Sensors & transducers Choosing a VOC sensor
In this article, Arthur Burnley, sales and marketing director of Alphasense, explains the factors affecting the choice of VOC sensor - for both end- users and manufacturers monitoring instruments. Burnley also discusses the key questions that must be addressed
V
OCs (Volatile Organic Compounds) perform many vital roles as fuels, solvents, cleaners, feedstock, sterilants
etc. However, they can be harmful to health and the environment, so it is often necessary to monitor their concentration. By definition, organic compounds contain the element carbon, and exhibit similar chemical properties, which is advantageous from a monitoring perspective. However, these properties unfortunately vary widely between the many thousands of different VOCs. The table on the opposite page provides an overview of the technologies that are currently available.
WhAt Is the mAIn ApplICAtIOn? This is the most impor tant consideration because it impacts the choice of technology. For example, the ability to measure a specific VOC may be required, and this would rule out many of the technologies if other interfering VOCs are likely to be present. Similarly, whilst the cost might be attractive, the potential presence of cer tain inorganic gases may mean that Metal Oxide sensors are unsuitable. However, in applications such as process monitoring the identity of other gases may be known so the response of a specific type of sensor may be solely attributable to the VOC of interest. Regulatory monitoring of VOCs in
applications such as industrial stack emissions and ambient air quality necessitate cer tain technologies such as GC/MS and FTIR. However, these technologies are less well suited to applications such as leak detection, surveys, workplace safety, personal safety, Hazmat etc. due to cost, power requirements and por tability. The most popular technologies for these applications are electrochemical, metal oxide and PID, and by offering all three technologies, Alphasense is able to recommend the most appropriate technology for these applications, taking into account a wide variety of factors such as: • Sensitivity • Range • Speed of response • Specificity • Accuracy • Interferences • Maintenance requirements • Longevity • Cost
eleCtrOChemICAl VOC sensOrs
With resolution from 10 to 50 ppb, electrochemical cells are low cost, low power, compact sensors. Electrochemical sensors need to be optimised for the target VOC because each VOC requires a different ideal bias voltage for best sensitivity. Also, electrochemical cells
respond in about 25 seconds, in comparison with one to two seconds for PIDs. Nevertheless, electrochemical sensors are suitable for some applications, where cost is important and performance characteristics are known. For example, Alphasense has developed an electrochemical Ethylene Oxide sensor for applications including fumigation of certain agricultural products and sterilisation of medical equipment.
metAl OxIde (mOs) VOC sensOrs Metal oxide sensors are compact and low cost but require more power than electrochemical sensors. Humidity sensitivity and baseline drift are all characteristics of traditional n-type MOS sensors, but Alphasense p-type metal oxide gas sensors have more stable baselines and very low humidity sensitivity. MOS are not as sensitive at low concentrations, compared with PIDs. MOS sensors also respond to high concentrations of some inorganic gases such
as NO, NO2 and CO. MOS may be a more suitable technology than PID in applications requiring the measurement of halogenated VOCs such as CFCs.
phOtOIOnIsAtIOn (pId) VOC sensOrs PIDs respond to most VOCs except for small hydrocarbons such as methane, and for some halogenated compounds. Each VOC has a characteristic ionisation potential and the peak photon energy generated in a detector depends on the PID lamp used. For example, a Xenon lamp = 9.6 eV, a Krypton lamp = 10.6 eV and an Argon lamp = 11.7 eV. Hence, the use of an argon lamp provides the largest detection range of VOCs, whereas a Xenon lamp can increase selectivity. Clearly, the choice of lamp is dictated by the
likely VOCs to be measured, lamp lifetime considerations, and the sensitivity and level of selectivity required. The Xenon lamp (9.6 eV) is suitable for
many aromatics and unsaturated VOCs containing at least six carbon atoms (C6+). For example, this lamp is commonly used for the selective detection of compounds such as BTEX (Benzene, Toluene Ethyl Benzene and Xylenes). The Krypton lamp (10.6 eV) detects most non-halogenated C2, most C3 and C4+ VOCs.
Continued on page 22... 20 February 2019 Instrumentation Monthly
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80
