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BT72i Relative Humidity Sensor User’s Guide
Short description
The CMA Relative Humidity Sensor BT72i measures relative humidity in a range
between 0 and 100%. The sensor consists of an integrated circuit, which uses a
thermoset capacitive polymer to sense humidity. The integrated circuit then
produces an output voltage, which varies with relative humidity.
The sensor is placed in a plastic box with holes, which provide air circulation. The
typical response time of the unit is 15 seconds in slowly moving air at 25
°
C. The box
not only protects the sensor, but also shields it from light. The sensor is slightly light
sensitive if the light strikes it in just the right way. For best results, shield the sensor
from bright light.
The Humidity 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
Order Code BTsc_1).
Sensor recognition
The Humidity sensor BT72i 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 Humidity 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 Relative Humidity sensor BT72i is supplied calibrated. The output of the
sensor is linear with respect to the measured humidity:
RH (%) = 32.26 * V
out
(V) -‐ 25.81
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 you can calibrate the sensor. Calibration can be done by
comparison to another instrument that measures relative humidity (hygrometer).
Another calibration method is based upon the use of saturated salt solutions as the
humidity generation source. If you place a saturated salt solution in a sealed
container, the air above it will reach a known relative humidity. The relative humidity
above the salt depends slightly on the temperature. A list of saturated salts and their
humidity values in different temperatures are shown in the table
1
.
1
Even though none of the salts listed here is especially dangerous, use normal cautions with these
chemicals.
