Part 2 Direct Reading Instrumentation

Direct Reading Instruments Many different instruments Many different operating principles including: –Electrochemical –Photoionisation –Flame ionisation –Chemiluminescence –Colorimetric –Heat of combustion –Gas chromatography Many different gases & vapour From relatively simple to complex

Uses of Direct Reading Instruments Where immediate data is needed Personal exposure monitoring Help develop comprehensive evaluation programs Evaluate effectiveness of controls Emergency response Confined spaces

Calculation of results Diffusion sampling: Conc (mg/m 3) = W (µg) x A r x t where W = contaminant weight (µg) A calculation constant = 1000 / Sampling rate r = recovery coefficient t = sampling time in minutes Conc (ppm) = W (µg) x B r x t where W = contaminant weight (µg) B = calculation constant = 1000 x / Sampling rate x mol wt r = recovery coefficient t = sampling time in minutes

Uses of Direct Reading Instruments (cont) For difficult to sample chemicals Multi sensors Multi alarms Stationary installations Fit testing of respirators Video monitoring

Limitations Often costly to purchase Need for frequent and regular calibration Lack of specificity Effect of interferences Cross sensitivity Need for intrinsically safe instruments in many places Battery life Sensors –Finite life, poisoning, lack of range

Advantages Direct reading Continuous operation Multi alarms Multi sensors TWA, STEL & Peaks Data logging

Other Limitations Catalytic combustion detectors –React with other flammable gases –Poisoned by Silicones Phosphate esters Fluorocarbons

Single Gas Monitor Interchangeable sensors including: O 2, CO, H 2 S, H 2, SO 2, NO 2, HCN Cl 2, ClO 2, PH 3 STEL, TWA, peak Alarm Data logging Source: Industrial Scientific Inc – reproduced with permission

Multigas Monitor 1 – 6 gases Interchangeable sensors: LEL, CH 4, CO, H 2 S, O 2, SO 2, Cl 2, NO, ClO 2, NH 3, H 2, HCl, PH 3 STEL, TWA, peak Alarm Data logging

Gas Badges Two year maintenance free single gas monitor Sensors include CO, H 2 S, O 2 and SO 2 Turn them on & let them run out Alarms Some data logging ability Source: Industrial Scientific Inc – reproduced with permission

Photo Ionisation Detectors (PID) Dependent on lamp ionisation potential Typically non specific VOCs or total hydrocarbons – Some specific eg benzene, NH 3, Cl 2 Not for CH 4 or ethane Affected by humidity, dust, other factors Source: Airmet Scientific-reproduced with permission

Flame Ionisation Monitor Similar to, PID but flame Non specific, broad range Less sensitive to humidity & other contaminants Poor response to some gases Needs hydrogen (hazard) Source: Airmet Scientific-reproduced with permission

Portable Gas Chromatograph –Highly selective –Range depends on type of detector used –Complex instrument requiring extensive operator training –Non continuous monitoring Source: Airmet Scientific-reproduced with permission

Infra-red Analyser Organic vapours Specific Portable Expensive

Mercury Vapour Detectors UV –Interferences: Ozone Some organic solvents Gold Film –High cost –Gold film needs regular cleaning

Maintenance & Calibration Source: Industrial Scientific Inc – reproduced with permission

Guidelines for Using Gas Detection Equipment Bump or challenge test –Daily before use, known concentration of test gas to ensure sensors working correctly Calibration –Full instrument calibration, certified concentration of gas(es), regularly to ensure accuracy & documented Maintenance –Regular services provides reassurance instruments repaired professionally & calibrated & documented

Typical Basic Instrument Checks Physical appearance Ensure instrument is within calibration period Turn instrument on and check battery level Zero the instrument Bump test (functionality test) instrument Clear the peaks

Standard Gas Atmospheres Primary Gas Standards Are prepared from high purity 5.0 Gases ( %) or 6.0 gases ( %) by weighing them into a gas cylinder of known size Secondary Gas Standards Are prepared volumetrically from these using gas mixing pumps or mass flow controllers Source: University of Wollongong

Intrinsic Safety (cont) IECEx Standards Equipment for use in explosive or Ex areas eg –Underground coal mines –Oil refineries –Petrol stations –Chemical processing plants –Gas pipelines –Grain handling –Sewerage treatment plants

Intrinsic Safety (cont) Gases, vapours, mists DustsExplosive atmosphere is present Zone 0Zone 20Most of the time Zone 1Zone 21Some time Zone 2Zone 22Seldom or short term Classification of zones Source: TestSafe – reproduced with permission

Intrinsic Safety (cont) Group 1Equipment used underground methane & coal dust Group IIEquipment used in other (above ground) hazardous areas IIA - least readily ignited gases eg propane & benzene IIB – more readily ignited gases eg ethylene & diethyl ether IIC – most readily ignited gases eg hydrogen and acetylene Gas or Explosive Groups

Intrinsic Safety (cont) Temperature classes Group ISurfaces exposed to dust less than 150°C Sealed against dust ingress less than 450°C Group II Temp ClassMax permissible surface temp °C T1450 T2300 T3200 T4135 T5100 T685 Source: TestSafe – reproduced with permission

Intrinsic Safety (cont) Levels of protection Suitable for use in “ia”Zones 0, 20 (safe with up to 2 faults) “ib”Zones 1, 21 (safe with up to 1 fault) “ic”Zones 2, 22 ( safe under normal operation) Levels of Protection & Zones Source: TestSafe – reproduced with permission

Intrinsic Safety Markings Example Smith Electronics Model TRE Ex ia IIC T4 Cert 098X Serial No iaequipment suitable for zone 0 application IICequipment is suitable for Gas Groups IIA,IIB, IIC T4equipment is suitable for gases with auto ignition temp greater than 135°C

Detector Tubes - Colorimetric Tubes Change in colour of a specific reactant when in contact with a particular gas or vapour Source: Dräger Safety – Reproduced with permission

Advantages Relatively inexpensive & cheap Wide range of gases and vapours – approx 300 Immediate results No expensive laboratory costs Can be used for spot checks No need for calibration No need for power or charging

Limitations Interferences from other contaminants Need to select correct tube & correct range Results should NOT be compared to TWA Correct storage Limited shelf life

Colour Tubes / Badges Available For Instantaneous short term measurement Long term measurements – pump Long term measurements – diffusion CHIP system Based on colour reaction, but with digital readout of concentration

End of Part 2

Part 3 Personal Air Sampler

Air Sampling There are various locations at which one may wish to take an integrated sample of a chemical in the plant air. A general plant air sample is useful to give an overall measure of plant contamination. One might also be concerned with escape of chemical at a known or suspected point source, such as an open vat, a spraying operation, or a valve. Measurements made at a source of contaminant escape should not be used as values representing overall contamination of plant air. Air collected at a point source will later be diluted by plant air or may be removed effectively by the ventilation system. However., such a reading indicated hazard to a worker at the location and estimates the effectiveness of systems that clear the air.

Air Sampling A variety of stationary devices are available that either collect a sample for later analysis of give a direct reading of the contamination of the air at that location. Such devices may depend on appearance of a specific absorption of infrared light, change in the transparency of a filter, change in the pressure drop across a filter, scattering of light by airborne particulate of variety of other techniques. Devices are available to take samples automatically at timed intervals.

PERSONAL AIR SAMPLERS The most important air to sample is the air inhaled by the individual worker. Such air must be collected near the face. Unless we wish to attach the worker by a tube to a large stationary device, which would restrict the free movement of the worker and thereby distort the results of the study, the entire apparatus must be small and lightweight enough to be carried about conveniently by the worker. Such personal air samplers are available and are in common use. They consist of a small, battery-powered air pump that can be worn on the belt, to which a trapping device is attached. A tube pinned to the clothing near the face carries the air to the trapping device.

PERSONAL AIR SAMPLERS The most important air to sample is the air inhaled by the individual worker. Such air must be collected near the face. Unless we wish to attach the worker by a tube to a large stationary device, which would restrict the free movement of the worker and thereby distort the results of the study, the entire apparatus must be small and lightweight enough to be carried about conveniently by the worker. This device, in spite of its small size, must meet adequate standards for analysis. Such personal air samplers are available and are in common use. They consist of a small, battery-powered air pump that can be worn on the belt, to which a trapping device is attached. A tube pinned to the clothing near the face carries the air to the trapping device.

End of Part 3

Part 4 – Practice Problem

Practice Problem 1