How the STS Siloxane Monitor works
The STS Siloxane Monitor has been designed in house by STS to provide the most cost effective , reliable and simple monitor available. STS were able to approach the issues of detecting and measuring siloxanes from a completely conceptual starting point - the monitor is not based on an existing device modified to also "do siloxanes", it was conceived , designed , built and tested with only that purpose in mind.
This has enabled the design to be both results led and cost driven- resulting in the use of robust but simple design with an emphasis on practicality.
The monitor works by taking a sample of the biogas, usually approx 1litre which is passed through a GC concentration column and then to vent. This GC column media selectively separates the siloxane molecules from the other contaminants in the gas which pass straight through the media.
The column is then heated which releases the siloxanes from the media and they are carried by a N2 carrier gas into the measurement cell. The cell comprises a Infrared source, filters and an pyroelectric detector mounted behind specialised windows. As the carrier gas passes through the cell the siloxanes cause absorbance of the IR signal so the received signal by the detector falls and produces and equivalent voltage drop. This is recorded and processed to produce a mass of material which when calculated with the sample volume produces a reading in mg/m3.
Siloxanes are particularly difficult to measure due to their "sticky" and persistent nature. Part of the challenge of making accurate measurements is ensuring that the measurement cell and concentrator are not contaminated after taking a reading. To alleviate this the monitor is purged with nitrogen to remove any residues and the concentrator is heated to drive off any residue which may not have been released leaving it clean for the next cycle.
The monitor will record one reading per hour under normal conditions - of that approx 15mins are the sampling phase, 10 minutes the measurement phase and the rest is cleaning and purging ready for the next sample.
The monitor produces a total siloxane reading which is the sum of the various species from L3 through to D6. The unit is usually calibrated using either D4 or D5 dependent on the site as these tend to form 80%+ of the siloxanes seen and are also the most damaging being the larger molecules.
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