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Development of bespoke detection system

In developing the STS Siloxane monitor the STS design team had to come up with a simple, cost effective but accurate method to measure Siloxane contamination in a gas stream which had many constituents and a high methane content making direct measurements difficult.  The system also had to be robust enough to use on line in the field and small enough to fit into a 19" rack housing.

The teams research was led by David Ward who has a 20 year background in analytical chemistry.  Early candidates for a detection system were GCMS and FTIR which would give the greatest level of accuracy and offer a wide range of potential substances to be analysed.  However the basis of the Siloxane Monitor was to produce a low cost solution which could be easily used by an unskilled operator.  GCMS and FTIR instruments are both very expensive and require experienced technicians to operate them and as such were quickly ruled out.

However parts of the technique in both instruments were identified as being suitable for the measurement challenge.  The GC element would allow the separation of the target molecules from the sample gas stream whilst rejecting the those not of interest.  When built into a concentration column this allowed a stream of gas to be passed through a special media and to concentrate a sample over a period of 6-10 minutes.  This technique means that the achievable sensitivity is much greater than will a smaller sample and also that the detection components can be less sensitive, and therefore simpler/more robust and cheaper.

Having established a method to obtain a good quality sample of sufficient volume to be able to measure accurately attention was then turned to how the actual measurement would be made.  Here STS turned to the other technology first assessed -FTIR, taking the implied sensitivity to IR measurement of the siloxanes group it was decided to use an IR source with pyroelectric detector to make the measurement.  To do this research and testing was carried out using a number of different filter options to restrict the visible bandwidth of the detector and therefore ensure its specificity to the molecules of interest.  Many factors contributed to the decision making process including the length of the cell, the type and power of the IR source being used, composition of measurement windows which isolate the detector and source from the gas stream and so on.

The measurement relies on the absorption of the IR light by the siloxanes molecules, this in turn is seen by the detector as a drop in signal which is converted to a voltage, the result is therefore an inverse peak as the siloxanes absorb the light and the signal falls. There are actually over 700 measurements taken to produce each result with an algorithm that calculates the starting baseline and ending baseline to adjust for any loss should the reading not return fully to the starting value before the measurement cycle completes.  

Measurement graph.jpg

One of the challenges quickly established was the propensity for the siloxanes to "stick" to surfaces - particularly the larger molecules D5/D6, particular care was therefore given to materials used which would minimise contamination potential, temperature control of the housing to ensure that all components are kept above any potential dew point and a clean and purge routine to ready the measurement chamber for the next cycle.

This entire development including building a whole instrument with gas control, circuitry, pcbs, display, telemetry and field testing took under 24 months with the first operational units being in place just 3 months later.

Mechanical Design

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