Short-term Detector Tubes

To use a short-term detector tube for measurement of instantaneous concentration of a contaminant, the pointed ends are broken off and one end of the tube is inserted into a special hand pump supplied with the kit. The pump is then used to pump a specified amount of air through the tube. The technique is relatively simple and effective, although the results may not as precise as with more sophisticated methods. Depending on the contaminants and the conditions of use and the skill, there can be as much as a 25 percent error in some situations. 

Long-term detector tubes are used in a similar manner to short-term tubes except a sample is pulled through the detector tube at a slow, constant flow rate by an electrical pump. The pump should have a stable low flow rate of 2-50 ml/min for... 

Table 10.18(a) Substances for which colour detector tubes are available from one supplier (Drager) - short-term tubes... 

Detector tubes are available for CO covering a measurement range from a few ppm to several thousand ppm. Some are suitable only for short-term sampling whilst others may be operated over a period of hours with the aid of a low-flow battery operated sampling pump or by passive diffusion. 

The last term, u ec, points out that substantial peak broadening may occur outside the column. To avoid this, the transfer lines from sample injection to column and column should be as short as possible. Stagnant areas in the system must be avoided. These can occur, for example, if the connection between two pieces of tubing bows out. The detector cell should have a low dead volume. 

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