RKI Instruments NP-204 Manual de usuario

INSTRUCTION MANUAL

Model NP-204

PORTABLE METHANE GAS DETECTOR

The accompanying instrument is sold and serviced in the USA by RKI Instruments, Inc. Please contact

RKI Instrumentsfor any follow up service needs, including questions, warranty issues, repairs, and spare

parts and sensors. Thank you for selecting this fine instrument for your use. With proper care and

maintenance, it will provide you with many.

NP-204 RKI Instruments, Inc.

Table of Contents

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

DESCRIPTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

COMPONENTSANDCONTROLS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Case . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Control Panel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Switches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Buzzers and Earphone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Battery Compartment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Circuit Boards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

OPERATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Alarms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Safety Self-Check . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Emergency Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

CALIBRATION AND MAINTENANCE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Calibration Supplies and Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Calibration Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Sensor Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

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NP-204 RKI Instruments, Inc.

INTRODUCTION

The Model NP-204 is a proven gas detection instrument, in use internationally for detection of natural

gas levels over the full range of concentrations. The NP-204 is compact, easy to use, and offers the

following features:

•Battery operated, the NP-204 can use either alkaline or Ni-Cad cells. Batteries are

easily field replaceable.

•The NP-204 detects natural gas over two ranges : 0-5% volume (or optionally 0-

100% LEL), and 0-100% volume.

•Gas levels displayed on an easy to read analog meter.

•Two sensor types are utilized: Platinum catalytic combustion sensors for low levels, and

thermal conductivity sensors for high level detection.

•Sample drawing by use of an aspirator bulb provides quick, positive sampling. A

probe and hose allow sampling from hard to reach locations.

•Ease of operation. Controls easily accessible for user operation and adjustment.

•Vinyl carrying case with shoulder strap provided.

•Extremely rugged construction assures that the instrument will provide years of

troublefree operation.

WARNING

The NP-204 is designed to detect combustible gas levels which could be life threatening. Users

must follow the instructions and warnings in this manual to assure proper and safe operation of the

instrument. Read and understand instruction manual before use.

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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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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).

LThis 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.

HThis position selects the HIGH range (0-

100% gas by volume range). When the

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NP-204 RKI Instruments, Inc.

switch is in this position the instrument

will detect and display the high range only.

2. ZERO ADJ.

This adjustment is used to Zero the reading for the LOW range. This is generally done

before each use of the instrument. (The zero adjustment for the % gas range only

rarely needs adjustment, so is not externally adjustable).

3. VOLT ADJ.

This adjustment is used to set the voltage for the detectors. When the “RANGE” switch

is in the “Volt Adj.” position, this knob can be turned to bring the meter reading to

match the red “V” line, which sets the detector voltage to the proper level.

4. LIGHTS

LWhen this light is lit, it indicates that the

instrument is turned on and that the LOW

range sensor is energized.

HThis light indicates that the HIGH range

sensor is energized.

5. Meter Mechanical Zero

Located centered directly below the meter, this adjustment is used if necessary to

adjust the meter Mechanical Zero while the power is turned off to the instrument.

Battery Compartment

The NP-204 battery compartment is located on the bottom of the instrument. It has a removable panel

for easy replacement of the batteries. The panel is held in place by one screw located on the bottom of

the instrument.

The NP-204 uses two “D” size cells; alkaline batteries will run the NP-204 for approximately 5

hours, rechargeable nickel-cadmium batteries will run for approximately 4 hours. (Rechargeable

batteries must be removed from the NP-204 and charged separately.)

Circuit Boards

The NP-204 has one circuit board. The circuit board contains the screw connectors for the replaceable

detectors, adjustment potentiometers for High range Zero and Span and for Low range span, and wiring

to the external switches and adjustments. The circuit board is internally mounted directly to the back of

the meter.

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NP-204 RKI Instruments, Inc.

OPERATION

Preparation

Normally the NP-204 requires very little preparation before use. Before using the instrument, follow

the steps listed below:

Start-up

1. Mechanical Zero

Before turning the instrument on, set it on a flat surface and verify that the meter

reading is exactly on zero. If necessary, turn the Mechanical Zero adjustment to bring

the reading to zero. The mechanical zero adjustment is located centered directly

underneath the meter.

2. Battery Check

Turn the range switch to the VOLT ADJ. position. Lift and turn the VOLT ADJ. knob to

bring the meter reading exactly to the red ”Volt Check” line indicated on the meter by

a red “V” and line near the top of the scale. Turning clockwise will increase the voltage

reading.

If you are unable to adjust to the voltage up the red “volt Check” line, replace or

recharge batteries before use. Do not attempt to use the instrument if the

voltage cannot be set high enough.

3. Flow System

Attach the hose to the inlet fitting on the left hand side of the instrument, and the

sample probe to the end of the hose. Assure that the hose and probe fittings are secure.

Close off the inlet to probe with finger, and squeeze aspirator bulb. Bulb should remain

collapsed while probe inlet is sealed off with finger. If not, re-check hose and probe

fittings and re-tighten if necessary.

With probe inlet sampling from fresh air, squeeze aspirator bulb 5 times to flush out

any remaining gas from the instrument and sample line.

4. Zero (Low range)

Turn range selector switch to “L”. Lift and turn the ZERO ADJ. knob to bring the meter

reading to exactly zero. If unable to adjust to zero, filaments may need to be replaced

or sample system may not be fully flushed with fresh air.

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NP-204 RKI Instruments, Inc.

5. Verify Response (Low Range)

To verify detection of combustibles use a controlled source of flammable vapor, for

example a bottle of isopropyl alcohol. For accurate testing of calibration, use a known

calibrating gas source, such as 50% LEL methane. (See calibration section of this

manual for calibrating instructions).

After verifying response, squeeze the aspirator bulb 5 times again to flush fresh air

through the system.

Caution

Do not use gas from a cigarette lighter to test response to combustibles. Do not use a gasoline

container or tank to verify response. Exposing the sensor to uncontrolled high concentrations of

gas in this manner will reduce response and sensor life.

WARNING

If the NP-204 does not respond to these verifications, take it to a known “fresh-air” environment,

Sample the fresh air and Re-Zero the unit. Repeat the “Verify Response” procedure before using the

NP-204 in a potentially hazardous location. Do not attempt to use the instrument unless the

response is verified.

6. Zero (High range)

Turn the range selector switch to the “H” (High) position. Allow a few seconds for

instrument to warm up, and verify the reading is exactly on zero. If it is not on Zero,

adjust the reading to zero using the internal adjustment before using the instrument.

Refer to the “Internal Adjustments” section of this manual for instructions on how to

adjust the High range zero.

7. After the above steps are successfully completed, the instrument is ready to take to the

job site and use. It may be turned off until needed. When using, refer to the Normal

Operation section of this manual below.

Normal Operation

The NP-204 can now be used to check atmospheres for gas levels. Before each sample is taken, the user

should follow the following steps:

1. Turn unit on to Volt Adj., and adjust voltage to the red Volt Check line on the meter,

using the Volt Adjust knob.

2. Turn Range switch to “L” position, and adjust the Zero to be exactly zero using the

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NP-204 RKI Instruments, Inc.

Zero Adj. knob.

3. Generally, when testing an unknown space or sample, it is recommended to have

the range selector switch in the H position. This helps to extend the life of the

Low range sensor by not stressing it with exposures to very high concentrations.

4. Attach hose and probe and check for leaks by holding finger over

probe inlet while squeezing aspirator bulb. Bulb should remain

collapsed if there are no leaks in the system.

5. Insert probe into the space or atmosphere to be tested, and squeeze aspirator bulb

4 to 5 times. Watch meter and observe the maximum reading.

If reading is very low, and range selector switch is on the “H” position, remove probe

from test space and place in fresh air, squeeze bulb 5 times, then repeat steps 2 and 5

above.

6. When test is completed, remove probe from the test space and place in fresh air.

Squeeze bulb 5 times to flush any test vapors out of the system, and turn instrument

off.

WARNING

If repeated tests are to be taken, it is necessary to periodically verify the battery voltage is

properly set (to the red Voltage Check line). If the voltage is not set properly, the instrument may

not provide correct readings.

Alarms

The NP-204 contains no audible or visual alarms.

CALIBRATION AND MAINTENANCE

The NP-204 requires only a few user adjustments. These are the zero and span for the low range and

high range sensors. The Low range span should be checked periodically ( preferably every use for

critical applications). The high range span needs to be checked only when the sensor is replaced.

The sensors need to be replaced if the span cannot be set to read the proper value, or if they cannot be

set to read Zero with the appropriate zero adjustment.

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NP-204 RKI Instruments, Inc.

Calibration Supplies and Equipment

To adjust the NP-204, you will need the following supplies and equipment, some of which is available

in an RKI Calibration Kit:

•Known calibrating sample of 2.5% Volume (50% LEL) natural gas (methane) in air.

•Aflowmeter with a range of 0 - 1.0 lpm (0-2.0 SCFH).

•Acontrol valve for the cylinder, and a tubing length to connect the gas to the NP-204.

•Asource of 100% natural gas or methane.

All the above items except the 100% natural gas source are available from RKI in a test kit. The 100%

natural gas source can be obtained by bleeding off a sample from a gas company, as piped to homes for

stoves or furnaces.

Calibration Procedure

Combustibles Span (Low range span)

1. Take the NP-204 to a non-hazardous environment before calibrating.

2. Loosen the two large screws on the top of the instrument, and separate the top

half of the case from the bottom case. Locate the potentiometer on the PCB marked

“SPAN (L)”

3. Turn the instrument on to “VOLT ADJ.”, and adjust the voltage to the red Volt

Check line near the top of the meter. Turn the Range switch to “L”, allow two

minutes warm-up, and then Zero the reading using the external zero adjustment.

4. Connect the hose and probe onto the instrument, and connect the span gas tubing

to the probe tube.

5. Open the valve connected to the 50% LEL Methane span gas cylinder, and,

verifying flow with flowmeter, flow about 0.5 lpm through instrument.

6. Adjust the SPAN (L) potentiometer if necessary to match the meter reading to

the span gas concentration. Turning the potentiometer clockwise increases the

reading.

Note

The combustible gas sensor is a general hydrocarbon sensor that responds to most flammable vapors

and gases; the response will vary depending upon the substance. See Appendix A for Relative

Response Curves for common gases and vapors, relative to a methane calibration.

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