Answers to Four Questions about Carbon Dioxide Self

Answers to Four Questions about Carbon Dioxide Self
/ APPLICATION NOTE
Answers to Four Questions about Carbon
Dioxide Sensor Self-Heating
What Is Self-Heating and
Why Does It Matter?
Figure 1: Error in
relative humidity
reading resulting from
+1°C (1.8°F) and +2°C
(3.6°F) self-heating
at room temperature
(20°C, 68°F).
The self-heating of electronics in
carbon dioxide (CO2) instruments
usually originates from two main
sources: the power required to take
the CO2 measurement (sensor infrared
source) and the energy required to
generate the output signals.
Incandescent light bulbs – typical
infrared sources in CO2 sensors –
consume a significant amount of
power and thus generate heat. The
heat spreads within the instrument
enclosure. Because the enclosure
limits thermal exchange with the
surrounding environment, the
temperature within the instrument
is always slightly higher than the
ambient temperature.
Self-heating is practically irrelevant
for sensors that only measure CO2.
However, when the instrument also
measures temperature, self-heating
disturbs the measurement by
inducing a temperature measurement
error, which is typically around 1°C
(1.8°F), or even more.
How Does Self-Heating
Influence Relative
Humidity Measurement?
Just like temperature, relative
humidity can’t be reliably measured
in close proximity to a heat source.
Relative humidity is a temperaturedependent parameter, so the
accuracy of measurement results
will be affected by self-heating.
Figure 1 shows how +1°C (1.8°F)
and +2°C (3.6°F) temperature errors
distort relative humidity readings at
room temperature (20°C, 68°F). The
horizontal axis shows the humidity
level in the environment and the
vertical axis the error in relative
humidity (%RH) measurement.
The humidity measurement error
rate grows as a function of increasing
relative humidity. Increased selfheating also results in a greater
error. At 50%RH and 20°C ambient
temperature, the error is -3%RH for
instruments with +1°C (1.8°F) selfheating and -6%RH for instruments
with +2°C (3.6°F) self-heating.
Can Self-Heating Be
Compensated For?
Sensor manufacturers may consider
automatic compensation to correct
the negative effect of self-heating on
temperature measurement. In theory,
this can be done by subtracting an
average correction factor from the
measurement results.
Compensation may work in some
conditions; however, the amount
of self-heating is not constant in all
conditions. Instead, it is dependent
on airflow and the wall material
behind the sensor. Moreover,
applying a correction factor larger
than the specified measurement
accuracy is highly questionable.
Also, compensation of humidity
measurements is even more
difficult than that of temperature. In
conclusion, compensating for selfheating is not recommended.
Can I Perform Sensor
Self-Heating Tests?
It is simple and straightforward to
perform self-heating tests, even
without investing in expensive
equipment:
1. Install the sensors on the wall
along with a passive temperature
sensor (e.g. Pt100 sensor or similar
with minimal self-heating).
2. Turn on the device. Immediately
compare sensor temperature
readings with the passive
temperature sensor output. Record
the difference between the two
readings. For a temperaturecompensated device, the initial
readings are often lower than those
of the passive sensor.
3. Leaving the power on, record the
temperature (and humidity) readings
from time to time. It can take from
15 to 40 minutes for the self-heating
effect to fully develop.
Figure 2: Example of
a self-heating test. The
transmitter is allowed
to reach equilibrium
with the surrounding
environment.
Figure 3: Transmitter
temperatures before power-up.
Figure 2 shows an example of the
expected self-heating test behavior.
The relative humidity reading
decreases as the temperature
increases due to self-heating.
Self-Heating Test Results
of Vaisala GMW90 Series
CARBOCAP® Carbon
Dioxide, Temperature,
and Humidity Transmitters
Vaisala GMW90 Series Carbon
Dioxide, Temperature, and Humidity
Transmitters were tested for
the tendency to self-heat. The
results were compared with two
competitors’ instruments (Devices
1 and 2, marked as Sp2 and Sp3).
Vaisala’s GMW90 transmitter was
marked as Device 3 (Sp4).
Background
temperature (Sp1)
Device 1 (Sp2)
Device 2 (Sp3)
Device 3 (Sp4)*
Figure 3 shows the thermal images of
the tested transmitters before powerup. All transmitter temperatures (Sp2,
Sp3, and Sp4) are in equilibrium with
the background wall temperature (Sp1).
Figure 4 shows the thermal images
of the transmitters 30 minutes after
power-up. The temperature readings
were taken from the estimated
location of the temperature sensor
within the instruments. Clear
differences between the transmitter
temperatures can be observed.
The test results are collected in Table
1. To conclude, there are significant
differences in the self-heating
tendency of the three transmitters. The
highest self-heating device (Device 1)
was 1.3°C (2.34°F) and the lowest
(Device 3) was only 0.2°C (0.36°F).
Figure 4: Transmitter temperatures
30 minutes after power-up.
The Vaisala GMW90 transmitter
outperforms the competition due
to its unique low-power microglow
infrared source. Its power
consumption is only 25% that of
traditional infrared sources. Learn
more about microglow technology at
www.vaisala.com/microglow.
Temperature
unpowered,
°C (°F)
23.6 (74.5)
Deviation from
background
temperature, °C (°F)
Temperature 30 min.
after power-up,
°C (°F)
23.2 (73.8)
Difference to
background
temperature, °C (°F)
Self-heating,
°C (°F)
23.5 (74.3)
23.4 (74.1)
23.4 (74.1)
-0.1 (-0.2)
-0.2 (-0.4)
-0.2 (-0.4)
24.4 (75.9)
23.9 (75.0)
23.2 (73.8)
1.2 (2.2)
0.7 (1.3)
0 (0)
1.3 (2.3)
0.9 (1.6)
0.2 (0.4)
Table 1: Self-heating test results.
* Vaisala GMW90 transmitter
Please contact us at
www.vaisala.com/requestinfo
www.vaisala.com
Ref. B211384EN-A ©Vaisala 2014
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