RKI Instruments GD-K8A - Manual (Page 8)

4 • Model GD-K8A Diffusion Detector Head
Sensor Bracket
The painted steel sensor bracket is used to install the sensor and amplifier into the GD-
K8A housing. It is secured to the housing with two screws. The mounting holes in the
bracket are slotted allowing removal of the bracket for sensor or amplifier replacement
without removing the two mounting screws.
Sensor
The sensor is mounted in the sensor bracket with two screws through the back of the
sensor bracket. The sensor face protrudes slightly through a gasketed opening in the
housing door to sample the ambient air by diffusion. Through a series of electrical and
chemical reactions, the sensor produces an electrical current that is proportional to the
detection range of the target gas. Electrodes within the sensor are surrounded by liquid
electrolyte. The chemical and electrical reactions are facilitated by the electrolyte.
Amplifier
The amplifier is to the right of the sensor (see Figure 2). The amplifier converts the output
of the sensor to a 4 to 20 mA signal that is proportional to the detection range of the GD-
K8A and transmits the signal to a controller. The amplifier includes the span pot, zero pot,
gain pot, and output check pins.
Span pot
The span pot is near the top of the amplifier. Use the span pot to adjust the GD-K8A’s
response output during the calibration procedure.
Zero pot
The zero pot is below the span pot. Use the zero pot to adjust the GD-K8A’s fresh air
output during start-up and calibration procedures.
Gain pot
The gain pot is below the zero pot. It is used to make coarse span adjustments. The gain
pot is factory set and for adjustment by a field service technician only
.
Another factory adjust pot may be located below the gain pot. Do not adjust this pot.
Output check pins
Two output check pins, marked 1 (+) and 2 (-), are below the pots. A 4 - 20 mA signal can
be read at the pins using a milliampmeter. Use the output check pins to read the signal
output of the GD-K8A during the start-up and calibration procedures.
Auto Sensor Keeper (ASK)
The ASK is in the top part of the housing. It consists of a battery compartment which
holds two AA size alkaline batteries mounted to a circuit board which maintains a bias
voltage on the sensor when the GD-K8A is not receiving incoming power, such as during
shipment or storage. If the GD-K8A is off power for an extended period and the batteries
in the ASK are dead, the sensor operation will be affected when the GD-K8A is started up.
See Storing the GD-K8A in the Maintenance section for battery replacement
recommendations.