RKI Instruments NP-204 - Manual (Page 5)

NP-204 RKI Instruments, Inc.
100% Volume Gas Sensor
The Volume Percent Gas sensor also consists of a filament set and wires. The filaments are
similar to the LEL sensors in that they are inserted and attached to the internal, flame
arrestor protected flow chamber. The flow chamber brings the sample gas to the filaments
when the aspirator bulb is squeezed.
100% Volume gas sensor - Principle of Operation
The 100% Volume gas sensor is a Thermal Conductivity (TC) type of sensor. It consists of
two electrically identical TC elements. The active element is placed in the sample chamber
in the gas stream, and the reference element is placed outside of the gas stream. The two
elements form half of a balanced Wheatstone Bridge. When voltage is applied to the elements,
the elements heat up. “Thermal conductivity” is a measurement of the rate a gas can carry
heat away from a hot object. High levels of natural gas (methane) in the sample stream
carry heat away from the hot reference element in the gas stream faster than heat transfers
from the reference element which is surrounded by air. This causes a difference in
temperature between the active and reference elements, which causes a difference in the
electrical resistance of the elements. This causes an imbalance in the Wheatstone Bridge
circuit. The imbalance is measured by the circuitry of the NP-204 and converted to a
measurement of the % Gas concentration.
Control Panel
The control panel is at the top of the instrument for easy access when the NP-204 is carried. It
contains the display, range lights, and switches that control the functions of the NP-204. The switches
are “lift to turn” type knobs, to reduce the possibility of accidental turning.
Switches
1 . RANGE SWITCH
The range switch has the following selections:
OFF This position turns the power off to the
instrument.
VOLT ADJ. This position is used to set the proper
voltage for the detector elements (used in
conjunction with the “VOLT ADJ.”
potentiometer).
L This position selects the LOW range (0-5%
gas by volume or 0-100% LEL ranges).
When the switch is in the “L” position the
instrument will detect and display the low
range only.
H This position selects the HIGH range (0-
100% gas by volume range). When the
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