Gas Detection 41 Table 1 Examples of standard test gas concentrations and minimum requirements for the lower limit of measurement for detectors tested to EN 45544–2 (2015) Gas Ammonia (NH3 )
Carbon dioxide (CO2 Carbon monoxide (CO)
) Chlorine (Cl2 )
Hydrogen sulphide (H2 Nitric oxide (NO)
Nitrogen dioxide (NO2 Ozone (O3
) Sulfur dioxide (SO2 ) S) )
Volume fraction of standard test gas (ppm V/V) Lower limit of measurement (ppm V/V) 20
2
5000 30
0.5 5
25 3
0.1 2
Table 2 Commonly used gas detectors for workplace measurements
Operating principle Chemiluminescence Colorimetry
Electrochemical (EC) Flame-ionization (FID) Gas chromatography (GC) Infrared photometry (IR)
Ion mobility spectrometry (IMS) Mass spectrometry (MS) Photo-ionization (PID) Semiconductor (SC)
Ultra-violet visible photometry (UV) * Other gases can be detected
Typical gases detected* NO, NO2
, O3
Isocyanates, inorganic hydrides CO, H2
S, NO, NO2
hydrides VOCs***
VOCs CO2 , NH3
VOCs VOCs VOCs
CO, VOCs, H2 VOCs, O3
** Gen – general gas detection; Exp – exposure measurement *** VOCs - Volatile organic compounds
S , NO2 , SO2 , Cl2 , Cl2 , NH3, inorganic
Measurement tasks** and portability Mainly Exp; transportable Gen; portable, transportable
Mainly Gen but also Exp if no interferents; personal, portable, fixed
Mainly Exp with GC; portable, transportable
Mainly Exp, used as speciation technique with FID, PID, MS; portable, transportable
Mainly Gen but also Exp if no interferents; personal, portable, transportable, fixed
Mainly Gen; portable, transportable, fixed Exp and Gen; portable, transportable Gen; personal, portable, fixed Gen; portable, fixed; alarm only Mainly Gen; transportable
500 3
0.1 0.5 2.5 0.3
0.01 0.2
themselves, although they are more likely to consult the manufacturer’s guidance where the manufacturer can draw on and incorporate information from the standard.
The number of toxic gases occurring in the workplace is very large, and this is matched by toxic gas detectors employing a variety of sensor operating principles. In practice, however, the majority of measurements are conducted using only a few such types of detector. Eleven types of gas detector which could be used for general gas detection and exposure measurement are listed in Annex A of the revised EN 45544-4. This is a reduction from 16 in the previous version to reflect current practice. I have annotated in Table 2 below some brief information regarding the detectable gases and usage of the 11 detector types.
Examples of commercial detectors certified to EN 45544 include those incorporating electrochemical sensors for carbon monoxide and hydrogen sulfide, and infrared sensors for carbon dioxide. These are currently the main workhorses for general gas detection in the workplace, especially for use in confined spaces (together with flammable gas and oxygen sensors in multigas instruments).
A key part of ensuring adequate performance is maintained during the working life of the detector is for the user to regularly test and maintain it. EN 45544-4 provides guidance on inspection and functional checks: “It is strongly recommended that these checks are performed before each day of use”. However, there is recognition that this may not be feasible for certain applications, e.g. emergency response, where there may not be sufficient time to conduct inspection and functional checks before use. In these cases, alternative plans for inspection and functional checks should be implemented which provide an equivalent level of safety. The routine inspection and functional checks should include checks that ensure:
• the detector is within its calibration period;
• the detector and associated equipment appear to be satisfactory from a visual inspection;
• there are no leaks in the sampling system for aspirated (e.g. pumped) detectors;
• the batteries have sufficient charge; • the zero reading in clean air is accurate;
• the apparatus responds correctly to gas, which can be undertaken by using a field calibration kit with the target gas or a surrogate gas (subject to the manufacturer’s recommendation);
• the display and audible and/or visual alarms are working, if fitted.
The results of these checks should be recorded and any remedial action should be completed and verified prior to use.
The provision of an alarm is probably the most important safety function of a general gas detector whether it is a personal, portable, transportable or fixed monitor. Updated guidance on alarm setting is provided in EN 45544-4 to reflect this. Alarm set points and the actions to be taken when an alarm is activated should be specifically defined by the employer for each hazard scenario arising from a risk assessment. Gas detectors typically have at least two independent, instantaneous alarms that are usually operated as a pre-alarm and a main alarm. The pre- alarm allows for taking early intervention options before the gas concentration reaches the main alarm set point, e.g. inspection of the area, activation of a ventilation system. Activation of the main alarm may initiate further, drastic action, e.g. evacuation.
Personal and portable detectors usually have, in addition to instantaneous alarms, time weighted average (STEL and TWA7
Coupling connections
each test. Moreover, their measuring range is defined by the manufacturer. Toxic gas detectors for exposure measurements have more stringent requirements: not only must they be subjected to tests and requirements comparable with those for general gas detectors but their relative expanded uncertainty, i.e. the sum of uncertainties (deviations) from the tests, expressed as a percentage, must be within limits prescribed by EN 482. For measurements for comparison with limit values and periodic measurements, the relative expanded uncertainty must be ≤ 50 % or ≤ 30 % depending on the measuring range and reference period (short term, e.g. 15 min, or long term). While these tolerances may initially seem generous, it should be borne in mind that (a) they are the sum of uncertainties derived from 13 tests; and (b) variation of exposure to chemical agents in the workplace can be significantly greater than indicated by the uncertainty of a single measurement calculated according to EN 482, due to the temporal and spatial variability of workplace exposure.
Additionally, Part 2 of the standard specifies the measuring of the detector, with both the lower and upper limits of
measurement being constrained; and the uncertainty of the zero measurement (i.e. that in clean air) must be within a specified limit. Some examples of the lower limit of measurement with associated values of the standard test gas concentration, extracted from the table in EN 45544-1, are shown in Table 1 (above).
If detectors for gases other than those listed in the table in EN 45544-1 are required to be tested, then values for the standard test gas concentration and lower limit of measurement, can be agreed between the manufacturer and the test laboratory, bearing in mind any limit values, should they exist.
Guide for use (Part 4)
The discussion above has been concerned essentially with performance requirements (Parts 1-3 of EN 45544) and therefore more relevant to the manufacturer (and test laboratory). EN 45544-4 (2016) is the guide for use and therefore contains information of value to users directly, if they read the standard
)
alarms. Typically, instantaneous alarms are used to warn against short term (acute) exposure to a dangerous concentration of toxic gas (i.e. safety related), whereas STEL and TWA alarms may be used to warn against longer term (chronic) effects (i.e. health related).
The alarm set points of the detector should be set specifically for its application. The set point should be low enough so that associated protective measures can be effective, while high enough to avoid false alarms, as far as possible. Frequent false alarms can lead to alarms being ignored, leading to complacency and possibly a potentially dangerous escalation of a toxic gas release. It is therefore recommended in the revised standard that the lowest alarm set point should not be set below twice the lower limit of measurement for detectors conforming to EN 45544-2 or below 10 % of the measuring range for detectors conforming to EN 45544-3. When determining the alarm set points, all delays (e.g. due to gas dispersion, time of response of the detector, the protective measures adopted) should be taken into account. Earlier alarm activation can be achieved through the use of lower alarm set points.
The revised standard also illustrates through diagrams the effect of gas concentration on the time of response and time
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