1-11
1.5.4 Analog Inputs (STD-A/D)
The temperature, flow, and pressure transducers are measured by the Analog to Digital
Converter (STD-A/D). Each of these is discussed further in the following sections.
1.5.4.1 Temperature Measurement
Reference Drawing 91S25912
Four 10KΩ thermistors are used for temperature readout and control with continuous real
time display by the computer. All are easily removable for calibration.
ATB Channel #
1. Saturation Temperature (RTD0) (indication and control)
0
2. Presaturation Temperature (RTD1) (indication and control)
1
3. Expansion Valve Temperature (RTD2) (indication and control)
2
4. Test Chamber Temperature (RTD3) (indication only)
3
The thermistor temperatures are measured by the STD bus Analog to Digital Converter Board
(STD-A/D) with a ratio of approximately 0.01 °C/bit. Since the temperatures measured by
the STD-A/D board are based on ideal R-T curves, further calibration to actual temperature
values is performed by the STD-CPU prior to use or display (refer to 4.2.2 for calibration).
A reference resistor of approximately 10KΩ is connected to channel 4 of the ATB, and is
used to compensate for short and long-term drift of the temperature measurement electronics
of the STD-A/D circuitry. Deviations from the reference resistor's nominal value are used to
mathematically offset the measured values of the four thermistor probes.
1.5.4.2 Mass Flow Transducer (T1)
Reference Drawing 91S25912
The mass flow control system utilizes a thermal type Mass Flow Transducer. Mass flow rates
through the generator are manually setable from 2 to 20 L/min by keypad entry. The mass
flow rate is displayed on the Liquid Crystal Display (refer to 4.2.5 for calibration).
The flow transducer is powered by +12 VDC from the STD power supply (STD-PS).
Transducer output is 0 to 2.5 VDC for a mass flow rate of 0 to 20 L/min. The output voltage
is connected to channel 7 of the ATB for measurement by the STD-A/D circuit board.
1.5.4.3 50 PSIA Pressure Transducer (T2)
Reference Drawing 91S25912
The 0-50 PSIA Pressure Transducer is of the piezoresistive type. When saturation pressures
are above 50 PSIA this transducer continually monitors chamber pressure. When the
saturation pressure is below 50 PSIA it monitors saturation pressure, but switches to chamber
pressure (barometric) once every five minutes by pressure select solenoid SOL5. The output
voltage is connected to channel 5 of the ATB for measurement by the STD-A/D circuit board.
Transducer output is 0 to 5 VDC for 0 to 50 PSIA (refer to 4.2.3 for calibration).
