1-11
1.5.4
Analog Inputs
The temperature, flow and pressure transducers are measured by the Analog to Digital
Converter. Each of these is discussed further in the following sections.
1.5.4.1
Temperature Measurement
Reference Drawing 95S39912, 95S39915 & 916
Two thermistors are used by the system for continuous real time temperature monitoring.
A 1KΩ thermistor probe, RTD1, is connected to channel 1 of the Analog Termina
l Board,
ATB. It is used to measure and control the actual saturation temperature.
A 10KΩ thermistor probe, RTD2, is connected to channel 2 of the ATB. It is used to
measure the test temperature, which is utilized for calculation and control of various humidity
parameters, such as %RH. The computer senses that the probe is connected by monitoring
terminal A2 of TIB.
When the Test Temperature probe is connected, a logic low is transferred from pin 1 to pin 2
of the probe connector, CN2, then to terminal A2 of TIB. When disconnected, terminal A2 is
internally pulled high.
The thermistor temperatures are measured by the Analog to Digital Converter card (A/D)
with a resolution of approximately 0.01 °C/bit. Since the temperatures measured by the A/D
card are based on ideal R-T curves, further calibration to actual temperature values is
performed by the CPU prior to use or display (refer to 4.2.2 for calibration).
A reference resistor of approximately 10KΩ is connected to channel 3 of the ATB, and is
used to compensate for short and long term drift of the temperature measurement electronics
in the A/D circuitry. Deviations from the reference resistor's nominal valu
e are used to
mathematically offset the measured values of the two thermistor probes.
1.5.4.2
Test Pressure Transducer
Reference Drawings 95S39912, 95S39915 & 916
The Test Pressure Transducer TR5 is powered by +15 VDC from the ±15 VDC power supply
PS1. The output, 0-5 VDC for 0 to full scale (typically 50 psiA), is connected to channel 7 of
the ATB for measurement by the A/D card. When connected this transducer continually
monitors the test or barometric pressure. The computer senses that the probe is connected by
monitoring terminal A3 of TIB. When the probe is connected, a logic low is transferred from
pin 1 to pin 2 of the probe connector, CN3, then to terminal A3 of TIB. When disconnected,
terminal A3 is internally pulled high.