RKI Instruments GX-82 - Manual (Page 6)

2 • Components and Controls Model GX-82 Instruction Manual
Components and Controls
This section describes the GX-82’s components and user controls.
Case
The GX-82 has a rigid plastic case that is durable and shock-resistant. The sensors are
housed in a metal diffusion head at the bottom of the GX-82. The touch-pad control panel
and display are located on the top. The carrying case has openings for the diffusion grill,
buzzer, remote alarm, earphone jack, and control panel. The case includes a belt loop and
shoulder strap loops.
Sensors
The sensors are mounted in the diffusion head at the bottom of the GX-82. The diffusion
grill protects the sensors. The grill easily snaps off the instrument case so you can
conveniently replace the sensors.
Combustible gas sensor
The combustible gas (LEL) sensor is contained in a metal cylindrical shell, with a bayonet-
pin base for easy replacement. The perforations at the end of the shell allow atmosphere to
diffuse into the sensor.
Combustible gas sensor - principle of operation
The LEL sensor is a platinum-coated resistive element, with a non-catalytic, electrically
identical reference element, to compensate for temperature variations and other
environmental factors. The elements are housed in a sintered stainless steel flame arrestor
that permits the atmosphere to diffuse inward but prevents flame to pass outward when
an explosive atmosphere is encountered. The two elements form half of a balanced
Wheatstone Bridge. When voltage is applied to the elements, combustible gas in the
atmosphere catalyzes on the platinum coating, raising the temperature and changing the
resistance of the element, causing an imbalance in the Wheatstone Bridge. The imbalance
is measured by the GX-82’s circuitry and converted to a measurement of gas
concentration.
Oxygen sensor
The oxygen (O
2
) sensor is contained in a cylindrical metal shell, with a bayonet pin base
for easy replacement. The shell forms one electrical contact, and a button on the base is the
other contact. The perforations at the end of the shell allow atmosphere to diffuse into the
sensor.
Oxygen sensor - principle of operation
The O
2
sensor is an electrochemical cell. It consists of gold and lead electrodes in an
alkaline electrolyte. A fluorocarbon membrane covers the cell and allows atmosphere to
diffuse into the electrolyte at a rate proportional to the partial pressure of oxygen. The
oxygen reacts in the cell, producing a current proportional to the concentration of oxygen.
The current develops a voltage across a temperature-compensating thermistor/resistor
network. The voltage is measured by the circuitry of the GX-82 and converted to a
measurement of gas concentration.