This background briefing document by Neville Mitchell, MD of Thermal Detection, discusses thermocouple validation wire for pharmaceutical autoclaves
It is very easy to consider thermocouples as just temperature measuring devices.
In use every day in pharmaceutical, process and industrial applications they are in the front line of measurement and control but often they are not given a second thought.
These background notes have been produced to help both the technical reader and the less advanced student understand their manufacture and application.
The pharmaceutical thermocouple.
Thermocouples used for the validation process on pharmaceutical autoclaves must comply with EN258 and HTM 2010.
Anyone involved in hospital and pharmaceutical sterilization should be familiar with the details of these specifications.
Type 'T' (copper/Constantan) thermocouples, in accordance with BS EN 60584: Pt 2, are the preferred type and for validation purposes should only be produced from Class 1 material.
This has a tolerance value of +/-1.5C or 0.004 (t) within the range of -40C to +350C.
Although the convention is to use solid core conductors of 1/0.3mm size, due to the ease of breakage, solid core conductors of 1/0.5mm are also used.
Where it is necessary to run the thermocouples some distance from the autoclave to the measuring instrumentation, stranded conductors of 7/0.2mm are often selected.
The benefit of this is their inherent greater flexibility and resistance to conductor breakage.
Construction: high steriliser temperature and hygienic considerations require that the conductors are insulated with PTFE, laid as a pair (also known as a figure of 8) and protected by an outer jacket of PFA, a derivative of PTFE.
The PFA jacket is generally extruded over the conductor pair, but it can be supplied in a lapped format.
The extruded jacket type is generally preferred because moisture can only penetrate the jacket at the thermocouple hot junction; whereas it is possible on the lapped type to penetrate the lapped joints and, under pressure, percolate along the inner jacket.
However some users consider that the lapped type of jacket is easier to strip back than the extruded type.
Although it is suggested in HTM 2010 documentation and frequently carried out this way, producing the hot junction by simply twisting the conductors together does not produce a junction of high integrity.
Just moving the thermocouple pair itself can be sufficient to fractionally loosen the conductors and by that action produce a higher resistance at the junction and therefore an impaired measurement.
The hot junction should be formed by TIG welding within an argon envelope.
This produces a hot junction of the highest integrity through the conductors becoming properly fused together and the incidence of oxidisation considerably reduced by the blanket of Argon.
To ensure an extended hot junction life and to prevent moisture tracking up the inside of the jacket, the hot junction can also be encapsulated within Teflon.
Where validation is to be carried out on hot air tunnels the same type 'T' thermocouple is used, but the conductor insulation and jacket material must be Kapton which is able to be used at temperatures of up to 300C for short durations.
The same procedure should be carried out in forming the hot junction, but since the presence of moisture is not a problem, it is not necessary to encapsulate the hot junction.
