-pin-type-humidity-sensor-enabling-easy-replaceability/ (https://www.sensirion.com/en/environmental-sensors/humidity-sensors/sht85-pin-type-humidity-sensor-enabling-easy-replaceability/)
or SHT35 are quite good!

I have the same Pt100 (I think) and a chinese clone board of the Adafruit one.
Title: Re: PT100 calibration?
Post by: hmderek on August 29, 2021, 01:42:49 PM
Do you have a suggestion for a calibrated sensor?

As I wrote before, they are expensive...

I'm thinking I may be misusing the term calibrated. What I mean is something with enough quality to give proper results. Like an SHT35. I've got a bunch of those, and they agree with each other quite well without further calibration.

I've ordered some alternative cheap options along the lines of the SHT35 price. Going to see what they do.
Title: Re: PT100 calibration?
Post by: zoomx on August 31, 2021, 04:20:45 AM
If a sensor have a good precision (values are close) but bad accuracy, calibration can improve it.

When you use an analog sensor you have to take care of all components from sensor to analog/digital converter and take care of EM noise too.
Title: Re: PT100 calibration?
Post by: hmderek on September 02, 2021, 05:38:14 AM
Super weird thing going on. Whenever the temperature outside drops below 15 C, the PT100 starts returning null values for temperature. Next day when temps rise above 15 C, it starts reporing temperature again.  :eek:
Title: Re: PT100 calibration?
Post by: zoomx on September 02, 2021, 06:03:51 AM
I believe that there is a problem in your sketch, maybe the library. An hardware problem that fire below 15 degrees it's hard to happens but maybe if below 15 start water condensing somewhere.
Title: Re: PT100 calibration?
Post by: hmderek on September 12, 2021, 03:27:23 PM
A few weeks and several sensors later, I've got something running now that I'm happy enough with to keep running for a while.

https://blog.meteodrenthe.nl/2021/09/11/looking-for-stable-pt100-readings/

In the end the third sensor I got is giving me the best results. Have a bunch of other things to test, primarily data logging methods.
Title: Re: PT100 calibration?
Post by: zoomx on September 12, 2021, 04:12:53 PM
It's the cheap Pt100? Goode results, it seems!
Title: Re: PT100 calibration?
Post by: hmderek on October 01, 2021, 01:46:39 PM
For those interested, I tried an Adafruit MAX31865 with the PT100 I was using. Did a ice calibration test. Ended up with ~ 0.1 degrees C. This is a LOT better than the clones (often purple pcb's in stead of blue) I have been using. While testing the same sensor, two of the clones were off by 0.9 and 1.5 degrees. The Adafruit board is not very cheap, between 15 and 25 euro's, but it appears to be better quality. Probably precision of components like resistors and such.
Title: Re: PT100 calibration
Post by: hmderek on November 18, 2021, 03:11:10 AM
Been at it more lately. Added a 4-20 mA transmitter to read a PT100. Surprising results for me. Both the MAX31865 board from Adafruit and the 4-20 mA current loop transmitter agree about the behavior of the PT100's.

My 4-20 mA PT100 transmitter setup:
https://blog.meteodrenthe.nl/2021/11/15/using-a-4-20-ma-pt100-transmitter/

Initial findings of the MAX31865 vs the 4-20 mA transmitter:
https://blog.meteodrenthe.nl/2021/11/18/pt100-measurument-max31865-vs-4-20-ma/