Sensor Technology 9
tetrachloroethylene, which are extensively used as solvents in cleaning, and are prone to high retention times within soil. In industry, as with other VOCs, chlorocarbons are most commonly monitored over a workday’s exposure by personnel wearing a badge or patch on which VOCs are trapped. VOCs are subsequently thermally released and analysed. Such technology is not really appropriate in the home or public space.
Why monitor VOCs generically
Despite the variable thresholds recommended for indoor VOCs, there is strong justifi cation for non-selective VOC monitoring. Clean air is extremely free of VOC’s. So, as long as it doesn’t sense methane, a generic air quality monitor, can prospectively detect very low levels of any VOC without alarming excessively. This approach is supported by the fact that VOCs often co- occur: they appear together from a particular source. In these instances, a more abundant and less toxic VOC can often present itself as a ‘tag’ for one that is more toxic. For example, benzene is considered so carcinogenic that in the air quality guidelines tabled above ‘no safe level’ is recommended. But because benzene usually co-occurs with other less toxic VOCs at much higher concentrations, such as fugitive fuel vapours or volatilising paint solvents, it is reasonable to monitor and respond to such compounds at a higher and more practically measurable level. This principle is embraced in various guidelines, by providing a total indoor VOC concentration threshold, such as 0.3 ug/m3
(~100 ppb) for building ventilation. A single threshold
such as this enables a standard to be applied diversely, as well as providing a single performance criterion against which monitoring technologies can be assessed and calibrated.
Routine VOC monitoring is usually considered after every effort has been made to reduce or eliminate VOC sources. In a domestic environment, VOC removal is typically just a matter of opening a window to a ‘stuffy’ room. For large buildings, where VOCs arise from internal sources, (see Table, columns A to E) the primary means of their removal is by forced extraction. VOC monitoring then provides a means to regulate ventilation, safeguarding health whilst minimising the costs of air conditioning.
At some locations, external air may be prone to contain VOCs (see Table, columns E to I). In such cases, it may be fi lter replacement costs that are minimised by VOC monitoring.
Whatever the case, VOC monitors form part of a procedural system, in which reliable measurement is required both above and below target exposure limits: false alarms and failure to alarm are equally compromising.
Additionally, monitors are normally expected to require minimal service and maintenance, and cost is also critical, particularly in monitors that might be deployed in generic VOC monitoring.
Generic indoor air monitors
As is always the case with technology, there is a trade off between price and performance. But within performance, servicing must also be considered. As a high cost option, Flame Ionisation Detection (FID) provides the sensitivity, linearity and rapidity of response required for air quality monitoring. It delivers a fair measurement of total VOC in mg/m3
. However, the requirement
for a hydrogen fl ame has prevented the technology from being available without a continuous service burden. It also responds to 2000 ppb methane in air, which is liable to compromise the discernment of non-methane VOCs at lower concentrations.
Amongst low cost sensing options, all of which provide an adequately fast and reliable response, IR sensors are fairly non-VOC discriminating, but lack sensitivity to sub-ppm (<1000 ppb) levels.
Metal oxide sensors are more sensitive, but non-linear and over- selective to specifi c VOCs. Additionally, they are subject to drift.
The generic VOC monitor of choice is photo-ionisation detection (PID) engaging a krypton lamp (‘PID-Kr’). It responds to few inorganic compounds such as ammonia and sour gas. It detects any VOC which ionises at less than 10.6 eV. This includes all VOCs containing two carbons or more except for most fl uorocarbons, saturated chlorocarbons, ethane, ethyne and propane. PID is now available as compact sensors which resolve all the compounds except formaldehyde in the table at levels of a few ppb (1 ppb = 0.001 ppm) or less.
Summary
Clean air is largely defi ned by the absence of VOCs apart from methane, and particulates. The VOCs of concern are chemically diverse, vary in their toxicity, and arise from several sources, both from indoors and out of doors. None the less, the concept of ‘total volatile organic compounds’ is well recognised, being stipulated for example, in building ventilation guidelines. A few technologies are capable of detecting VOCs non-selectively. Of these, PID is the best suited and most widely deployed.
Contact Details Peter Morris, Head of Sensors, Ion Science • Email:
sensors@ionscience.com • Web:
www.ionscience.com
How can we help identifying natural gas leaks when H2 is added?
TBM and THT are the most commonly used odorants to make odourless natural gas smell, so that people realize there is a gas leak. However, control of these odorants is necessary to maintain their effectiveness. Only last year, Sensorix offered its electrochemical (EC) gas sensors for mercaptan and THT detection - Sensorix TBM 50 and Sensorix THT 100.
But due to various power to gas initiatives, more and more H2 will be added to natural gas. For this reason, a gas measurement device with high-performance gas sensors and no H2 interference is required. We are very pleased to confi rm that the Sensorix TBM 50 is already suited for this application. To complete our respective range of sensors, we have developed the Sensorix THT 100 low H2, so we now offer a perfect solution for the detection of mercaptans and THT in natural gas with added H2.
The Sensorix TBM 50 detects mercaptans with zero cross sensitivity towards H2, CO, H2S, THT and is ready for use in H2 containing natural gas (“power to gas”). The Sensorix TBM 50 gas sensor has excellent repeatability (standard deviation of sensitivity = 0.5 % for 10 consecutive measurements at 10 mg/m³ TBM). It also shows excellent baseline stability from -10 to 40°C and in changing humidity (<0.1 mg/m³) and its unbiased operation will increase battery life in your portable instrument.
The 3-electrode sensor, Sensorix THT 100 low H2 is suitable for power-to-gas applications because it has a very low H2 interference (THT signal only 1 mg/m3 in 20% hydrogen), has no H2S cross interference and it requires 300 mV bias.
All sensors are available in various standard formats - 4S, 7S, Mini, Classic, Smart, as well as in customer specifi c mechanical adaptations.
More information online:
ilmt.co/PL/R201 and
ilmt.co/PL/Y0YY For More Info, email: email:
58263pr@reply-direct.com
For More Info, email: email:
WWW.ENVIROTECH-ONLINE.COM
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