Reliable temperature measurement is key to the sterilisation cycle

The elimination of micro-organisms to the point at which they are no longer detectable in standard media culture, is a basic and essential process within the fields of pharmaceutical manufacturing, research and in health care organisations. The process within which this is carried out is sterilisation, of which there are various forms, but perhaps the most prominent of these is steam or moist heat sterilisation. Neville J. Mitchell reports.

In the steam sterilisation process, time, pressure and temperature are the three main parameters that contribute to a successful sterilisation cycle. Of these three, reliable temperature measurement is the parameter most generally regarded as the one that causes most problems and frustrations.

As with most modern industries, pharmaceutical production regimes are constantly being exhorted to attain an ever improved manufacturing performance, in order to retain or increase the competitiveness of the product. Unlike many other industries though, this situation is then aggravated by a seemingly unending flow of regulatory requirements. Equipment reliability in these situations then becomes of paramount importance.

The temperature sensors used in modern autoclaves generally fall into two distinct types, thermocouple and resistance thermometer. Autoclave thermocouples these days are invariably based upon the type aT' thermocouple.

These consist of a positive leg in copper with the negative leg in a proprietary metal alloy of copper and nickel, known as Constantan. They should only be produced from Class 1 material, which is the highest tolerance grade of commercially available material. The thermocouple offers a sensor which is generally more robust, but with a shorter, long term stability characteristic and it is less accurate than the resistance thermometer.

Thermocouples generate an eletromotive force (emf) when their opposite junctions are at differing temperatures. These junctions are usually referred to as the measuring junction (hot junction) and the reference junction (cold junction). The voltage produced is very small, measured in microvolts, and follows a known, but non-linear curve. Because of the very low signal level, care must be taken to ensure that the thermocouple leads do not run close to power lines or equipment operating high voltages, otherwise the signal may give rise to errors induced by these higher voltages.

However the majority of autoclave sensors used these days tend to be based on resistance thermometry and in particular the Pt100 (a platinum winding exhibiting 100ohms at 0°C) which has an excellent long term stability characteristic. Unfortunately the trade off in this instance is that the PT100 detector, unlike the thermocouple, is more susceptible to physical shock and when continually subjected to this, will show a decline in accuracy and an increase in drift.

The following chart shows the comparative tolerances of a type aT' thermocouple and the Pt100 resistance thermometer:

Although 1/10DIN is a considerable improvement on a Grade aA' tolerance, in practical terms, by the time the full sensor assembly has completed manufacture, this tolerance will invariably have been degraded slightly.

The autoclave chamber or load probe is the sensor that invariably causes the most problems because of the need for operator handling. The other fixed sensors such as the air detector and drain will usually provide excellent service with minimum drift over long periods and for many cycles.

Because of the need for the operator to handle the chamber/load probe at the start and conclusion of each sterilisation cycle, it is necessary for at least a section of this probe to be of a flexible construction. In order to provide this flexibility, a seal of high integrity is necessary at the transition point of the rigid and flexible sections of the probe.

However the environmental conditions within the autoclave chamber are harsh, extremely demanding and conspire against most sealing arrangements.

Temperature cycling within the autoclave chamber causes the various materials of construction in the probe assembly to expand and contract at differing rates and amounts.

The stresses and strains that this imposes on the chosen type of sealing arrangement will soon be evident if an inadequate seal has been used. This will manifest itself in the form of erroneous data being received by the instrumentation, caused by the chamber environment penetrating the seal and allowing the ingress of moisture or steam contamination of the detector.

This situation can only be avoided or delayed by careful selection of the appropriate materials and the manner in which they are applied.

The design of the chamber/load probe assembly must also take into account the opposing ideals of ruggedness to resist the attentions of the operator, whilst being sensitive enough to respond to temperature variations in the chamber within an acceptable time period.

Having met these various conditions the chamber/load probe should provide a good and acceptable service life.

However, the autoclave chamber/load temperature sensor has long been treated as the aCinderella' of autoclave components.

Demanded by purchasing managers to have minimum impact on their budget, but at the same time expected to provide a long and reliable service ­ then generally subjected to physical mistreatment by the autoclave operators.

The consequence of this imbalance of requirements has been that most suppliers of these devices endeavour to provide a product that satisfies the purchasing manager only.

When finally put to use, the product is then often found to display questionable reliability and generally falls well below an acceptable confidence level.

The cost of lost production through down time, caused by unreliable temperature measurement on a production autoclave, frequently contributes to overhead costs of no less than E1500 per hour. In addition to this, is the costly probability of having to reject a spoilt product load. This general attitude regarding autoclave temperature sensors, in many cases stems from the autoclave manufacturer's sensor choice being based on cost, thus often resulting in the inclusion of inappropriate sensor assemblies.

This decision is frequently driven by the need to maintain a competitive edge, coupled with the client's main interest being centred on the control specification and the other more obvious aspects of the autoclave.

It is in the interest of both the autoclave manufacturer and the end user that appropriate temperature sensors designed for the application, should be specified in order to ensure the confidence of reliability, so necessary in today's pharmaceutical industry.

Enquiry No 71

Neville J Mitchell is managing director with Thermal Detection Ltd, Stockton-on-Tees, England. www.thermal-detection.com

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