Calibration Uncertainties in Infra-Red Radiation thermometry

1. Calibration Uncertainties in Infra-Red Radiation thermometry Joint EUROMET CCT-WG5 meeting BNM-INM, Paris, September 2001 By Dr Mark Ballico, National Measurement Laboratory, Australia EUROMET-CCT.WG5 meeting Paris, Sept 2001

2. Outline • Calibration Schemes: only consider pyrometer calibration here, but the same terms appear for calibration of a clients’ BB. • Definition of major uncertainty terms • My view: It is essential to agree on a set of terms for the uncertainties, so that we understand each other, and don’t double count terms. • Some experimental techniques for measuring major terms • The best way to define these terms is operationally, i.e. define them by demonstrating how they are measured. • My view: Major uncertainty terms should be experimentally determined by the laboratory: i.e manufacturers specs not enough. • A few examples • n.b. examples give U95 or k=2 estimates • The examples are typical only: not best / recommended • Summary EUROMET-CCT.WG5 meeting Paris, Sept 2001

3. Two Basic Calibration Schemes: 1. Calibrated reference pyrometer Ref. pyro. Pyro. to be calibrated 2. Calibrated sensor Pyro. to be calibrated thermocouple 650 oC EUROMET-CCT.WG5 meeting Paris, Sept 2001

4. TSP uncertainty termsexample: NML’s 850nm std. pyro. at Zn pt EUROMET-CCT.WG5 meeting Paris, Sept 2001

5. Interpolation error • Sakuma-Hattori and other schemes: ie. I=a.exp(-l/(b+cT) • basically assume leff a weak function of T • OK for narrow band pyrometers but • Low T => low signal => wide bands • Interpolation schemes: don’t explicitly assess RSR • Out of band transmission may creep up unnoticed. • 2 approaches used at NML: • 1) numerical simulation using a range of “likely” RSRs and band leakages • 2) Use redundant fixed points: • eg. 850nm, calibrate at Zn, Al, Au • check at Ag and Ag-Cu eutectic • eg. 1.6mm, calibrate at In, Sn, Zn • check at Pb EUROMET-CCT.WG5 meeting Paris, Sept 2001

6. DUT uncertainty terms: (typical 7-14mm pyro. at 400oC) EUROMET-CCT.WG5 meeting Paris, Sept 2001

7. SOSE measurement • We can specify reference test conditions: • We can specify pyrometer–aperture distance and aperture diameter • But: for practical BBs it is not so clear cut. e0.9 , compared to 1 near base Temperature gradient zone • 1: Axial scan of pyrometer position • 2: Adjustable aperture on BB • 3: Adjustable greybody radiator Focal plane EUROMET-CCT.WG5 meeting Paris, Sept 2001

8. SOSE : 1) scan pyrometer Pyrometer reading position • Advantage: Simple “bad” typical • Disadvantage: Hard to separate SOSE from BB uniformity (i.e. how to turn the measured variation into an uncertainty) EUROMET-CCT.WG5 meeting Paris, Sept 2001

9. SOSE : 2) vary BB aperture • Advantages: • Allows explicit determination of SOSE characteristic • Explicitly fold in uncertainty of effective radiating region diameter • (At NML we use this technique for our TSPs) Largest BB diameter Pyrometer reading typical “bad” Aperture diameter EUROMET-CCT.WG5 meeting Paris, Sept 2001

10. SOSE: 3) adjustable radiating surface • OK, but.. .. • BB aperture diameters restricted to about 50mm • BUT: many commercial intruments have significant SOSE out to over 200mm • Solution used at NML • Don’t need a BB, just a variable diameter greybody radiator • (just need to watch out for stray thermal radiation and multiple reflections) EUROMET-CCT.WG5 meeting Paris, Sept 2001

11. Correction for 95% e • Many commercial pyrometers have a fixed e setting. • Calibrate w.r.t. BB at known T • Compute T for a 95% e radiator giving same band integrated signal at a detector (incl. 5% ambient radiation!!) • But what is the uncertainty in this correction? • Approach: • Numerical simulation • Vary ambient by a few oC • Vary “guessed” SR of pyrometer DUT • Eg. Correction uncertainties used at NML for nominally 7-14mm pyrometers EUROMET-CCT.WG5 meeting Paris, Sept 2001

12. BB sensor uncertainty terms: (typical BB at 600oC) •  These are not the same thing!!!! EUROMET-CCT.WG5 meeting Paris, Sept 2001

13. Sensor Conduction Errors • TCs widely used in common commercial 50-1100oC BB sources. • TC usually has small immersion: Conduction errors • Conduction errors are exponential with depth, decrease by about e in 2-3 diameters. • => move 3 diameters (of sheath!) and note the change. • Usually negligible for fluid bath BBs Full immersion Error estimate Conduction error EUROMET-CCT.WG5 meeting Paris, Sept 2001

14. TC homogenaity errors EMF zone • Wire damage is usually in the in-use heated zone • But: TC is usually calibrated at deep immersion in a calibration block of bath. • SO: repeated calibration just tells us about the calibration in the “cold” zone of the TC • Assess homogenaity of the in-use gradient zone using an oil or salt bath. • New wire ±0.02%, but >0.1% not uncommon EUROMET-CCT.WG5 meeting Paris, Sept 2001

15. BB uncert. terms: (NMLs Cs heatpipe at 600oC @ 8mm) •  one needs a BB emissivity model • For TSP transfer calibration, only e(lref)-e(ldut) and not e(ldut) is important, but in practice just use 1- e(ldut), as this is usually the worst case. • Note: for BBs below ambient, ambient irradiance is significant! EUROMET-CCT.WG5 meeting Paris, Sept 2001

16. BB gradients • 1) Direct measurement: at NML we use a range of “homemade” special TCs • Problem: conduction errrors ca. 2oC, 3cm inside BB at 600oC • 2) Indirect measurement: Use a high F/# pyrometer to scan the walls • Problem with multiple reflections (eeff 0.9 at aperture => 1 near base, =>50oCapparent radiance T increase) • Problems with “specularity” of cavity walls at glancing incidence • 3) Design: Incorporate multiple sensors in the BB design • (we use this at NML in our Alcohol and Oil BBs) TC junction pressed to surface Kaolin covered alumina foam EUROMET-CCT.WG5 meeting Paris, Sept 2001

17. Gradients  BB surface • Heat flux to/from ambient causing a gradient in the surface of the BB base • Estimate by calculation, ...and/or… • Estimate by measurement – eg. one approach: • view with a pyrometer • block BB with insulating wool • remove insulation and watch transient temperature evolution. Insulation removed BB surface cooling to new equilibrium (short time constant, few seconds) signal Error estimate BB cavity cooling to new equilibrium(long time constant, minutes) time EUROMET-CCT.WG5 meeting Paris, Sept 2001

18. Summary Dominant terms should be measured not estimated! EUROMET-CCT.WG5 meeting Paris, Sept 2001