Neville J Mitchell discusses thermocouple materials for validation within pharmaceutical manufacturing.
The regulatory requirement regarding the validation of autoclaves and lyophilisors, is a necessary process within pharmaceutical manufacturing and one which can involve the employment of a rather large number of thermocouples.
The preferred thermocouple type for this activity is the type aT' thermocouple (cu/Con), consisting of a pair of conductors of differing materials. The positive leg in copper and the negative leg in a proprietary material composite of nickel and copper, known as Constantan. Different grades of this material are available, but for validation purposes this should always be in Class 1 material which offers a tolerance of ±0.004t , where at' equals the operating temperature.
Although thermocouple wire is available in different conductor diameters, the most common size used in validation applications is a pair of single solid conductors of 0.315mm diameter, known as 1/0.3mm. Due to the small size of these conductors, relatively long runs of this material are very prone to damage and where longish runs are expected to be used, the same thermocouple material can be obtained in a stranded format. This consists of seven twisted solid conductors, each of 0.16mm diameter in each leg and known as 7/0.2mm.
For the construction of both the solid and stranded thermocouple arrangements, the pair of conductors are laid side by side, insulated with extruded Tefloni and known as a flat pair, an outer jacket of PFA is then further extruded over the pair of conductors.
The principle of operation of a thermocouple is simply that when their opposing junctions (hot junction as the measuring junction and the cold junction as the reference junction at the instrument terminals) are at different temperatures, then an emf or electromotive force will be generated. Although this generated voltage is only in microvolts it follows a known, non-linear curve which meets Internationally accepted standards.
This low signal is then amplified by the receiving instrumentation to enable recording and data logging to be carried out. However it is important that the thermocouple wires running between the point of measurement and the instrumentation are not located close to power lines or equipment operating high voltages, otherwise the signal may give rise to errors induced by these higher voltages.
When carrying out validation procedures, the thermocouple hot junction is often produced on site, simply by twisting the conductor ends together by hand, a fast but unreliable method. Although this action will often allow the thermocouple to provide a measurable output, it is not good practise and is something that should be avoided. The action of twisting the conductor ends together by hand will never produce a credible and lasting hot junction for several reasons. Additionally a hot junction made in this manner will invariably be contaminated by grease imparted by the fingers, will never be mechanically sound and will be subject to increasing oxidisation.
A thermocouple hot junction that is intended to be used for the collection of credible data should only be formed by welding the two conductors together to form a small uniform ball , whilst in an inert atmosphere. This procedure is usually carried out using a TIG (tungsten inert gas) or a capacitance type welding machine, in both cases with the proposed hot junction being maintained within an envelope of Argon gas during the welding procedure.
An irritating consequence of using a thermocouple with a bare hot junction for autoclave validation, is that under chamber pressure, steam (or water in a water cycle autoclave) will be forced up the inside of the thermocouple outer jacket. Capillary action assists this and when reaching the cooler side of the chamber the steam condenses and invariably evidence of this will be seen by a constant drip of condensate at the thermocouple terminals.
To prevent this occurrence, the hot junction is encapsulated within a Teflon cap which forms a permanent seal, thus preventing steam or water penetrating the thermocouple jacket. The cap also gives mechanical protection to the hot junction and reduces the effects of oxidisation. Thermocouple wire with encapsulated hot junctions are usually supplied in multiples of one metre lengths and they can be supplied with miniature thermocouple connectors fitted to the cold junction.
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Neville J Mitchell is with Thermal Detection Limited, Stockton-on-Tees, England. www.thermal-detection.com
