An often ignored consequence of using longer than average lead wire lengths for pharmaceutical autoclave chamber probes, is the sacrificing of accurate temperature measurement
When planning to upgrade or purchase an autoclave incorporating temperature probes with longer than average conductors it is therefore essential to give this factor due consideration.
Detector types and construction.
PT100 detectors are now available in two formats: the conventional type which uses a wire wound construction, and the thin film type.
This latter type is based upon a ceramic substrate with a deposition of high purity platinum, laser etched to give 100W at 0.0C.
This is then sealed within a glass adhesive.
Detectors are commercially available to several different tolerance levels.
Class B and A are according to BS EN 60751:1996, and the remainder are based on fractions of the Class B tolerance.
In general only the wire wound types are available in 'fractional' DIN tolerances and this is achieved by selection following manufacture.
The higher tolerance of the wire wound type is due in part to it's construction because the platinum winding of the detector component is freely 'suspended' within a ceramic housing.
This 'freedom' provides the ability for the platinum to self anneal after temperature cycling which also aids long term stability.
The disadvantage of this type of construction is that this freedom of movement makes it more susceptible to mechanical shock, which induces drift and in extreme cases a break in the winding, causing an open circuit.
Thin film PT100 detectors show greater resilience to mechanical shock, but they are only manufactured successfully to class 'A' tolerance and they lack the long term, drift free characteristic of the wire wound types.
Autoclave chamber probes.
In selecting a detector for use in an assembly, which is destined for autoclave chamber use, it is necessary to achieve a balance between the optimum tolerance and the resilience of the detector to frequent handling by the operators.
For all pharmaceutical applications Thermal Detection uses only Class A detectors or those with a higher tolerance.
Unless otherwise specified by the customer the thin film Class A type of detector is incorporated within chamber probes to provide greater resilience to the effects of operator handling.
Principles of measurement.
PT100 detectors are connected to the measuring instrumentation using any one of three different connection principles.
Two wire, with no lead wire compensation, three wire with partial compensation or four wire which provides full compensation.
Autoclave chamber probes fall into a different category than most other PT100 applications since a high proportion of their connecting lead wires reside within the autoclave chamber during the sterilising cycle.
Therefore a certain length of these connecting lead wires are exposed to the same temperature as the detector with the remainder being at ambient temperature.
Since the actual measurement of a PT100 resistance thermometer is a resistance measurement, it follows that in any measurement that involves the detector legs not being directly connected to the measuring bridge, then the additional resistance of the connecting lead wires will also be seen by the bridge unless it has a high impedance.
In basic terms, with a two wire connection, if the connecting lead wire resistance is known and subsequently subtracted from the overall measurement, then theoretically a reasonably accurate result could be achieved.
However, this is on the assumption that the connecting lead wires are at the same ambient temperature along their entire length.
The lead wires used for autoclave chamber probes by Thermal Detection are PTFE insulated silver plated, copper conductors, stranded at 7/0.16mm.
They exhibit a resistance of 0.130W/metre when at an ambient temperature of 22C, increasing to 0.168W when the temperature rises to 120C.
The average chamber probe with a lead wire length of 1.7metres within the chamber and 3.0metres outside the chamber, shows an additional 0.129W at the measuring bridge during sterilising temperature.
1.7metres x 2 = 3.4metres.
0.168W - 0.130W = 0.038W.
3.4 metres x 0.038W = 0.129W.
This additional resistance is equivalent to 0.34C and this of course increases in proportion with a longer length of connecting lead wire.
Since the two wire method is impractical, the three wire compensation method was adopted using a modified Wheatstone bridge.
In this configuration two matched resistors are located on one side of the bridge and a 100W resistor on the other side.
The detector lead wires (one from either side of the detector) are connected to the opposite sides of the bridge (one to the 100W resistor and the other to the matched resistors), effectively compensating for each other, with the third wire supplying power to the bridge.
However this will produce a thermal gradient along the leads themselves.
Although the effect of Joule heating is reduced, it cannot be eliminated altogether because the heat transfer conditions at the detector will be different from those of the matching 100W resistor in the bridge circuit.
In fact there will only be one situation when this effect is eliminated entirely and that is when both the 100W resistor and the PT100 detector are at the same temperature.
To enable more accurate measurements to be made, particularly when the connecting lead wires are relatively long and passing through varying ambient temperatures, two types of 4 wire system were developed.
The first is known as the four-wire loop, but this is not used as much as the true four-wire system, which also provides the greatest accuracy.
In the true four-wire system the bridge is again modified, but now it incorporates two pairs of matching resistors on opposite sides of the bridge with the 100W resistor located between the two pairs.
A lead wire from each side of the detector is connected to each side of the 100W resistor with the second pair of lead wires, from each side of the detector, being connected to each of the matching resistor pairs in the bridge.
This last pair of lead wires takes the constant current power source to the detector with the first pair of lead wires being used to measure the actual voltage drop across the detector.
Therefore by using a constant current source and being able to simply measure the change in voltage across the PT100 detector rather than a change in resistance (Ohms Law), any fixed or varying lead wire resistance is totally eliminated.
When calibrating three and four wire connected probes in an oil bath or a hot block the difference in readings between these two connection types may appear to largely disappear.
This is due to there being no temperature effect on the leads of the three wire connection.
On an autoclave with a large chamber and a consequently lengthy chamber/load probe that requires an input to the chart recorder, a further consideration arises.
Unlike most control system manufacturers, no chart recorder manufacturer, currently or in recent times has manufactured a chart recorder that incorporates a bridge that will accept a true four wire arrangement.
However, virtually all chart recorder manufacturers can provide an input card that accepts the industry standard 4-20mA signal and this can be accommodated by fitting a temperature transmitter.
Most transmitters can be provided with a DIN rail mounting facility and several are available with the required four wire connection.
