Challenges in HVAC CO2 Measurement

Challenges in HVAC CO2 Measurement
/ APPLICATION NOTE
Challenges in HVAC CO2 Measurements
Operation Verification
CARBOCAP® Technology
Sensor stability, spaces occupied 24/7, transmitters’
verification of operation, and temperature dependence pose
the most common challenges in HVAC CO2 transmitters. Due to
the unique sensor technology, Vaisala has resolved these issues
with its CARBOCAP® sensors.
Stability
Most HVAC instruments are rarely
serviced or calibrated although they
are still used for control purposes
for many years. Automatic offset
correction - if it works at all - only
compensates for offset drift but not
for sensitivity changes. Light sources,
light paths, filters and detectors often
have different aging properties.
The optimal way to eliminate most
drift sources is using a single light
source, single light path and single
detector together with an adjustable
filter, typical in Vaisala HVAC
transmitters. By using these design
solutions excellent long-term stability
can be achieved: the specified
stability for Vaisala’s intelligent CO2
module, GM10, is ± (15 ppm + 2% of
reading) over 5 years.
Spaces Occupied 24/7
Many sensors depend on the CO2
concentration dropping to 400 ppm
at night to allow automatic offset
correction in the transmitter. Without
this corrective measure stability
of the measurement is poor. The
challenge here is that many rooms
are constantly occupied and do
not reach the level of 400 ppm at
any time of the night. Vaisala HVAC
transmitters, however, have the
unique CARBOCAP® sensors that use
an internal reference measurement
instead of the offset correction, see
figure 1 above for an illustration. On
every measurement cycle the light
intensity at the detector is measured
at two wavelengths:
1. A reference wavelength that is not
absorbed by CO2
2. A wavelength at which CO2 light
absorption is strong
These two wavelengths are filtered
out of the same light beam using a
tunable band pass filter. Both light
intensities are measured using a
single light detector, exactly identical
light paths and the same light source.
By calculating the ratio of these two
light intensities most sources of
instability can be eliminated.
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After sensors are installed, typically
correct operation must be verified.
It may take several weeks before
the sensors with automatic offset
correction show proper values.
Due to the internal reference
measurement, Vaisala CARBOCAP®
sensors show correct values
immediately when powered on; all
Vaisala HVAC CO2 devices have full,
specified accuracy within 10 minutes
after power-on.
Temperature Dependence
While the internal reference
measurement of the CARBOCAP®
sensors eliminate most of the sensors
temperature dependence this is not
enough to get correct ppm values.
Measuring CO2 absorbance is not
enough – it is important to know
the gas temperature, too. Without
temperature compensation, you
get a 0.3% of reading temperature
dependence per °C. To overcome this
challenge, temperature measurement
for compensation of gas properties
has been included in all Vaisala
GMW80- and GMW90-series products
for compensation purposes.
Conclusions
The unique Vaisala CARBOCAP®
CO2 sensors present an optimal
and cost-efficient solution to the
main challenges presented by HVAC
measurements: reliability, long
lifetime and ease of installation
and operation. Learn more at
www.vaisala.com/hvac
Ref. B211464EN-A ©Vaisala 2015
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in any form without the prior written consent of Vaisala is strictly
prohibited. All specifications — technical included — are subject
to change without notice.
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