Health Monitoring of SPRTs

1. Health Monitoring of SPRTs

2. SPRT - Standard Platinum Resistance Thermometer Your subject photo here What is it?

3. SPRTs • Interpolating instrument for the ITS-90 temperature scale • 13K to 961.78C - Capsule, Long Stem, HT SPRTs • Repeatability – 0.5 to 1.0mK • LT stability (100 hrs) – 0.03 to 1.0 mK possible • Quartz glass (or metal) sealed sheath • Specific gas mixture • 25.5 ohm, 2.5 ohm

4. SPRTs Characteristics DESIGN FEATURES • Strain-free, well-annealed wire • Pure platinum • Protection from contamination • Sealed sheath • Four-lead construction • Good electrical insulation • Standard dimensions • REQUIREMENTS • Stability • Reproducibility • Measurability AS A RESULT, SPRTs • are precise • are fragile • have relatively large-sized resistors • have relatively low-valued resistors

5. SPRTs The Fluke SPRT design includes the following(to provide the best stability of any commercial SPRT in the industry): • High quality, well-annealed platinum wire held by an optimum design quartz structure. • Proprietary gas mixture to provide protection against sensor contamination, at the same time ensuring low oxidation • High quality quartz tubing with sand blasted exterior to prevent ambient radiation from reaching the sensor • Lead wires exit through sealed quartz glass (not epoxy)

6. SPRT - Standard Platinum Resistance Thermometer Your subject photo here How/why does it drift? What will damage it?

7. SPRT Drift, Damage • Drift from Oxidation • Normal and reversible through annealing • Drift from thermal cycling, vibration, light mechanical shock • Causes strain • Can be reversed if not too bad • Cold quenching – cooling too fast from above 480C • Sometimes reversible • Serious mechanical shock • Causes shorting / strain • Not usually reversible

8. SPRT - Standard Platinum Resistance Thermometer Your subject photo here What can you do to prevent drift or fix damage?

9. SPRT drift prevention It helps to purchase a stable SPRT to begin with… The best design yields better long term stability • Quartz sheath construction • Gas mixture • Too much oxygen promotes too much oxidation of the platinum sensor • Some oxygen protects the sensor from contamination • Sensor and lead wire holding structure • Allow strain-free expansion/contraction of the platinum wire, but still hold the sensor/wire in place • Sheath sealed against air leakage over time • Quartz material melted around the exiting platinum lead wires • How to solve the problem of mismatch of expansion rates for quartz and platinum? • Epoxy has beenshown not to be a very good long term solution to prevent leakage

10. SPRT Handling & Maintenance Avoid contamination of the sensor • Furnace should not contain any base metals • Protect SPRT using a platinum sheath • Use graphite or alumina annealing block • Clean the sheath using alcohol before it is exposed to high temperature

11. SPRT Handling & Maintenance Avoid mechanical shock • Do not leave in baths or furnaces for extended periods of time • Handle carefully • Train staff

12. SPRT Handling & Maintenance Avoid cold quenching • Cool slowly (less than 100 degrees/hour) to below 480C • The SPRT can be taken directly to room temperature when below 480C

13. SPRT Handling & Maintenance Annealing • At highest useable range for the sensor • Above 450C, preferably above 550C • For many hours (10-100) • In a non-contaminating furnace/block

14. SPRT Handling & Maintenance Keep a control chart • Record Rtpw regularly • Use to regulate calibration cycle Check Rtpw when return from calibration • Drift during shipping • Hand carry where possible

15. SPRT Handling & Maintenance • Shorting due to coils touching each other cannot be repaired • Excessive change in resistance from misuse cannot be repaired

16. Conclusion • Recent design improvements have made SPRTs more stable • Challenges remain in sourcing pure platinum • Research continues • Drift is mainly due to handling issues