Reducing risk with

Exposure Monitoring

Equipment

What we know

What we don’t know

What we must assume

What we must never assume

Components of risk assessment

What is the conclusion of risk assessment?

The definition of acceptable risk

PID and LEL monitors

Do they keep you safe?

These machines are DUMB!

The value displayed is a

TOTAL concentration of

all DETECTED

compounds AS IF (selected compound)

Indicative at best

Calibrate to Chocolate: Display 500 Chocolate

Display Vanilla (response factor 8): 4000 Vanilla

Display Cake (response factor 0.002): 1 Cake

300 Chocolate (RF 1)

200 Flour (RF 0.25)

200 Sugar (RF 0.47)

6 Cocoa (RF 1.16)

150 Sunflower oil (RF 0.1)

300 Soured cream (RF 0.08)

2 Egg (RF N/A)

1 Vanilla Extract (RF 8)

PID : Photo Ionisation Detector

Broad spectrum

Sensitive

Non destructive

Low cost

Cannot see smaller/lighter compounds e.g. methane

UV Lamp ionises the gas sample. The free electrons are then

counted and this value is converted and displayed as gas concentration (typically in ppm).

LEL and close space monitors

Lower Explosive Limit

Catalytic diffusion sensor burns

available volatiles and converts

the energy value to display %LEL

Broad spectrum

Low cost

Can detect smaller/lighter compounds (methane)

O2

H2S

CO

LEL

%VOL - %LEL - PPM EXAMPLE: Pentane

1%VOL―75%LEL―10,000ppm

1%LEL―0.13%VOL―133ppm

1ppm―0.0075%LEL―0.0001%VOL

Typical resolution for LEL monitors 1.0%LEL

Typical resolution for PID 1ppm

Best resolution for PID 1PPB!

Correction factors: Calibration gas and Action alarms

Correction factors are not absolute

Sensor type/manufacturer

Calibration gas:

Pentane/Methane?

Isobutylene?

Are alarms preset?

Is this arbitrary?

Other factors

Weather conditions

Sensor inhibitors: Methane, Nitrogen, Carbon monoxide

Sensor poisoning: Silicones

Sensor reaction time: Pentane/methane

Personal proximity

ZERO

Nominal drift

Who do you trust?

Warm up period

Ambient or absolute?

Carbon filter

Bump testing and Calibration Ensure adequate warm up period

Pump stall test

Bump test first

Potential issues: No gas,

blocked regulator, cold machine,

contaminated filters, incorrect

gas selection

When to calibrate?

Potential issues: as above

Gain?

Are these machines DUMB?

It is not the tools we use

which make us good, but

rather how we employ

them.

These tools help you to

redefine the risk and

revaluate your own

acceptable risk