This is an instrument used for evaluating the tank atmosphere, prior to man entry, specifically for the presence of toxic gas. Hollow, glass tubes are inserted into a hand-held bellows pump through which a metered volume of the sample to be tested is drawn.
The tubes are coated internally with a reagent and sealed at both ends. Different tubes are required for each type of toxic gas. The tubes are graduated externally. Just before use, the sealed ends are broken and the tube is fitted to the pump.
As the sample is drawn through the tube by a recommended number of strokes of the bellows pump, the toxic gas discolours the reagent by chemical reaction. The amount of discolouration (as read off from the graduated scale) gives the toxic gas content in parts per million (ppm) by volume.
The DRAGER GAS DETECTOR together with the DRAGER TUBES is one of the most common and widely used instruments in the industry today. Information is given in the instrument booklet with respect to each gas detection tube consists of the following points:
e.g. Hydrogen Sulphide tube 5/b (Note: Hydrogen Sulphide tubes are available in a, b or C models, each having different specifications)
1. Drager type: Hydrogen sulphide 5/b.
2. The standard range of measurement: 5 to 60 ppm hydrogen sulphide.
3. Number of strokes of the gas detector pump: n=10
4. Relative standard deviation: 10 to 5%
5. Description: Scale tube: white indicating layer reagent: lead compound – colour change to brown.
6. Reaction principle: H + PbX –>PbS + H2X Hydrogen Sulphide + lead compound-> brown lead sulphide
7. Cross-sensitivity: In the presence of SO, the H2S indication is too high (e.g. a mixture of 10 ppm H2S and 10 ppm SO2 gives an indication of about 15ppm H2S) SO2 alone does not discolour the indicating layer.Extension of the range of measurement: The number of strokes can be arbitrarily varied between n=1 and n=100.
Range of measurement with 1 stroke – 50 to 600 ppm Range of measurement with 100 strokes – 0.5 to 6ppm
Concentrations below 0.5 ppm H2S can be determined if the number of strokes is increased above n=1 00. 500 strokes are possible but it must be ensured that the indicating layer does not dry out during the test since this would lead to diffuse discolourations, which are difficult to evaluate.
ERROR IN INDICATION
These errors are classed as INCIDENTAL and SYSTEMATIC.
The standard deviation is a measure of the difference of the incidental values from their mean value.
Eg. Mean value: 300 ppm. Standard deviation: 45ppm Relative standard deviation = 45/300 x 100 =15%
Tubes are subdivided into six groups as per their relative standard deviation i.e.:
(1) 10%-5%, (2) 15%-10%, (3) 20%-15%,
(4) 30%-20%, (5) 40%-30%, (6) above 40%
Detector tubes with a relative standard deviation 10%-5% represent an optimum level.
Systematic errors cannot be calculated but can be avoided.
The causes of systematic errors are:
1. Tubes not correctly calibrated: These can largely be excluded since tube manufacturers use approved and reliable calibration methods.
2. Changes in properties of filling preparation during storage: These effects occur when the tubes are exposed to temperatures above 60° C. for prolonged periods or to direct sunlight. The shelf life of natural gas test tubes is 1 year while that of others is 2 years.
3. The pump does not supply exactly 100 cm This may be due to leaks in the bellows which can be checked. It may also happen if the bellows are not fully compressed before expansion. Leaks are to be checked by fitting a sealed tube into the pump, after compressing it fully. If the bellows are leaky, the pump will open out; otherwise, it will stay deflated.
4. Bellows do not have full suction capacity: This is almost exclusively due to blocked suction channels and can be detected by operating the pump without inserting a tube. The bellows should open suddenly after releasing the pressure. If the suction channels are blocked, the bellows will open relatively slowly.
5. Errors if a suction device other than the pump is used: Pumps and detector tubes of other manufacturers are not interchangeable. To test the atmosphere at lower levels in confined spaces, flexible tubes of various lengths are provided. Drager tubes can be fitted at the far end of these flexible tubes, while the rear end is fitted with the hand pump.