High resolution temperature measurement with no accuracy requirement - Page 1
Forum section: electronics / general

author: Sarke
author_rank: Newbie
timestamp: December 14, 2022, 12:04:58 pm
content: Hi,I'm looking for ways to improve my current temperature measurements using sheathed thermocouples and WebDAQ dataloggers. The current limiting factor is both quantization and especially signal to noise (the noise is currently around 100 mKpp).I'm trying to achieve something like:Measuring in a liquidAccuracy and drift: no requirement (ish)Resolution: 0.1-0.2 mK @1kHz data rateTC: <1 secondMeasurement range: ~20 K (during testing I will be designing for 10-30 °C)I have found two relevant threads:https://www.eevblog.com/forum/metrology/measuring-mk-with-small-rtds/https://www.eevblog.com/forum/metrology/diy-precison-thermistorntc-temperature-measurement/The design presented in the second thread looked pretty promising, and I have calculated that using a thermistor with a high beta (~10k) would result in a 4 ksps, 2 uK resolution (at center of range) measurement. Obviously I have not even started looking at what noise levels I would be getting, and there are many relevant improvements listed in the first thread to improve this that I could look at, but I wanted to check in here first if I'm on the right track? Designing something like this with absolutely no requirements to accuracy and drift is kinda weird and unlike 99% of what I have found when researching this. Maybe there are other measurement techniques more suited when only high resolution and frequency measurement is required?

author: David Hess
author_rank: Super Contributor
timestamp: December 14, 2022, 12:36:42 pm
content: Drift still matters because it cannot be distinguish from a slowing changing signal. Thermisters are one of the better options because of their high sensitivity. RTDs have lower sensitivity, but also may have lower drift and better stability.If a thermocouple is used, then not only is noise an issue because of low sensitivity, but the drift of the cold junction compensation is added to the thermocouple drift.

author: Sarke
author_rank: Newbie
timestamp: December 14, 2022, 12:42:42 pm
content: Quote from: David Hess on December 14, 2022, 12:36:42 pmSorry, I was just trying to emphasize that I want to focus mainly on resolution, sample rate and noise. To be more specific, I don't really know the measurement window yet, but it probably will never be longer than one minute, and the data will undergo a lot of signal processing anyway so compensating for constant drift is unproblematic. Of course, it cannot be so significant that the baseline moves out of the measurement range during the measurement window.

author: David Hess
author_rank: Super Contributor
timestamp: December 14, 2022, 05:14:53 pm
content: Quote from: Sarke on December 14, 2022, 12:42:42 pmWhat I am getting at is that the drift is important because it limits useful resolution. It creates an error band around the nominal value which is greater than the potential resolution from all other effects like noise.Now I know an RTD can deliver millikelvin resolution and commensurate drift because I have done it and it is not difficult. Of course that is not millikelvin accuracy, but temperature accuracy at that level is questionable for reasons other than the RTD and meter.A thermister should deliver even better resolution because its sensitivity is so much higher, but its drift and stability may be worse than an RTD limiting it to lower useful resolutions. That I do not know.At the beginning of the year I looked into how to build an *absolute* temperature calibrator without sending it out for calibration. An RTD with detailed specifications is one way, but Mouser sells some inexpensive 0.05C and 0.1C tolerance thermisters. I was thinking of using them in a bridge configuration, so two thermisters and two precision resistors, to generate a differential ratiometric output, which doubles the sensitivity and removes supply/reference voltage errors. The reason to select a higher accuracy part is that it should also have lower drift.

author: miro123
author_rank: Frequent Contributor
timestamp: December 14, 2022, 07:48:34 pm
content: - Normaly high beta NTC are less stable- I dont understand the specs of 1Ksample/sec . What is the thermal constant of the while system/bandwidth? Are you searching for sensor that can measure fast chenging temeratures - e.g 10degree with 1..50msec?I'm afraid that requiremnt are very strong. If answer of my question is yes- NTC does not like fast temerature changes even more their package has high thermal conductivity- thin film RTD are better in handling high BW temeratures

author: Sarke
author_rank: Newbie
timestamp: December 15, 2022, 09:55:28 pm
content: Quote from: David Hess on December 14, 2022, 05:14:53 pmMy measurement only needs to capture the waveform and amplitude. I might be misunderstanding, but I believe sources of error/inaccuracy like offset error and near-linear gain errors will not affect the quality of my results for my use, as the offset is not relevant and I can characterize and compensate for the gain error as long as it isn't changing too rapidly.Quote from: miro123 on December 14, 2022, 07:48:34 pmI mentioned the sample rate because the ADs I have looked at so far all have programmable OSR, and I'm okay with using that to improve resolution and noise at the expense of samples/sec, but the final data needs to be at least 1 ksps. As an example the AD I normally use has a sample rate of 4 MS/s, but if OSR is set to 256 it outputs 24 bit at 4096 S/s.The temperature I want to measure in theory should have the shape of a right skew normal distribution (temp vs. time at probe position). I probably won't have a rate of change higher than about 10°C/s, with a max amplitude of 5°C. The signal duration is ~3 to 30 seconds.Quote from: miro123 on December 14, 2022, 07:48:34 pmThat's interesting. Do you know why? I'll do some research into thin film RTDs and thermistors tomorrow, would love any tips for papers, app notes or handbooks you have.

author: Anders Petersson
author_rank: Regular Contributor
timestamp: December 16, 2022, 09:23:51 am
content: Hi SarkeWith a data rate of 1 kSps I guess you're measuring turbulence or mixing. I designed a circuit to support atmospheric turbulence measurements using very small thermistors. I got 2 mK noise-free resolution with a theoretical max sampling rate around 500 Sps.Measuring in a liquid is easier thanks to the higher thermal conductivity compared to air. 1 kSps is possible, but I'm not sure about 0.1-0.2 mK resolution as the TC of the filters needs to be low which will give more noise.Your sensor needs a TC of much better than 1 second but fortunately this is possible if you can live with an expensive and fragile sensor. FP07 specifies time constant of "Plunge into Water" as 7 msec:https://www.amphenol-sensors.com/hubfs/Documents/AAS-920-267C-Thermometrics-NTC-TypeFP07-031814-web.pdf?option=com_edocman&task=document.download&id=116Your resolution requirement probably leaves thermistors as the only option. Naturally you would optimize the resistor values for maximum sensitivity in your range of thermistor resistances. Perhaps a bench multimeter can give lower noise than the off-the-shelf ADCs that MiDi and I use.On a second thought, you could improve the signal-to-noise ratio by increasing the bias current (self-heating should still stay small in liquid). Also, you can generate a reference voltage equivalent to the voltage at the center of your temperature span, then use an op amp with gain or use internal gain on the ADC to amplify the signal for the narrow range of expected temperatures.

author: HighVoltage
author_rank: Super Contributor
timestamp: December 16, 2022, 09:31:40 am
content: I have developed circuits for Automotive Mass Air Flow Sensors and they require a fast reaction time. Your application seems similar. After a lot of research, we ended up using small mass 20 Ohm platinum resistors and they worked very well with reaction times of around 100 ms.

author: miro123
author_rank: Frequent Contributor
timestamp: December 16, 2022, 01:09:52 pm
content: I'm quite short and cryptic at my post.Here are some links1. NTC long term drifts -https://ieeexplore.ieee.org/document/72814602.https://www.ti.com/lit/an/sbaa275/sbaa275.pdf?ts=1671175462668&ref_url=https%253A%252F%252Fwww.google.com%252F- very good article. eevblog circuts has some major mistakes in capacitive decoupling. and TI do it righ ang gives calculation methods.===========My experience- the project has more phisical mechanical constains than electronics- If i have to put some number of time spend in diferent area- phisics sundametals - 25%- temperature senesors properties theory - 25 %- sensors experiments - 35%- software - 14,9%- electronics HW - 0.1% - CPU STM32G474/ ADS1112/ADS1256=============Tips- it seems that you have mixed area and tight requirements. - Kalman filter with NTC + PT100 sensors array + Fluid speed meter can do the job. Fluid speed chnges the thermal conductivity.= Does the fluid has constant thermal properties - e.g water vs petrol vs industrial coolant- follow application note from TI and Analog devices.- last but not least - scientific studies are great source

author: RoGeorge
author_rank: Super Contributor
timestamp: December 16, 2022, 01:30:49 pm
content: I've tried once aTMP112out of curiosity. It was so sensitive it could feel the heat while breathing (62.5 mK resolution).TI has many digital temperature sensors, didn't check recently. I don't recall the refresh rate. The IC sensor is very small, therefore low thermal inertia and fast response. Can be put in a tiny protective tube for harsh environments.

author: 1audio
author_rank: Frequent Contributor
timestamp: January 01, 2023, 12:53:11 am
content: Here is a different way to get therehttps://www.axtal.com/English/Products/PiezoSensors/TemperatureSensors/HP basically invented it but discontinued the products years ago,http://hparchive.com/Journals/HPJ-1965-03.pdf

author: RoGeorge
author_rank: Super Contributor
timestamp: January 01, 2023, 09:33:33 am
content: The description of Fig.5 in the pdf says the resonator is sealed in He gas. I've read He is problematic to obtain and/or to contain. Why not something that is inert and abundant, like N2?

author: nfmax
author_rank: Super Contributor
timestamp: January 01, 2023, 01:31:16 pm
content: Probably because adsorption of the gas molecules onto the quartz surface affects the resonance, and helium is the best gas to minimise the effect.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 01, 2023, 02:03:33 pm
content: The gas will dampen the vibration, and here a lighter gas has some advantage. So hydrogen might be an alternative. The other point is the thermal contact when used as a thermometer.He gas finds even tiny leaks, but once really sealed one can get a He system tight. As long as only He goes out it would still ne be that bad: reduced pressure He is not that bad. And other gasses would only come in if there is a larger leak.Humidity / water can be a nasty factor with quarz: it absorbes and adheres quite strong and the effect on the frequency / damping may be visible. This may produce hysteresis effects from humitiy moving between the can and the crystal itself.

author: ace1903
author_rank: Regular Contributor
timestamp: January 01, 2023, 04:34:31 pm
content: Helium is used probably because it has several times greater thermal conductivity when compared to other gases(second after hydrogen).Helium is not problematic to obtain at all, at least there is no scarcity of helium balloons. Some isotopes that are used in lasers and special equipment are problematic but regular helium probably is OK.Anyone has info about price range of those sensors? Are they available through regular distribution channels(Mouser, Digikey, Farnell...)?

author: Kleinstein
author_rank: Super Contributor
timestamp: January 01, 2023, 05:43:28 pm
content: The extra quarz crstals for temperature measurement are no longer build - at least not HP. One can still get crystals for use in an oven that could be useful for a limited range (especially below RT). The sensitivty would still not be high. As this is only a different orientation of a crystal that could still be a crstal manufacturer that can make some - no real high tech, just a special order/exotic part.I don't expect them to be available from the regular distributors, more a used part or custom cut / special part from a small company.I would not consider the quarz thermometers competative anymore. Chances are PT100 or NTCs are the better choice.Normal Helium is quite cheap - it's only tiny quanteties. The only other relevant isotope is He3 and that is quite expensive AFAIR some $5000/g. The main use is for very low temperatures (below 100 mK).

author: ace1903
author_rank: Regular Contributor
timestamp: January 01, 2023, 07:19:34 pm
content: Question for price was for the link provided for RKOV206 RKTV 206 sensor pair.I was lazy to think and just sketched measurement system in my head consisted of few logic gates, TDC7200 time measurement unit and STM32.Self heating will be minimal compared to NTC and PT100 but I am not sure for the sensitivity. Temperature to Frequency is not so simple to guess if mK is doable.

author: 1audio
author_rank: Frequent Contributor
timestamp: January 01, 2023, 08:34:45 pm
content: More on the Quartz technology herehttps://www.eevblog.com/forum/testgear/hp-2804a-quartz-thermometer-teardown/

author: Kleinstein
author_rank: Super Contributor
timestamp: January 01, 2023, 08:40:47 pm
content: The quarz thermometers also have an effect of the oscillator circuit and parasitic capacitance. If not careful even the cables in the supply to the oscillator can have an effect. The actual frequency measurement is the least problem. With the active part of the oscillator at the crystal one does have quite some self heating. With extra cables to the sensor there is the problem with cable capacitance and additional heat flow through shielded (maybe triax) cables. They gave up on it for a reason (actually multiple weak points).The sensitity depends on the frequency TC and thus the oscillator. As a side effect I have looked at a (rather large) MEMs oscillator frequency effected by temperature: this "worked" down to better than mK resolution, but with some nasty effects from residual water that effects the surface oxide with delays. The mems part may have less sensitivity from cables - if you could get a bare resonator.

author: tszaboo
author_rank: Super Contributor
timestamp: January 02, 2023, 03:16:38 pm
content: 0. Realize that RTDs are the way to measure if accuracy is involved.1. Get a 24 bit ADC from TI or AD, which has current generator built in for RTDs2. Get an platinum RTD, class AA if you can afford, 4 wire3. Get a 1K or 10K reference resistor that you can afford. At around 40 EUR you will be able to get very nice ones from VPG.4. Put the reference resistor in series with the RTD and make a ratiometric measurement.5. Bonus point, get an extra resistor like for point 3 at 100Ohm or 1000Ohm to calibrate the input instead of the RTD.Congratulations, you joined the tempnut crowd, welcome.You can also just connect a 4 wire PT1000 to a Keysight 34465A and get instant 1mK resolution.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 02, 2023, 04:38:39 pm
content: If noise / resolution is more important than accuracy than a measurement type NTC (e.g. 1K or 10 K) is a real option. The sensitivity is about 10 x higher than an RTD and the useful range is still something like -50 C to 200 C. Another possibly advantage is that relatively small sizes are available.

author: laichh
author_rank: Regular Contributor
timestamp: January 03, 2023, 05:43:12 am
content: For long-term stability, go for RTD.For sensitivity, go for NTC thermistor.

author: tszaboo
author_rank: Super Contributor
timestamp: January 03, 2023, 12:37:54 pm
content: Quote from: Kleinstein on January 02, 2023, 04:38:39 pmI would bring it up, that sensitivity doesn't really matter. With the required 1KSPS, these ADCs provide ~20 noise free bits, so able to resolve 1PPM, while a platinum RTD has the sensitivity of ~4000PPM/K. Turning on the internal gain blocks, you can easily amplify the signal by 128.My issue with thermistors is their composite nature. An RTD could be a piece of platinum wire, an inert, non-corrosive metal. While a PTC will have humidity, stress and other effects. Maybe microphonics. But then again, I never tried to use it for precision measurements, so prove me wrong?

author: mzzj
author_rank: Super Contributor
timestamp: January 05, 2023, 06:19:59 pm
content: Quote from: tszaboo on January 03, 2023, 12:37:54 pmPlatinum RTD in its ideal form is close to just a piece of platinum wire but then we are talking about metrology grade SPRT sensors starting at 2000 eur ++Anything else and we got stress, drift, hysteresis, dielectric absorption, poor insulation, humidity effects and whatnot. I have got for example some suprising results on industrial PRT's with dielectric absorption requiring over 2 second settling times.While SPRT's are free of most of these effects the construction is extremely sensitive to shocks and vibration. Any temperature lab metrologist can tell you horror stories of Tinsley Pt-25 SPRT with shorted out platinum coils after someone talked too loudly in the lab.

author: Echo88
author_rank: Frequent Contributor
timestamp: January 05, 2023, 09:43:38 pm
content: Would love to hear those stories mzzjWouldnt have expected DA in PRTs.

author: mzzj
author_rank: Super Contributor
timestamp: January 05, 2023, 10:35:05 pm
content: Quote from: Echo88 on January 05, 2023, 09:43:38 pmxDevs website had a good example of (Tinsley) SPRT construction:https://xdevs.com/doc/Tinsley/SPRT/1.pngPlatinum wire coil is basically hanging freely in that twisted quarts tube. If youlookat it wrong it falls down or clumps at some part of the bore with adjacent turns touching each other.Coils on xDevs example seem to be really close to each other but it could be still ok. The examples I have seen have been in worse shape, now part of each labs (very expensive) collection of non-functional SPRT'sThe quarts tubing/sheath is actually really robust compared to what is inside! "It has been said that, if one puts an SPRT down onto a workbench hard enough to hear ittouch, it will probably be strained."

author: Kleinstein
author_rank: Super Contributor
timestamp: January 06, 2023, 08:53:12 am
content: The construction with the spiral in the tube does not look that sensitive. It would need quite some shock to make the turns actually touch. However it would not need much for a simple relative movement against the sliding friction. I would not expect this to have a significant stress effect.The problem with a wire type PTR is that they are usually slow - so not an option for a fast measurement.For a fast measurement it would be thin film PTR. It is not so clear wether a thin film PT1000 is more stable than a good NTC. There are quite stable NTCs.For lowest noise one should measure the resistive sensor with AC, to avoid 1/f noise and unwanted thermal EMF. This is especially true for the usually relatively low resistance PT sensors.A higher sensitivity of the sensor makes it easier to get away with DC. For a limted temperature range higher restance is also not that bad.

author: HighVoltage
author_rank: Super Contributor
timestamp: January 06, 2023, 11:11:19 am
content: If you are only after accuracy, I can recommend the Keithley DMM7510 with a 4 wire RTD, you get 0.1 mK resolution. And with a good platinum probe, like a Fluke 5616-12 you even get good accuracy.I was amazed a few years ago, how sensitive such setup can be.Here in this picture I had the temperature stabilized in a glass water and used a small laser pointer from like 3 m away, pointing through the glass and water at the temperature probe tip.

author: Echo88
author_rank: Frequent Contributor
timestamp: January 06, 2023, 02:14:08 pm
content: Ah, that tight wound platinum coils explain the necessary careful handling.I own a very old SPRT similar to this onehttps://www.nist.gov/sites/default/files/styles/960_x_960_limit/public/images/2016/12/22/flukeprobe.jpg?itok=qrxma0naand only knew about this more rugged construction type.Which temperatur readouts do you use mzzj? Something in the direction of Fluke 1590/Anton Paar MKT50/MicroK or rather AC bridges from WIKA/ASL?

author: mzzj
author_rank: Super Contributor
timestamp: January 06, 2023, 06:16:34 pm
content: Quote from: Echo88 on January 06, 2023, 02:14:08 pmTwo older ASL bridges and Fluke 1594A. Tinsley 25r and 100r references for bridges.IIRC there is also 1 ohm tinsley reference resistor that was previously used with 0.25 ohm high temperature SPRT.