Thermocouple Response times for EN285 Compliance during Qualification

The Qualification of Porous Loads / Equipment Loads to European Standards (EN285:2015) requires fast responding temperature measurement to demonstrate good air removal. Equilibration time is measured as the time between the drain temperature and the load temperature achieving the sterilisation temperature band and must be ≤15 seconds for small chambers and ≤30 seconds for large chambers.

To record the Equilibration Time accurately requires a fast data logger and fast responding temperature measurement. EN285 is quite specific in this regard with detailed specification for logging interval (1 second) and the response time of temperature measurement probes used.

Bill Bingley our Instrumentation, Calibration, Controls SME recently completed a study for a client where they had been challenged by an auditor for this information. Some of that information is shared here with the permission of the client.

The key compliance requirements from EN285 for the temperature measurement are as follows:-

• The cross sectional area of any part of the probe ≤ 3.1mm2
• The Temperature Response time T90 ≤ 0.5 sec

The maximum cross sectional area ≤ 3.1mm2 requires a probe diameter of less than 2mm which is achievable.

The Temperature Response time is much harder to achieve. Test Method:-

• Stabilise temperature probe at 20C
• Insert into water at 90C
• Record the time for the temperature probe to reach 90% (T90) of the difference. If moving from 20C to 90C the temperature change is 70C. 90% of 70C is 63C. Therefore the time to move 63C (From 20C to 83C) should be less than 0.5 seconds. (T90 ≤0.5 seconds).

Since all of the industry benchmark data loggers used for qualification log at 1 second intervals, the decision was taken to use such data loggers for this testing. Our Kaye AVS system was used for the testing.

A range of thermocouples were tested, all Type T class 1 accuracy:-

1. Single core single insulated twisted pair thermocouple wire welded tip.
2. Multi strand PTFE double insulated thermocouple wire welded tip.
3. Multi strand PTFE double insulated welded and encapsulated.
4. Stainless steel 4mm probe

The test was performed using a stirred water bath with temperature stability at 90C +/- 0.1C. The probes stabilised at room temperature 21C before insertion. Since 1 second logging interval is used the fast acting thermocouples can be seen to have a near step change with the 1 second accuracy. However the encapsulated tipped thermocouples and the larger 4mm Stainless Steel sheath thermocouple have measurably slower response times and significantly away from the EN285 requirement. Many runs were performed to demonstrate repeatability and the results were very consistent with response times as follows:-

1. Single core welded tip < 1 second
2. Multi strand welded tip < 1 second
3. Multi Strand welded and encapsulated tip 8 seconds +/- 1 sec
4. Stainless Steel Sheathed 4mm probe. 6 seconds +/- 1 sec

The conclusion; The tip encapsulation with some polymer material is significant insulation and therefore impact on response time. The large 4mm stainless steel probe is actually faster than the thermocouple with encapsulated tip. When measuring a short equilibration time this is a significant impact, the site conclusion and feedback to auditor in this instance is that encapsulated thermocouple will not be used for Porous Load Qualification.

It is often argued that provided all the thermocouples have an equally slow response then this doesn't matter? But it is certainly a long way from EN285 compliance and also a potential variable in the critical measurement that should be eliminated if possible.