Calibration and uncertainties of secondary IPRT's, and more. - Page 1
Forum section: electronics / general

author: Personwithhats
author_rank: Contributor
timestamp: November 13, 2024, 06:19:29 am
content: I am currently in the process of scoping out the required equipment I would need to calibrate some custom industrial sensors upwards of about 420°CAiming for a final calibration uncertainty of 0.25°C or so, will temper expectations depending on what I can get. For 'hobby/side-gig' purposesGeneral plan is to use some small stirred water/oil baths, a DIY dryblock calibrator, and a 'secondary' reference PRT.No TPW cell yet, will be using the dewar/siphon/shaved-ice method for an Ice Point reference.The best-fit probe appears to be Isotech's 935-14-116 (~$1.2k). Maybe the Fluke's 5615-12-L (~$1.7k) would work as well.Latter to be built better but both claim similar specs. Hard to say.Will be using a Tektronix DMM4040 (6.5 digit) as well as a standard reference 100ohm resistor, TBD.Maybe an IET-labs SRX-100 or Isotechs 456? I don't know of any affordable accurate ones, would prefer to spend more on the probe than on the ref resistor xDAbsolute accuracy might not even be critical if I exclusively do resistance ratio measurements, just stability.Looking to build a DIY bipolar constant current source (1mA and sqrt(2) at least), maybe I can find plans for the one CERN used in Metron Designs I-REF2Some kind of thermal scanner and a junction box to automate the measurements.Low-thermal emf is nice but optional thanks to bipolar current.Well, that's the general setup. Now for the calibration questions:1 -Is it possible to have the secondary probe calibrated using the ITS-90 interpolation scheme? There are numerous papers that detail this as preferred over CVD even for low-alpha IPRT's.I've seen some papers mention that some labs already do this though the specifics of the calibration process elude me.I'd like to follow a similar method for the IPRT's I would calibrate, mostly to report in terms of W rather than pure resistance.This would allow future measurements at R(0.0°C) to minimize drift in the IPRT calibration.2 -For an ITS-90 calibration of a secondary SPRT using a primary SPRT in a series of baths, what would be the process? (And same for calibrating IPRT's using secondary SPRT)I believe a standard method is least-squares fit to derive ∆W coefficients but *which* points to use isn't discussed anywhere.A few used comparison points similar to Fixed point temperatures while others used e.g. a comparison point every 20°Celsius for their test.3 -What about the uncertainty propagation of the above method?Closest parallel I can come up with, assuming comparison points were close to fixed points, is to follow (C.23) in "Uncertainties in the Realisation of the SPRT Subranges of the ITS-90 "Where I could multiply by some factor R(0.0°C) and use as 'Client' TPW point uncertainty.....all of this is quite complicated but once again advice is welcome.4 -I've seen the TPW Propagation factor mentioned briefly, not sure what it is or its usage.Only seen in Appendix 2, some calibration certificates, and briefly mentioned in "Guide to Secondary Thermometry: Industrial Platinum Resistance Thermometers"

author: Overspeed
author_rank: Frequent Contributor
timestamp: November 13, 2024, 08:21:42 am
content: HelloAn interesting article on the subjectIn your inventory the oven / temp well as both your sensor and the probe shall be at the same temperature .RegardsOS

author: mzzj
author_rank: Super Contributor
timestamp: November 13, 2024, 04:49:16 pm
content: -you can get reasonably cheap and stable 100 Ohm reference resistor if you buy Vishay hermetically sealed Z-foil resistor and get it calibrated. After initial drift you should be within few ppm/year.-6.5 digit multimeter gets you pretty far even if you just use the reference resistor to verify the DMM. DMM with 200ohm range would be preferred to something like say 120 ohm range.-DIY dry block with enough small gradients to get 0.25cel total calibration uncertainty could be harder than it looks at first.1. Certainly possible. With secondary/tertiary standards there is little advantage of using actual fixed points so calibration points can be spread out bit more freely and you can get better coverage with calibration points.Instead of ITS-90 fixed points I'd just slap plenty of calibration points with comparison calibration to primary SPRT over liquid bath temperatures. Calibrations above oil bath temperatures (300C) are better done with fixed points but $$.2. A Use same calibration points as defined for ITS-90 subrange in question and least squares fit with matching subrange correction coefficients. If you follow the ITS-90 definitions You get "perfect" fit but that won't tell anything how well or badly the data fits.or B Select "plenty" of calibration points to cover the whole range. With this method you can also use higher number of polynomial correction function as long as you have enough calibration points. (ie use 0...660C subrange coefficients for 0-20-40-60-80-100 cel calibration)3. See 6.2.1 Fixed-point sensitivity coefficients and propagation equation without correlationshttps://www.bipm.org/documents/20126/41773843/Guide_ITS-90_5_SPRT_2021.pdf/c4bbbe56-4118-eef7-47cb-3ea234db40b8or "Traceable Temperatures: An Introduction to Temperature Measurement and CalibrationAuthor(s):J. V. Nicholas, D. R. White" (Introduction.. yea right)Having a lots of calibration points gives you good picture if one calibration point is off-the line and the propagated uncertainty is controllable and it helps with the nasty curves the IPRT's have habit of making.Actual math of freely selected calibration point uncertainty propagation gets too nasty for my liking but I prefer to have at least 2 additional points over the minimum needed to calculate the deviation function.4. You can visualize the TPW propagation by considering that the Pt100 RTPW would drift by 1%. Now as a result the resistance value at 200Cel is also off by 1% and the sensitivity factor is the ratio of R200c/RTPW.Pt100 sensor resistance is 200 ohms at 261cel and 1mK error at TPW results 2mK propagated error at 261Cel.

author: mzzj
author_rank: Super Contributor
timestamp: November 13, 2024, 04:59:18 pm
content: One of the weakest links is going to be the IPRT if calibrated up to 420cel.I'd normally start with characterization and aging test of multiple sensors:1. measure R02. measure ~200c in oil bath3. anneal/age at 440 cel for 1-24hour4. without completely cooling down calibrate in ~200c oil bath to find out amount of hysteresis5. re-measure R0repeat stages 1-4 until you have enough data to estimate the stability or lack of it.IPRT's are going to be total hit and miss and only way to know what you got is to test and test them more.I have done the stages 1-4 sometimes 10 times for a batch of sensors only to find out that they were not enough stable for my purposes/liking.Wirewound IPRT's have less hysteresis and are typically more stable than thin-film sensors.Pretty often you have to ask the manufacturer if the probe is built from wirewound or thin-film sensor.

author: Personwithhats
author_rank: Contributor
timestamp: November 13, 2024, 08:51:14 pm
content: Quote from: Overspeed on November 13, 2024, 08:21:42 amThat's where I first saw the CERN current source mentionedQuote from: mzzj on November 13, 2024, 04:49:16 pmFor the Vishay, something like Y4733100R000V0L? I think Vishay also offers NIST traceable cals w reasonable pricing.Available off the shelf from 3achips but a cal lab would likely cost more than Vishay xdWill double check assembly and all that. Worried about overheating the resistor during assembly, maybe should use Indium solder.What's a good source for the low-emf posts atm? iirc pomona wants $80 each........For dry-blocks, there is plenty of media for ad-hoc laboratory blocks and Isotech/etc. have some published media on their own block construction.Three-zone heating, bifilar wound coils, proper monitoring RTD placement (Match inserted sensor element depths) etc.It will take trial and error, will likely make a lower-temp cheap aluminum variant first.Still cheaper than the 5-10k+ price tag for a 'Portable' one or dual zone heater.Only part I am really unsure of is insulating the windings without damage over frequent thermal cycling.Cotronics has a wide selection that might work.Quote from: mzzj on November 13, 2024, 04:49:16 pmHaven't seen anyone use an order higher, interesting. I actually have the digital book and tbh it *is* a basic introductionStill learning uncertainties though, and speaking of which -6.2.1 is covered in more detail in the attached file. MSL technical guide 21 is also a good resource, though it uses a simplified formulae.My main concern is I do not see how to properly propagate uncertainty when using ITS-90 subranges and arbitrary points.How do you define an uncertainty at a specific "fixed-point" if you are using arbitrary points? Or using extras for the coefficient determination?iirc NIST uses extra fixed point tests for SPRT's only as a redundant check that the derived deviation formulae is within tolerance, not as extra points.As an aside, "Traceable Temperatures" mentions a cubic CVD equation I haven't seen anyone use.....Might address concerns about excessive error in CVD above 250°CQuote from: mzzj on November 13, 2024, 04:49:16 pmUsing the "TPW propagation factor" from Appendix 2https://www.bipm.org/documents/20126/70380091/Guide_ITS-90_5_SPRT_Appendix-2_2018.pdf/4a6e12cd-0f7d-4924-2dc3-b6d284c86141I cannot derive the same values NIST reports on pg.52https://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.250-81.pdfI'm not sure what the math is or how it is reported.Especially when you consider that the full uncertainty equations such as in 6.2.1, or C.23, already account for errors in TPW measurements.Quote from: mzzj on November 13, 2024, 04:59:18 pmI'll be manufacturing the probes myself, one variant is based on the TMP119 PN-junction, another is using thin-films from IST that conform to the ITS-90.Off-the-shelf probes of reasonable quality cost hundreds (a WW element alone is $50+) and are usually too large for the uses I am considering.Boguhn have some information regarding these relatively new thin-films.Most of the improvements are in CTE matching the substrate/bonding/Passivation layers and additional Pt doping.Multiple thermal cycles to minimize hysteresis are a must, not sure about accelerated aging via current alterations.......The Jursic report has a lot of interesting information, I struggle in general to find any thin-film specific info.For example, I have conflicting reports on the effectiveness of thin-film barrier layers.Some are reported immune to metal-ion contamination as well as oxidation, others differ. Might need to speak with the manufacturer for more information.Not looking forward to sealing in an Argon/Oxygen mix (Oxygen 2kPa), with a quartz sheathFor now I've decided to simply pre-age/oxidize some Stainless sheaths and see how it goes.A finicky issue I've been considering is how to hermetically seal the small capsule RTD. Glass is out of the question and most epoxies wont be able able to handle the temperatures or temperature shocks. There might be a few I can test but that's to be seen.

author: Overspeed
author_rank: Frequent Contributor
timestamp: November 14, 2024, 07:39:51 am
content: Helloas current source ''stable'' on this forum there is a thread on ultra stable 1 mA sourcehttps://www.eevblog.com/forum/metrology/a-stable-battery-powered-1ma-current-source-for-measurement-of-10k-standard-resi/msg5555995/#msg5555995I have recorded some pics , performances looks niceRegardsOS

author: mzzj
author_rank: Super Contributor
timestamp: November 14, 2024, 08:12:11 pm
content: These might be interesting for someone: different approaches to fit industrial IEC751 pt100 calibration data with various options:

author: Personwithhats
author_rank: Contributor
timestamp: November 14, 2024, 11:30:34 pm
content: There are a lot more that do similarCCT's "Industrial Platinum Resistance Thermometers" has a good breakdown of available interpolation methods.I attached a few below, as well as the latest Manuscript regarding uncertainties from Dr.WhiteUnfortunately most tests only cover interpolation related errors and deviations. Nothing regarding uncertainties from using said formulae.Overall I think ITS-90 based formulae is a better fit than CVD, esp past 250C.Cubic or quartic polynomials appear to fit even better regardless of test. Unfortunately uncertainties aren't as detailed, plus the dependency on an absolute resistance value rather than W ratio limits resulting calibrations. I've yet to find anyone using a polynomial based on W, barring the cubic CVD equation mentioned in Traceable Temperatures.

author: Overspeed
author_rank: Frequent Contributor
timestamp: November 15, 2024, 10:38:11 am
content: HelloUncertainity on measurement is not linked to the sensor onlyThe GUM defines measurement uncertainty as a "parameter, associated with the result of a measurement, that characterizes the dispersion of the values that could reasonably be attributed to the measurand''.I link an interesting doc in PDF the GUMRegardsOS