14 Gas Detection AUTHOR DETAILS
Dr Alan Doncaster,
C.Chem., M.R.S.C., Clairair Ltd,
Little Braxted Hall, Witham Road, Little Braxted, Witham, Essex, CM8 3EU Tel: +44 (0) 1376 516414 Fax: +44 (0) 1376 516415 Email:
info@clairair.com
Methane response is normally affected by poisoning before responses to other gases. This is because methane has a higher activation energy for combustion. Therefore a sensor which is being poisoned could show a low methane response but retain a normal response to other gases such as propane. It is therefore normal to calibrate using methane and check for responses to other gases relative to methane rather than calibrate using a different gas and estimate the methane response.
So there are several factors that have an influence on the pellistor response and also on the relative responses. Normally, the larger the molecule the larger the effect and the lower the response. The response of a pellistor to hydrogen is normally affected least and indeed it is extremely difficult to poison a pellistor to hydrogen. Although hydrocarbon infrared sensors are replacing pellistors in many applications, there is still a place for a pellistor as it measures flammability, non-hydrocarbon explosive gases and gases that have very weak infrared signatures.
References:
1. J. G. Firth, Alan Jones and T. A. Jones, Combustion and Flame, Volume 20, Iss 3, 303-311, (1973)
2. IEC 61779–1: “Electrical Apparatus for the Detection and Measurement of Flammable Gases – Part 1: General Requirements and Test Methods”, International Electrotechnical Commission, 1998
3. Sidney W. Benson and Jerry H. Buss "Additivity Rules for the Estimation of Molecular Thermodynamic Properties" J. Chem. Phys. 29: 546. (1958)
4. S.W. Benson, "Thermochemical Kinetics, 2nd Ed.", John Wiley & Sons, New York, 1976.
5. J.O. Hirschfelder, R.B. Bird and E.L. Spotz, Trans. Am. Soc. Mech. Eng. 71, 921 (1949).
6. Neufeld et al., J.
Chem.Phys, 57, 1100 (1972)
New Gas Detection Rack Mount Control System
The NEW Digital Modbus Rack Mount Controller TA2048MB designed and manufactured by Milram Technology (USA),
features a 48channel system with 3 separate 16 channel Modbus Networks. Local or remote power supply options. Multidrop smarter Transmitter (sensor) Networks, which eliminates separate costly wiring to each Transmitter. RS485 digital Modbus RTU; half or fullduplex configuration. Four relays (SPST), 10 Amp. common to all channels on each Network. Select energised /nonenergised, latching/nonlatching function. Low, Mid, High and Fault relays. LED alarm indicators. Audible buzzer alarm. Backlit 16 characters x 2 lines LCD display. Autoscrolls to provide channel data and fault conditions. Simple front panel, push button channel setup. Redundant storage of configuration parameters for highest reliability. Rack mount or wall mount enc- losures. Explosion proof available. Continuous Diagnostics. Optional: Remote Relay Modules, Remote Analog Output Module, and Remote Digital Output Module, Modbus enabled, can be installed at any point on RS485 network to minimise wiring. Use with any MilRam smarter Transmitter Toxic/LEL/VOC/ Oxygen: hundreds of different gases and vapors.
Reader Reply Card no 39
Alphasense ‘A Series’ NDIR Sensor - Second to None! Alphasense’s (UK) contemporary designed miniature CO2
infrared sensors are selfcontained, plug in sensors with
integrated optics contained within standard sized miniature (20.0 mm) gas sensor housings. There are no moving parts within the sensors. An internal temperature sensor is included to enable the temperature to be monitored at the detectors, thereby providing the best means of compensating for temperature effects, including ideal gas law corrections. In order to derive a signal from the Alphasense infrared sensors, the lamp should be pulsed at a constant frequency and duty cycle and power should be applied to the on-chip FET transistors. The signal can then be extracted from the on-chip FET transistor sources and the internal sensor temperature can also be monitored continuously.
Arthur W. Burnley – Sales and Marketing Director stated: “Alphasense has a broad and rapidly growing range of customers. They are demanding different techniques and sensing capabilities of the highest quality to further their own growth and developments. The introduction of these NDIR sensors is part of a much wider release schedule from Alphasense that also included a unique PID sensor, developments utilising the latest Nano- technology and exotic toxics in a variety of formats, especially our unique and highly successful D-Series of truly miniature sensors.” Alphasense maintains its commitment to customers by continuing to supply the highest quality sensors with on-time delivery every time allied to exemplary support.
Reader Reply Card no 41 New Portable Multi-Gas Solution
BW Technologies by Honeywell announces the arrival of the latest portable multi-gas solution – the GasAlertMax XT. Simultaneously monitoring for hydrogen sulfide (H2 oxygen (O2
S) carbon monoxide (CO),
) and combustibles (%LEL), the GasAlertMax XT features a non-intrusive integrated sampling pump and SmartSample diaphragm pump technology. By offering one-button operation, the GasAlertMax XT provides ease of use and simple compliance to industry safety standards. Tamper-proof, user adjustable options enable the instrument to be customised to suit your specific application.
The GasAlertMax XT is ideally suited to a range of applications and customers including confined space entry, oil and gas, marine and shipping, utilities, wastewater treatment plants, power generation and municipal needs.
Powered by rechargeable batteries that provide enough power for an entire 13 hour shift, the compact GasAlertMax XT comes with standard event logging and datalogging. Calibration is simple with an automatic procedure. The GasAlertMax XT is compatible with the BW MicroDock II automatic test and calibration system. Multi-language support is available in English, French, German, Spanish and Portuguese.
Electrochemical Gas Sensors
Gas detector manufacturers the world over have great appreciation of the direct contact they have with our team. Every year there are more and more who count on our expertise, support and the quality of our sensors.
We have more than 100 sensors, made to measure 16 gases with your:
personal safety monitors fixed-point detection systems
flue gas analysers
New Infrared Methane Sensor City Technology (UK) is pleased to announce the expansion of its range of IRceL®
a methane sensor – IRceL® CH4 Reader Reply Card no 42
infrared sensors with the addition of
. Infrared sensors are being increasingly used in place of pellistors for detecting flammable gases in both fixed and portable gas detectors. Although infrared sensors are not directly analogous to pellistors (they don’t measure combustion and will not detect H2
for instance) they have the benefits of long-life, not
being susceptible to poisoning, operating in anaerobic conditions and they fail “safe”. Consequently infrared based sensors for detecting a range of hydrocarbons are being increasingly used in certain applications. City Technology’s new methane sensor, IRceL®
CH4
breath checkers air quality monitors portable gas detectors
The independent Swiss sensor developer and producer
MEMBRAPOR AG BIRKENWEG 2 CH-8304 WALLISELLEN / ZURICH TEL. +41 43 311 72 00
WWW.MEMBRAPOR.COM
info@membrapor.ch
Reader Reply Card no 40
intrinsically safe standards. The IRceL®
CH4 , is in the industry standard 4 Series format and is fully certified to flameproof and
has been designed to deliver the best possible performance and uses high quality optical components comprising a long-life tungsten filament bulb as the IR source and a dual channel pyroelectric detector (reference and detector). The novel non focusing optical design, offers significant improve-ments in performance over other optical designs enabling the sensor to operate reliably and reproducibly across a wide temperature range. Like all the IRceL®
range, the methane sensor contains an embedded EEPROM that contains device specific calibration
factors which, when used together with City Technology’s algorithms for signal linearisation and temperature com - pensation, allow the instrument to perform with industry leading accuracy virtually unaffected by temperature.
Reader Reply Card no 43
IET
Annual Buyers Guide 2009
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