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BT24i CO
2
Gas Sensor User’s Guide
Short description
The CO
2
Gas sensor BT24i is used to monitor gaseous carbon dioxide levels in the range between 0
and 5000 ppm
1
. The sensor uses the Non-‐Dispersive Infrared Detection (NDIR) method and detects
the net increase or decrease of light that occurs at the wavelength where absorption of carbon
dioxide takes place. The light intensity is then correlated to CO
2
concentrations.
CO
2
gas moves in and out of the sensor tube by diffusion through the six vent holes in the sensor
tube. The tube contains an infrared source at one end of the sensor tube and an infrared detector at
the other end. The detector measures infrared radiation absorbed in a narrow band centered at 4.26
µm. The greater the concentration of the absorbing gas in the sampling tube, the less radiation is
detected by the IR detector.
When the sensor is collecting data, the IR source blinking on and off – it takes a reading about every 5
seconds.
The sensor is delivered with a 250 ml sampling bottle and rubber stopper to attach the gas-‐sampling
bottle to the sensor tube. Do not place the sensor tube directly into any liquid. The sensor is intended
only for measuring gaseous,
not aqueous
, CO
2
concentrations. The CO
2
sensor needs to warm up for
90 seconds anytime when power is interrupted.
The CO
2
Gas sensor can be directly connected to the analog BT inputs of the CMA interfaces. The
sensor cable BT -‐ IEEE1394 needed to connect the sensor to an interface is
not supplied
with the
sensor and has to be purchased separately (CMA Article BTsc_1).
Sensor recognition
The CO
2
Gas sensor BT24i has a memory chip (EEPROM) with information about the sensor: its name,
measured quantity, unit and calibration. Through a simple protocol this information is read by the
CMA interfaces and the sensor is automatically recognized when it is connected to these interfaces. If
your CO
2
Gas sensor is not automatically detected by an interface you have to manually set up your
sensor by selecting it from the Coach Sensor Library.
Calibration
The CMA CO
2
Gas sensor BT24i is supplied calibrated. The output of the CO
2
Gas sensor is linear with
respect to the measured CO
2
concentrations. The supplied calibration function is:
CO
2
(ppm) = 2000 * V
out
(V).
The Coach software allows selecting the calibration supplied by the sensor memory (EEPROM) or the
calibration stored in the Coach Sensor Library. For better accuracy the sensor can be calibrated.
For the best accuracy (the sensor’s characteristics may change over time) before starting the
measurement the sensor should be calibrated at known CO
2
level using the calibration button on the
sensor box.
To calibrate the sensor:
•
Place the 250-‐mL sampling bottle delivered with your sensor in the air outside long enough to
ensure that its content is replaced with fresh air. The calibration will be based on this sample
1
The CO
2
sensor measures in parts per million (ppm). In gaseous mixtures, 1 part per million refers to 1 part by
volume in 1 million volume units of the whole. This unit can be recalculated to
percent
by dividing a value in
ppm by 10000. 5000 ppm is equal to 0.5 %.
The level of C0
2
in the Earth’s troposphere has gradually increased from 317 ppm in 1960 to current levels of
nearly 370 ppm. Exhaled human breath has a C0
2
concentration of about 50 000 ppm.
