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common concern) and have traditionally been quality sensors’. They are relatively cheap occupant comfort is affected detrimentally so
very costly
, although recently introduced to produce but reports indicate that they that productivity drops, leading to increased
‘system-on-a-chip’ NDIR have significantly need frequent replacement as they drift out operational costs and a potential perverse
reduced costs. Modern implementations of of calibration and lose sensitivity. They have increase in the carbon footprint. The use of
CO2 sensors use gas permeable membranes been successfully applied in a number of properly-controlled DCV systems can provide
to allow the air sample to be analysed whilst applications as a means of detecting occupancy, improved internal environments while saving
keeping larger particulate matter away from including in libraries, and were noted as being operational costs.
the sensor’s optical system. particularly successful in areas where there
By integrating microprocessor control, are food odours and cigarette smoke
. Since © Tim Dwyer
sensors can be recalibrated by automatically the sensor output provides an indication of
resetting to the known outdoor air CO2 levels, change, rather than a specific value, these
frequently (and sometimes erroneously) based sensors are particularly useful where there is Further Reading
on the assumption that the lowest reading of a step change in conditions that would signal a For more detailed coverage of the
the week is the outdoor concentration. If this change of use of a space requiring a shift in the contaminants see BSJ CPD Article, ‘Indoor
assumption is not true, then auto recalibration ventilation regime – for example, a dedicated Air Quality, December 2003 and Chapter 8 of
can drive the sensor into serious error
. smoking area at an airport. CIBSE Guide A 2006
Almost all manufacturers certify their sensors Since the installation cost and maintenance For an extensive review of IAQ sensing see
for five years or more – and one, notably, requirements of gas sensors may be Appendix A of [9].
provides a lifetime guarantee
. However prohibitive, multipoint space sensing may be
several studies reported on by Maripuu
used where one sensor is employed to sample
have shown that practical implementations many spaces. This uses a network of inert References
of the sensors in the field record significant flexible tubing around the building that draws [1] H.B. Rijal, P. Tuohy, F. Nicol, M.A.
and unpredictable errors. air samples from the different points (that Humphreys, J. Clarke, A window opening
Developed from metal oxide may include the outdoor air) back to a central algorithm and UK office temperature: Field
semiconductor (MOS) sensors used to sense monitoring sensor. The number of spaces that results and thermal simulation:field results
can be effectively sensed with one sensor is and thermal simulation. 2008.
limited by the time needed to draw air from a [2] Wyatt, Terry, Inadequate servicing: the
There’s little benefit in
space and take a representative reading. enemy of sustainability. 2009.
Movement detector technology using [3] Shepherd, K. Variable Air Volume Air
saving energy if air quality
passive infra-red or ultrasonic sensors are Conditioning Systems. Blackwell Science.
and occupant comfort is used to determine the presence of occupants 1999.
and have been widely applied to lighting [4] Carslaw, Nicola, An improved model for
controls. This can be usefully utilised for indoor air chemistry. Cranfield University:
VOCs the technology has recently been ventilation applications (such a classrooms) 2008.
applied to determine CO2 levels. Although where there is a known occupancy, but there [5] CIBSE Guide A – Environmental Design.
the sensitivity to selectively measure CO2 is some uncertainty as to the periods of use. CIBSE. 2006.
is traditionally low for MOS technology On detecting occupation, the ventilation [6] Bluyssen, European indoor air quality audit
new semiconductor doping materials have systems may be simply actuated or step project in 56 office buildings. 1996.Bluyssen
improved this and they are appearing in the changed. [7] Sherman, Nance E. Matson and Max
. It is still a relatively untested Intelligent movement detection is available H., Why We Ventilate Our Houses – An
technology for CO2 sensing. using infrared sensors combined with Historical Look. Lawrence Berkeley
Electrochemical sensors have also been advanced tracking and detection algorithms, National Laboratory.
used for CO2 measurement; however, to date to count the occupants as they enter and [8] Wolkoff, P., Wilkins, C.K., Clausen, P.A.,
their operational effectiveness is uncertain leave a space. This can provide a direct Nielsen, G.D., Organic compounds in
since they are a relatively recent addition to the measurement of occupancy (as opposed to office environments – sensory irritation,
HVAC marketplace. They are more resistant the previous inferred methods) and so, if set odor, measurements and the role of reactive
to dust and moisture than non-dispersive up correctly, can provide a reliable signal of chemistry. 2006.
infrared sensors and quicker to stabilise
. occupancy levels to the ventilation control [9] Maripuu, M.-L. Demand Controlled
MOS sensors are typically used as ‘mixed system. Ventilation Systems in Commercial
gas’ sensors. The gases in the sample are Buildings. Chalmers University of
absorbed by the sensing element’s surface Conclusion Technology. 2009.
so changing the resistance of the surface. By The successful monitoring and control of the [10] CIBSE Guide H – Building Control
varying the semiconductor material or by quality of room air is becoming increasingly Systems. CIBSE. 2009.
operating the sensor at different temperatures important as the application of novel [11] Warden, David, Supply Air CO2 Control
and temperature profiles, the response of building techniques and materials coincide of minimum outdoor air for multiple space
MOS sensors may be adjusted. with improved awareness of their effects on systems. 2004.
However, they provide a measurement only comfort and health. [12] Mark Hydeman, Taylor Engineering,
of the presence of a mixture of gases with no There is a prerogative to reduce the building Correspondence providing evidence
feedback of individual concentration, and so environmental impact, but there is little to California Energy Commission, Air
commercially they are used as general ‘air benefit in ‘saving energy’ if air quality and Resources Borad. 2003. September 2009 CIBSE Journal 61
CIBSEsep09 pp59-62 cpd.indd 61 8/20/09 7:10:01 PM
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