MAC MAC155L - Theory of Operation

MAC155 Rev. 12 ©Copyright MAC Instruments 2005. All Rights Reserved.
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The accuracy of the MAC155 is ± 1% of full scale over 20% to 100% of the
full scale range. The accuracy is reduced to some extent below 20% of full
scale.
The MAC155 contains a built in sensor calibration system that makes it
possible to do a daily two point calibration check. The calibration gas is
generated by mixing water vapor from an internal water reservoir with air in the
proper ratio. The probe is then swamped with this calibration gas so that the
sensor is totally immersed in the internally generated reference gas. The
calibration points are 40% and 80% of full scale.
The internal water reservoir holds approximately 17 ounces (500mL) of
distilled water. Under normal operating conditions, the water will last for over a
year. It is recommended however that the reservoir be drained and re-filled
at least once per year.
THEORY OF OPERATION
The MAC155 Probe contains a proprietary high temperature capacitive
sensor that responds to the partial pressure of the water vapor that is dissolved
in the atmosphere that is being monitored. An absolute pressure sensor
measures the total pressure at the moisture sensor. The percent of water vapor
by volume is then calculated by the internal processor using the following
equation:
% water vapor by volume = (water vapor partial pressure / total pressure) X 100
The total pressure is a function of the altitude above sea level, fluctuations of
barometric pressure with weather, and changes in operating conditions of the
equipment feeding the stack
.