lecting this fine instrument for your use. With proper care and maintenance,
it will provide you with many years of reliable service. Gas Detection For Life
RKI Instruments, Inc. 1855 Whipple Rd. Hayward, CA Phone (510) 441-5656 (800) 754-5165 Fax (510) 441-5650
World Leader In Gas Detection & Sensor Technology
www.rkiinstruments.com
1.
GENERAL DESCRIPTION
The model GP-204 HAND HELD PORTABLE COMBUSTIBLE GAS DETECTOR is a compact battery
operated portable instrument used for taking an air sample and indicating the presence and concentration of
combustible gas. Samples of the air under test are drawn by means or a rubber aspirator bulb and analyzed
for combustible gas content on a heated platinum filament in a Wheatstone bridge measuring circuit. A
built-in meter indicates combustible gas content in units of explosibility. Power for operation or the
instrument is provided by two size D alkaline or Ni-Cad batteries. A probe and extension hose permit
withdrawal of samples from remote locations and the instrument fits in a compact leather case with over the
shoulder carrying strap. The model GP-204 is suitable and recommended for testing tanks, manholes,
vessels, pressure cylinders, pipelines, and other closed systems or confined spaces to determine presence
or absence of combustible gas. It is a valuable aid for safety of operations whenever combustible gases or
vapors are handled.
2.
OPERATION
Before taking instrument on the job, check battery voltage. To check, put switch in VOLT ADJ position.
Meter should rise to the "CHECK" position near top of scale. Lift and turn VOLT ADJ control clockwise to
determine maximum voltage setting. If it cannot be set beyond mark, batteries need recharging or
replacement. Do not attempt to use instrument at all if voltage reading cannot be set up to mark.
If voltage is satisfactory, continue with the next steps of preliminary adjustment as follows:
A. Confirm operation of pilot light/meter illuminating lamp.
B. With sample inlet in fresh air, squeeze bulb several times to flush out any remaining gas.
C. Check zero setting by turning switch to ON position. Meter should read close to zero. Lift and
turn ZERO knob to bring reading to exactly 0.
D. Couple sampling hose to instrument inlet on left hand end, and connect probe to end of hose.
Hold finger over probe to block flow and squeeze bulb. Bulb should remain squeezed while finger
blocks inlet.
E. Admit a sample of some combustible gas to end of probe, and confirm that meter rises upscale.
Instrument is adjusted and ready to use. lt may be turned off and carried to the job.
To make a gas test, proceed as follows:
1) Turn to VOLT ADJ. position, adjust voltage if necessary, then turn to ON.
2) Hold probe within space to be tested. Squeeze bulb several times while watching meter, and
observe maximum reading.
3) After completion of test, remove probe from test space, flush with fresh air for 4 or 5 squeezes of
bulb, and turn off. Gas Detection For Life
RKI Instruments, Inc. 1855 Whipple Rd. Hayward, CA Phone (510) 441-5656 (800) 754-5165 Fax (510) 441-5650
World Leader In Gas Detection & Sensor Technology
www.rkiinstruments.com
3. INTERPRETATION
Meter readings are taken on a scale graduated 0 - 100% LEL. The abbreviation L.E.L. stands for Lower
Explosive Limit, and represents the lowest concentration, which can be ignited by a source of ignition, hence
the lowest concentration which can produce an explosion. This quantity is also spoken of as the Lower
Flammable Limit (L. F. L.).
The model GP-204 is calibrated before shipment to read directly in percent L.E.L. of methane in air, based
on the known L.E.L. for methane of 5.0% by volume. That is, a 5.0% by volume will produce a reading of
100%, and smaller concentrations will read in proportion.
Other combustible gases will read in approximate terms of explosibility. For maximum accuracy, the unit
should be calibrated to the gas intended to be detected.
Concentrations may also be interpreted in terms of volume percent by multiplying the percent L.E.L. in the
sample (determined from the meter reading and the curve) by the published figure for L.E.L. in volume
percent. as noted on the curve. The maximum concentration allowable in a space where men are working
or where welding operations are carried out is primarily a matter of local regulation and of judgement based
on knowledge of conditions. A maximum reading of 10% or 20% is usually allowed. If 20% is selected, this
is often spoken of as a factor safety of 5, as the concentration must be increased five times before explosive
conditions are reached.
4. MAINTENANCE
A. Calibration and Adjustment
In addition to the normal operating controls found on the top panel, the following auxiliary controls are
available.
Calibration Potentiometer
This adjustment is used to set the meter reading to the desired level, while sampling a known
concentration or combustible gas. To access this adjustment, the top plate must be removed by
taking out the screws in each corner. The calibration potentiometer is a slotted-shaft control located
above the right upper corner of meter. Turn clockwise to increase meter reading.
B. Sensor replacement
The sensor assembly consists of an active filament and a similar but enclosed reference filament,
potted into a flame arrestor. It should be replaced if zero cannot be set within the range of the ZERO
ADJ., or if reading cannot be set high enough on a calibration gas sample using calibration
potentiometer. To replace:
1) Loosen the two panel hold-down screws. Remove and invert top panel.
2) With switch off, loosen (do not remove) the three screws holding the terminals for red,
black and white wires. Pull wires from terminals.
3) Remove the four screws holding the rectangular sensor plate. Pull out sensor and replace
with the new one in same position. Check that the gasket is in place on chamber before
installation of new sensor. Install wires on terminals as before.
4) Turn instrument on and adjust zero. Gas Detection For Life
RKI Instruments, Inc. 1855 Whipple Rd. Hayward, CA Phone (510) 441-5656 (800) 754-5165 Fax (510) 441-5650
World Leader In Gas Detection & Sensor Technology
www.rkiinstruments.com
5) Recalibrate the new sensor using span gas.
C. Batteries
The model GP-204 is furnished with two standard size D batteries, either alkaline or Ni-Cad. These
batteries will give 4 to 5 hours of operating life before replacement or recharge.
When meter cannot be set as high as the "CHECK" line with switch in VOLT ADJ. position and VOLT
ADJ. knob all the way clockwise. batteries require replacement or recharging.
To replace batteries, remove instrument from leather case, and loosen the coin slotted captive screw
found in center of bottom plate. Remove plate, exposing batteries in their spring contact holders.
Pull old batteries out, and install new ones in the same position, observing polarity as marked on
holder.
D. Sample system
1) Hose
The hose used is teflon-lined, synthetic rubber jacketed, and immune to absorption or attack
by any combustible vapors or solvents. Keep hose clean and be sure that couplings make air
tight contact, checking occasionally by holding finger over hose inlet. Bulb should remain
flattened after squeezing if there is no leak. Extension hoses in various lengths are available.
2) Flame arrestor / Sensor assembly
The active filament is installed within a sintered bronze porous metal cup, which acts as a
flame arrestor to retain explosions that may occur when sampling explosive gas and air
mixtures. The flame arrestor is part of the sensor assembly, which may be removed by taking
out the four screws that hold the sensor plate. If the flame arrestor is dusty, wet, oily or
corroded, it must replaced by replacing the sensor assembly. Before reinstalling sensor in
instrument, be sure that the reaction chamber cavity and incoming lines are clean and dry.
E. Meter lamp
The meter lamp is on whenever the instrument is on, and provides illumination to permit meter
reading in dark places. If lamp fails, it should be replaced as follows:
Remove four screws holding top plate to the top panel. Take off top plate exposing lamp. Loosen
set screws, which lock lamp wires to terminals, and pull lamp out. Install new lamp in the same
position.
5. PRECAUTIONS AND NOTES ON OPERATION
A. Heated samples
When sampling spaces such as hot tanks that are warmer than the instrument, remember that
condensation can occur as the sample passes through the cool sample line. Water vapor condensed
in this way can block the flow system and corrode the flame arrestor. A water trap can be used to
control this, and is available as an accessory.
If heated hydrocarbon vapors of the heavier hydrocarbons (flash point 90° F or above) are present,
they may also condense in the sample line and fail to reached the filament. Thus an erroneous low
reading may be obtained.
B. Element poisoning Gas Detection For Life
RKI Instruments, Inc. 1855 Whipple Rd. Hayward, CA Phone (510) 441-5656 (800) 754-5165 Fax (510) 441-5650
World Leader In Gas Detection & Sensor Technology
www.rkiinstruments.com
Certain substances have the property of desensitizing the catalytic surface of the platinum filament.
These substances are termed catalyst poisons and can result in reduced sensitivity or in failure to
give a reading on samples containing combustible gas. The most commonly encountered catalyst
poisons are silicone vapors, and samples containing such vapors