PGC, Inc 500-1000 CFM Air Handler May 2006
Appendix A A-8
The position of the air bypass damper is determined by the PLC based on the air output
percentage. The input to the damper actuator is a frequency modulated (FM) square-wave signal.
The damper will be positioned based on the frequency of the FM signal. A yellow LED on the
PLC indicates the duty cycle. Zero percent (0%) air heat output will produce a 0.59 second On
time pulse, which instructs the damper to move to the full spray position. One hundred percent
(100%) air heat output will produce a 2.93 second On time pulse, which instructs the damper to
move to the full bypass position. An On time between 0.59 seconds and 2.93 seconds will
produce a proportional response in the damper position.
The water temperature is measured using a positive-coefficient 100Ω Platinum RTD temperature
sensor. This sensor consists of a very thin Platinum wire that is wound around a ceramic core. As
the temperature of this wire increases, the resistance of the Platinum element increases linearly.
This resistance is placed in a bridge network to produce a linear voltage proportional to the
change in temperature. This voltage is then converted to a digital value that can be used by the
PLC by and the A/D converter.
The air temperature is also measured using a 100Ω RTD located in the HygroClip T/Rh
transmitter. A circuit in the HygroClip converts this RTD input to a digital signal that is
transmitted to the PLC using a single-wire serial interface. The PLC receives this digital signal
and converts it to a digital value that the PLC can use in its calculations.
The HygroClip also converts the RTD input into a 0-1VDC analog signal that is scaled so that
0.1VDC = 1°C. This analog signal is applied to an amplifier built into the HygroClip connection
cable, which converts the 0-1VDC signal into a 0-5VDC signal (0-5VDC is required if the analog
signal is transmitted more than 10 feet). The 0-5VDC amplifier can be used to scale the analog
output for different ranges; the most common range is (0-5VDC) = (0°C to 100°C). However,
amplifiers are available for (-30°C to +70°C) and (-40°C to +60°C). This 0-5VDC analog
amplifier is not used by the chamber controller, and is only required when an analog device (such
as a chart recorder) is connected to the system. This analog output is available on the chart
recorder connector of the PLC.
The relative humidity is measured using a thin-film polymer capacitive element that changes
capacitance as moisture is absorbed or given off. The HygroClip T/Rh transmitter will convert
this change in capacitance into a digital value. This digital value is transmitted to the PLC in the
same single-line serial interface as the air temperature measurement. The PLC will decode the
digital value and convert it to a digital value that can be used by the PLC. The HygroClip T/Rh
transmitter will also convert the measured Rh to a 0-1VDC analog output. The same analog
amplifier that is used in the analog air temperature output will convert the 0-1VDC signal to a 0-
5VDC signal. In all instances, the Rh output will be scaled so that 0-5VDC will equal 0-100%
Rh. This analog output is also available at the chart recorder connection on the PLC.
A separate voltage to current converter is installed in the air handler electrical compartment for
both the air temperature and relative humidity. This device will convert the 0-5VDC analog
signal from the HygroClip to 4-20mA. The output will be scaled so that 4-20mA corresponds to
the same temperature or Rh range as the analog signal from the HygroClip (typically this will be
4-20mA = 0-100°C or 0-100% Rh). The 4-20mA signal is provided as an interface to a customer
supplied monitoring device. This converter is equipped with a zero and span calibration
potentiometer that can be field adjusted in order to match the 4-20mA output with the NIST
traceable temperature and Rh output from the HygroClip.
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