RKI Instruments NP-204 - Manual (Page 4)

NP-204 RKI Instruments, Inc.
DESCRIPTION
The RKI Model NP-204 is a rugged battery operated portable instrument for detection of combustible
gas (methane) levels over a wide range.The two ranges, 0-5% by volume and 0-100% by volume, are
displayed on a built in analog meter. The NP-204 utilizes a rubber aspirator bulb for drawing of a test
sample through a hose and probe. Typical uses are to test for gas levels in wells or pipelines at
landfills, natural gas handling plants, or other natural gas delivery systems.
The NP-204 is rugged and compact, and is supplied with a padded vinyl carrying case with shoulder
strap for easy transporting. The NP-204 is easy to adjust and maintain, with user-replaceable
batteries and sensors.
COMPONENTS AND CONTROLS
Case
The NP-204 has a cast aluminum housing that is durable and shock-resistant. The sensors are housed
inside the instrument in a sample chamber. The “lift to turn” protected controls are located on the top
face of the unit, along with the analog meter readout. The carrying case has openings for connection of
the sample inlet tube and aspirator type sample bulb, and the top flap of the case folds back during use
to provide access to the controls and meter. The case includes shoulder strap loops and a strap.
Sensors
The sensors are mounted inside the instrument to a flow chamber that sample flow passes through when
the aspirator bulb is squeezed.
Combustible Gas Sensor (0-5% range, or 0-100% LEL range).
The combustible gas (LEL) sensor consists of a filament set and wires. The filaments are
inserted and attached to the internal flow chamber. The flow chamber brings the sample gas
to the filaments when the aspirator bulb is squeezed. The chamber contains a flame arrestor
to prevent ignition of sampled flammable gases or vapors from exiting the instrument to the
surrounding atmosphere or to the sample line.
Combustible Gas Sensor - Principle of Operation
The LEL sensor is a platinum-coated resistive element, with a sealed, electrically identical
reference element, to compensate for temperature variations and other environmental
factors. The active elements are housed in a sintered stainless steel flame arrestor that
permits the atmosphere to diffuse inward, but prevents flame from passing 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 circuitry
of the NP-204 and converted to a measurement of gas concentration.
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