3 ms·

I have built a thermometer for an automation project with the requirement to be able to measure rate of temperature change 50 times a second with resolution of

I have built a thermometer for an automation project with the requirement to be able to measure rate of temperature change 50 times a second with resolution of 0.001K/s (between 50 and 140C). I can tell you, I spent probably a month just learning how difficult it is to measure temperature and what are all different ways you can screw up the measurement. You can't just stick a thermocouple anywhere you want. Everything has thermal mass, everything is thermal impedance, everywhere there is a thermal gradient. If you have ANY rate of change of temperature, your measurement method will always greatly impact the result regardless of how precise thermocouple and electronics you use. If they really can get nanokelvin resolution I would be more interested in understanding their measurement method, because resolution is pretty much worthless if you stick it in the wrong place.

semi-extrinsic 4y ago
Wow, 0.001K is impressive for something you've built yourself. Care to share what approach you used?
- twawaaay 4y ago
  It wasn't anything special. I am an amateur EE and not especially talented. I did some cursory search on the Internet, found a circuit that was looking promising and brute forced it by using best components I could find and then ovenized it for great stability and removed any sources of noise I could imagine (for example using linear power supply, etc.) I think just the measurement circuit with the oven was about $500 which is just saying I did poor at optimising for the cost. But for this project the price was not an issue and the real challenge and bulk of work was in building a model and control algorithm. There is couple of gotchas. For example if you use very low current through the RTD (PT100 in my case) you get a lot of noise so you need to use relatively high current. But this causes it to heat up. Rather than try to eliminate this effect I decided to make sure this heating up is as constant as possible. This is one of the reasons it had poor absolute precision but was good for measuring changes over time.
anfractuosity 4y ago
Out of interest what pt100 probe did you use? Would this be a 4 wire pt100? Also did you use a specialised chip for converting to digital readings? I vaguely recall looking at some fancy reference RTD thermometers with glass housings (just found them, they seem to be called SPRTs - https://isotech.co.uk/products/isotech-model-670/ https://isotech.co.uk/products/isotech-model-670/)
- twawaaay 4y ago
  I don't remember the exact PT100 I used but it wasn't very expensive (still probably one of the more expensive things I ever bought per unit of mass -- they are probably more expensive that diamonds). It was definitely 4 wire (you don't want to measure wire resistance change and 4 wire is only way to get rid of it). As to housing and type of RTD, a lot of the cost here is spent on making sure the measurements are stable over long time (so for example to prevent humidity and other gasses from putting it out of calibration). In my case I did not care a lot about it because I wasn't after absolute precision, for me the important part was rate of change. If you are after absolute precision you might want one of the fancy RTDs. But I cared that it is small and that it has very low thermal impedance interface with the measured object. Smaller means less thermal mass means faster response. For the chip I used a general purpose ADC. You also need a very good voltage reference, otherwise even best ADC isn't worth much.
  - ISL 4y ago
    If you're primarily chasing sensitivity, one can generally be much more sensitive with a non-platinum thermistor. They're less stable but exhibit a much greater change in resistance with temperature. A good voltage reference (or current source) is important, but even more important is building up a reliable bridge topology.
    - deleted 4y ago
      [deleted]
    - twawaaay 4y ago
      You might be right about PTCs though I now can't remember exact reason for choosing PT100. It probably wasn't just linearity. I think I had other problems like controlling amount of power dissipated on the sensor (large changes in resistance mean large changes in amount of power dissipated) and possibly other.
guenthert 4y ago
Hmmh? Some confusion of terms perhaps. You can expose a RTD or even NTC (to bring the point home) to the temperature in question, then measure the resistance in a bridge with nearly arbitrarily high resolution. Tadah! Temperature measurement with nK resolution. Of course it doesn't say anything about precision, much less about accuracy nor as you pointed out "The Temperature".
- twawaaay 4y ago
  Theory is closer to practice in theory than in practice. PT100 is 0,4 Ohm/K. Or 0.0000000004Ohm/nK. This means, at 100Ohm, you are measuring 4 parts per (american) trillion for that nK. Good Luck and please, report how many of those digits were pure noise!
  - airbreather 4y ago
    Instrumentation engineer here, but RTD is not linear? Near linear but usually even for industrial use there is a three term polynomial correction applied. There's a whole standard devoted to the subject, DIN/IEC 60751. Useful reference from TI : https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.ti.com/lit/pdf/snoaa67&ved=2ahUKEwjDuJqAvLf4AhW71DgGHQXQDk0QFnoECAsQAQ&usg=AOvVaw0CcxqxgXS48uFHNQ15W_Zr https://www.google.com/url?sa=t&source=web&rct=j&url=https:/...
    - twawaaay 4y ago
      Yeah, it is nonlinear and I did apply three term ploynomial correction as per manufacturers spec sheet.

semi-extrinsic 4y ago

Wow, 0.001K is impressive for something you've built yourself. Care to share what approach you used?

twawaaay 4y ago
It wasn't anything special. I am an amateur EE and not especially talented. I did some cursory search on the Internet, found a circuit that was looking promising and brute forced it by using best components I could find and then ovenized it for great stability and removed any sources of noise I could imagine (for example using linear power supply, etc.) I think just the measurement circuit with the oven was about $500 which is just saying I did poor at optimising for the cost. But for this project the price was not an issue and the real challenge and bulk of work was in building a model and control algorithm. There is couple of gotchas. For example if you use very low current through the RTD (PT100 in my case) you get a lot of noise so you need to use relatively high current. But this causes it to heat up. Rather than try to eliminate this effect I decided to make sure this heating up is as constant as possible. This is one of the reasons it had poor absolute precision but was good for measuring changes over time.

anfractuosity 4y ago

Out of interest what pt100 probe did you use? Would this be a 4 wire pt100? Also did you use a specialised chip for converting to digital readings? I vaguely recall looking at some fancy reference RTD thermometers with glass housings (just found them, they seem to be called SPRTs - https://isotech.co.uk/products/isotech-model-670/ https://isotech.co.uk/products/isotech-model-670/)

twawaaay 4y ago
I don't remember the exact PT100 I used but it wasn't very expensive (still probably one of the more expensive things I ever bought per unit of mass -- they are probably more expensive that diamonds). It was definitely 4 wire (you don't want to measure wire resistance change and 4 wire is only way to get rid of it). As to housing and type of RTD, a lot of the cost here is spent on making sure the measurements are stable over long time (so for example to prevent humidity and other gasses from putting it out of calibration). In my case I did not care a lot about it because I wasn't after absolute precision, for me the important part was rate of change. If you are after absolute precision you might want one of the fancy RTDs. But I cared that it is small and that it has very low thermal impedance interface with the measured object. Smaller means less thermal mass means faster response. For the chip I used a general purpose ADC. You also need a very good voltage reference, otherwise even best ADC isn't worth much.
- ISL 4y ago
  If you're primarily chasing sensitivity, one can generally be much more sensitive with a non-platinum thermistor. They're less stable but exhibit a much greater change in resistance with temperature. A good voltage reference (or current source) is important, but even more important is building up a reliable bridge topology.
  - deleted 4y ago
    [deleted]
  - twawaaay 4y ago
    You might be right about PTCs though I now can't remember exact reason for choosing PT100. It probably wasn't just linearity. I think I had other problems like controlling amount of power dissipated on the sensor (large changes in resistance mean large changes in amount of power dissipated) and possibly other.

guenthert 4y ago

Hmmh? Some confusion of terms perhaps. You can expose a RTD or even NTC (to bring the point home) to the temperature in question, then measure the resistance in a bridge with nearly arbitrarily high resolution. Tadah! Temperature measurement with nK resolution. Of course it doesn't say anything about precision, much less about accuracy nor as you pointed out "The Temperature".

twawaaay 4y ago
Theory is closer to practice in theory than in practice. PT100 is 0,4 Ohm/K. Or 0.0000000004Ohm/nK. This means, at 100Ohm, you are measuring 4 parts per (american) trillion for that nK. Good Luck and please, report how many of those digits were pure noise!
- airbreather 4y ago
  Instrumentation engineer here, but RTD is not linear? Near linear but usually even for industrial use there is a three term polynomial correction applied. There's a whole standard devoted to the subject, DIN/IEC 60751. Useful reference from TI : https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.ti.com/lit/pdf/snoaa67&ved=2ahUKEwjDuJqAvLf4AhW71DgGHQXQDk0QFnoECAsQAQ&usg=AOvVaw0CcxqxgXS48uFHNQ15W_Zr https://www.google.com/url?sa=t&source=web&rct=j&url=https:/...
  - twawaaay 4y ago
    Yeah, it is nonlinear and I did apply three term ploynomial correction as per manufacturers spec sheet.