Four-Gas Diffusion Personal Monitor — Training Guide

No pump, no hose. This instrument measures the air touching its own sensor face, which is the air you are standing in. Everything below follows from that.

About this simulator. A generic training model of a four-gas diffusion personal monitor. Inspired by instruments of this class; not affiliated with, endorsed by, or a reproduction of any manufacturer's product. Alarm setpoints, ranges and response times are representative of the class. Always verify the setpoints and procedure of the instrument you are issued against its own manual before relying on it.

1. Display requirements

Minimum 980 × 600 px. Recommended 1920 × 1080 (16:9) or larger.

2. Diffusion versus aspirated — the pairing

This simulator has a sibling, the Pump-Aspirated Confined Space Gas Monitor. They are a matched pair and are best taught together, because the difference between them is a safety decision, not a product preference.

	Aspirated	Diffusion (this one)
How gas reaches the sensor	A pump pulls it down a hose	It arrives on its own, by diffusion
Can it test a space you are not in?	Yes — lower the probe from outside	No — it only knows where it is
The delay you must respect	Hose transit time, then sensor T90	Sensor T90, plus whatever your clothing adds
Primary role	Pre-entry survey of a space	Personal exposure monitoring and evacuation alarm

The rule that matters. A diffusion monitor cannot clear a confined space for entry. To obtain a reading from inside the space, the instrument — and therefore the person wearing it — has to be inside the space. The reading arrives only once you are already exposed. Pre-entry testing requires an aspirated instrument that samples from a safe position.

3. The instrument

3.1 Channels and alarm points

Channel	Range	Resolution	1st alarm	2nd alarm	TWA	STEL
Combustible (CH4)	0–100 %LEL	1 %LEL	10 %LEL	50 %LEL	—	—
Oxygen (O₂)	0–25 vol%	0.1 vol%	19.5 vol% low	23.5 vol% high	—	—
Carbon monoxide	0–150 ppm	1 ppm	25 ppm	50 ppm	25 ppm	200 ppm
Hydrogen sulfide	0–30 ppm	0.5 ppm	5.0 ppm	30.0 ppm	10.0 ppm	15.0 ppm

3.2 The three switches

Switch	Short press	Hold
POWER / ENTER	Confirm a menu item (starts a bump test)	3 s — switch on or off
MODE	Step through PEAK / STEL / TWA / ALARM-P / BUMP / ID	—
RESET	Reset a latched alarm	Run the fresh air adjustment

The status strip carries a pilot indicator, a carry state flag reading WORN / COVERED / NOT WORN, the battery icon and the clock. Where the aspirated instrument shows a pump indicator, this one shows whether it can actually breathe — the equivalent thing a wearer must keep an eye on.

4. Carry position — the central skill

A diffusion sensor needs air to reach it. Obstruct that and the instrument still displays a confident number; it is just wrong, and late.

Carried	Reaching the sensor	Response	Verdict
Breathing zone (lapel/collar)	100 %	Normal	Correct
Clipped at the belt	92 %	1.4× slower	Acceptable
Chest pocket, partly covered	55 %	3.2× slower	Restricted
In a toolbag	18 %	7× slower	Useless
Left outside the work area	0 %	—	Monitoring nothing

Try this. Run the toolbag scenario and stand in the engine room. The true CO is 31 ppm. Bagged, the instrument settles near 6 ppm — comfortably below the 25 ppm alarm. Move it to the breathing zone and watch it climb past the alarm point. Nothing about the atmosphere changed; only where you were carrying the instrument.

5. Procedure

5.1 Before the shift

Check the battery will last the whole shift.
Inspect the sensor face — clean and unobstructed.
Hold POWER three seconds and let the warm-up finish. Watch the setpoint scroll; that is how you confirm the alarm points are what you expect.
In clean air, hold RESET to run the fresh air adjustment.
Apply test gas and run a bump test from the MODE menu.
Clip it in the breathing zone.

5.2 Bump test

A bump test proves each channel responds to gas. It is not a calibration and adjusts nothing — which is precisely why it is the check you do daily while calibration is periodic. A channel passes if it reaches 50–150 % of the applied concentration; oxygen must actually see the depletion.

Why it is not optional. A dead electrochemical sensor does not announce itself. It reads a calm, plausible zero — exactly what a safe atmosphere reads. The only thing that tells the two apart is presenting gas and watching for a response. Inject the CO sensor unresponsive fault and bump it: the display looks perfectly normal right up until the test fails.

5.3 During the shift

Keep it in the breathing zone and move through the work areas. Alarms latch: once a setpoint is reached the alarm stays up until you have genuinely left the gas. RESET pressed while still in the atmosphere is refused, and the simulator says so.

5.4 After the work — review the exposure

This is the part most often skipped and the reason the instrument exists.

PEAK — the worst instantaneous reading (the lowest for oxygen).
STEL — highest rolling 15-minute mean. Short excursions show up here.
TWA — dose averaged over an 8-hour reference period. It climbs and never falls, because the dose has already been taken.

The exposure that never alarms. A shift spent at 20 ppm CO never trips the 25 ppm alarm — and still breaches the 25 ppm TWA limit. No instantaneous reading would ever have told you. Only the TWA does.

6. Training scenarios

Pre-use check and bump test beginner

Objective: prove the instrument before work. Expected result: fresh air adjust accepted, all four channels bump-pass, monitor worn in the breathing zone. Pass: 70. Par: 10 min.

A shift of personal exposure intermediate

Objective: work through the engine room (31 ppm CO) and hot work area (78 ppm CO), then review peak, STEL and TWA. Expected result: the STEL climbs sharply in the hot work area and the TWA keeps its value after you leave. Pass: 75. Par: 15 min.

The monitor in the toolbag intermediate

Objective: the same work carried badly. Expected result: roughly a fifth of the true reading, arriving far too slowly; moving it to the breathing zone reveals the real atmosphere. Pass: 80. Par: 12 min.

Why this is not a pre-entry instrument advanced

Objective: an unventilated bilge at 17.9 vol% oxygen and 26 ppm H₂S. Getting that reading requires standing in it. Expected result: recognise the alarm, leave, and answer the question in the control bar correctly — this instrument monitors your exposure; it does not clear a space for entry. Pass: 85. Par: 10 min.

7. Instructor controls

Ctrl+Shift+F opens the fault panel: H₂S sensor abnormality, unresponsive CO sensor (fails bump test), CO zero drift (+18 ppm), low battery, and a fouled sensor face that halves the response.

8. Keyboard shortcuts

Key	Action
F	Fullscreen
Space	Pause / resume
Esc	Close a modal
?	Shortcut help
Ctrl+Shift+F	Fault injection panel
Ctrl+Shift+T	Trend plots
Ctrl+Shift+S	Score display

9. Scoring

Event	Change
Bump test completed before use	+5
Phase checklist completed	+5
Correctly identifying the instrument's role	+15
Alarm left unacknowledged over 30 s	−2
Claiming a diffusion monitor can clear a space for entry	−25

Grades: A 90+, B 75–89, C 60–74, D below 60.

10. Troubleshooting

Symptom	Cause	Action
Reading seems low for the area you are in	Sensor obstructed by carry position	Check the carry flag on the LCD; move it to the breathing zone
Reading changes very slowly	Covered sensor face, or a fouled face	Uncover it; inspect and clean the sensor face
`BUMP FAIL`	A channel did not respond to test gas	Do not use the instrument. It cannot warn you on that channel
`FAIL CAL`	Fresh air adjustment attempted in contaminated air	Move to genuinely clean air and repeat
RESET does nothing	The gas is still above the setpoint	Leave the area first — this is correct behaviour, not a fault
Channel shows `---`	Sensor failure	That gas is not being detected at all
Reading shows `∩∩∩`	Above the service range	Leave immediately; the true value is unknown and higher
TWA will not go down	Working as designed	A dose already taken cannot be undone. Only a new reference period resets it