FAQ for the topic clearance measurement – Chapter 2

In Part 1, we have learned some basic knowledge about how to choose the right measurement location and time when measuring air concentrations in confined spaces or tanks. Part 2 will follow with the technical knowledge of the sensor and measuring method of the machine.

7. How do I convert vol.-% to ppm or ppb?

Explosive gases such as methane, ethane and other hydrocarbons unfold their effect in the vol. % range. However, toxic gases (CO, CI2, H2S, HCN, …) in much lower concentrations are dangerous and are therefore measured in the ppm range.

Conversion Formula: 1 vol.-% = 10.000 ppm = 10.000.000 ppb

8. Why should carrying strap not be used for letting the gas detector down to the space or container, in which clearance measurement is to be performed?

Very simple: Because it is not possible to read the display, when the detector is hanging down from above. Sometimes people counter this argument with: If the detector let down to the container does not beep, there is no reason for an alarm and the container can be released for passing. This is not strictly true. What if the measured value is slightly below the threshold value? If you read this result on the display, you would surely be alarmed – even if the alarm threshold were not exceeded.

9. When taking a sample, how long does a hose have to be flushed, before the measurement can be carried out?

According to one rule of thumb, a typical flushing time of approx. three minutes is required for each metre of hose plus 30 seconds. According to another one, a flushing time of five minutes is required at every measuring point per 10 metres of hose.

What matters is: The exact duration of flushing may vary depending on the measuring task and diffusion behaviour. The density of the respective gases as well as the absorption and the conductivity of the hose wall must be considered when calculating the flushing time. Their level depends among others on the hose material: Viton®2 is chemically more resistant than vulcanised rubber, has a lower wall adhesion and is resistant to solvents.

10. Should gas detectors be used with accessories – e.g. hoses or pumps – produced by other manufacturers?

The following rule also applies for respiratory protective devices: No, never. Your Dräger gas detector would probably be compatible with a hose produced by another manufacturer. But in the event of an incident or errors, no legal warranty can be assumed for this. This is because gas detectors are only approved for combination use with certain specified accessories – these accessories are specified in the so-called ”Declaration of Conformity” (in the Technical Manual).

IMPORTANT: A rubber hose is not suitable for measuring hydrogen sulphide – the material lets H2S modecules diffuse to the outside

11. What should be documented in the clearance measurement protocol?

The protocol must state in which space or container and at which time clearance measurement was performed – and under which conditions. Thus, the following information is essential:

– Date and time and/or time period

– Container number and measuring point at the container, if there is more than one measuring point

– Measured hazardous substances

– Responsibilities (name of the person performing the measurement, supervisor)

– The instruments used for the clearance measurement, so that they are clearly traceable later on.

12. How do different sensor types differ from each other?

Electrochemical sensors work in a similar way to batteries. When the target gas gets to the sensor, a small electrical charge is produced chemically between the two electrodes and is then indicated in the transmitter. The signal amplitude is proportional to the concentration.

Good to know:

– There are electrochemical sensors for more than 100 toxic gases

– Ready for use in a temperature range from -40 to +65° Celsius

– Very robust, long service life

– Cross-sensitivities and thus false alarms can be reduced by selective filter

In catalytic sensors, the target gas in the sensors is combusted at a heated detector element, the so-called pellistor, using a catalyst. This oxidation produces a heat of reaction that increases the electrical resistance in a second pellistor. This pellistor measures the ambient temperature – the measured value is determined by the difference between the two signals.

Good to know:

– Catalytic sensors are able to detect more than 200 flammable gases and vapours – they are however not able to differentiate between those

– They require oxygen from the ambient air for the combustion process

– Risk of intoxication from sulphurous compounds (H2S, SO2) and halogenated hydrocarbons

– Heavy metals, leaded petrols, substances containing silicone and long-chain polymers can cover the catalyst

Source: Draeger Global

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