Hansen Technologies HGD-EC-NH3-100 Specifications, Applications, Service Instructions & Parts page 6

SENSOR ELEMENT
Typical sensor element life is based on normal operating
conditions. Exposure to the target gas will shorten
these times. The typical sensor element life of the
electrochemical gas sensor element is 2 to 3 years;
typical sensor life for semiconductor element is 4 to 6
years; and typical sensor life for infrared element is 5
to 7 years. Recalibration should be performed at least
every 6 months. Calibration can be performed locally
at detector installation site. If desired results cannot be
achieved or if signal reads <1.2mA >2.1mA, steady 3mA,
replacement of the sensor element may be required.
ELECTROCHEMICAL SENSOR ELEMENT
Electrochemical sensor element measure the partial
pressure of gases under atmospheric conditions. The
monitored ambient air diffuses through a membrane
into the liquid electrolyte in the sensor. The electrolyte
contains a measuring electrode, a counter-electrode and
a reference electrode. An electronic "potentiostat" circuit
ensures a constant electrical voltage between measuring
electrode and reference electrode. Voltage, electrolyte,
and electrode material are selected to suit the gas being
monitored so that it is transformed electrochemically on
the measuring electrode and a current flows through the
sensor element. This current is proportional to the gas
concentration. At the same time, oxygen from the ambient
air reacts at the counter electrode electrochemically.
The current flowing through the sensor is amplified
electronically, digitized and corrected for several
parameters (e.g., the ambient temperature).
SEMICONDUCTOR SENSOR ELEMENT
Semiconductor or metallic oxide sensor element (MOSs)
are among the most versatile of all broad-range sensor
element. They can be used to detect a variety of gases
and vapors in low ppm or even combustible ranges.
The sensor element is made up of a mixture of metallic
oxides. They are heated to a temperature between 150°
and 300° C depending on the gas(es) to be detected.
The temperature of operation as well as the "recipe" of
mixed oxides determines the sensor element selectivity
to various toxic gases, vapors, and refrigerants. Electrical
conductivity greatly increases as soon as a diffusion
process allows the gas or vapor molecules to come in
contact with the sensor element surface. Water vapor,
high ambient humidity, temperature fluctuations, and low
oxygen levels can result in higher readings.
INFRARED SENSOR ELEMENT
The infrared (IR) gas sensor element is designed to
measure the concentration of combustible gases and
vapors in the ambient air. The principle is based on
the concentration-dependent absorption of infrared
radiation in measured gases.
The monitored ambient air diffuses through a sintered
metal material into the enclosure of an optical "bench".
The broadband light emitted by an IR source passes
through the gas in the optical bench and is reflected
by the walls from where it is directed towards a dual-
element detector. One channel of the detector measures
the gas-dependent light transmission, while the other
channel is used as a reference. The ratio between
measurement and reference signal is used to determine
the gas concentration. Internal electronics and software
calculate the concentration and produce an output signal
A100h
JANUARY 2020
IMPORTANT: Certain substances in the atmosphere to
be monitored may impair the sensitivity of the sensors.
Such substances include, but are not limited to:
• Polymerizing substances such as acrylonitrile,
butadiene and styrene.
• Corrosive compounds such as halogenated
hydrocarbons (releasing halogens such as
bromine, chlorine or fluorine when oxidized)
and halogen hydride acids as well as acidic
gaseous compounds such as sulfur dioxide and
nitrogen oxides.
• Catalyst poisons such as sulfurous and
phosphorous compounds, silicon compounds
(especially silicones), and metal-organic vapors.
LED LOGIC
The Hansen gas detectors provide external indication
of their current operational state via audible visual
feedbackand also provide relays outputs. Visual
indication of the instrument status is provided by
a single tri-color LED (Green I Red I Orange) as
indicated below:
WIRING GAS DETECTORS
Install the Hansen Gas Detector in an area where
operating personnel can easily monitor it. Refer to
Gas Detector Location on page 3 for suggestions on
proper placement of Remote Detectors.
Use two-core shielded pair, 16 to 20 AWG wire to connect
power to the gas detector. Connect the power input
wires to terminal 4. Connect the analog signal wire to
terminal 6 and/or digital sign (MODBUS) to terminal 5. The
analog 4-20mA output signal is best for long distances
to the monitor and where electrical noise is a problem.
The voltage output is for short distances within 10 feet
(3 m) of the monitor. The maximum 4-20mA cable length
is 1000 ft (300 m).
Under no circumstances should the gas detector low
voltage signal wires be in a common conduit, tray or
wiring panel with power wiring over 48 volts. Do not run
wires near variable frequency drive (VFD) equipment.
Hansen recommends backup of gas detection system
with an uninterruptable power supply to provide battery
backup in the event of a power failure.
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