Detection tube
Detection tubes are easy-to-use measuring aids for the detection and measurement of chemical substances (e.g. pollutants ) in the air.
Detection tubes are still used today alongside modern electronic devices. With them z. B. the content of formaldehyde in living spaces can be measured. The Drägerwerk in Lübeck offers an extensive range of test tubes and accessories .
construction
A test tube consists of a thin glass tube that is sealed at the ends by melting. Inside, substances are applied to inert carrier materials which, after a chemical reaction with the substances to be detected, have an indicator function by changing color .
For the measurement, the ends of the test tube are broken off, then one end of the tube is inserted into the so-called gas detection pump . This can be a simple hand pump in which the air to be examined is pushed through the tube by squeezing it and the volume is determined by the number of strokes. Newer pumps simply pull a piston, like an air pump, to direct exactly 100 ml of air through the test tube. A modern variant are electronically controlled pumps that precisely meter the volume of air flowing through.
In principle, a distinction is made between scale test tubes and color comparison test tubes.
Scale test tube
For measurements with common test tubes, a precisely prescribed number of strokes is made with the pump; the number varies depending on the pollutant. If a pollutant is present, the indicator changes color and the pollutant concentration can be read off a scale on the tube. A marking on the tube usually shows the MAK value .
Color comparison test tube
The concentration of the pollutant is determined here based on the strokes made. There is a so-called color comparison layer in the tube. The gas detection pump is operated until the indicator layer is exactly the same color as the color comparison layer. A table now shows the number of strokes and the concentration of the pollutant next to it.
Areas of application and procedure
Breath alcohol determination
A typical area of application for test tubes are the tubes used for breath alcohol determination , which indicate the presence of alcohol in the exhaled air by changing the deep yellow color of the potassium dichromate present in the tube to green Cr (III) compounds and thus as a preliminary test for a subsequent blood alcohol determination can be used.
Standard tubes
Standard tubes are used for the quick investigation of known gases on site. Each of the precision gas detection tubes contains special reagents that are finely tuned to the target substance to be measured and that quickly form an exactly colored boundary layer. A total of approx. 300 standard tubes are available for the detection of more than 400 gases.
If a defined amount of air is sucked through the tube by means of a precise vacuum pump, the reagent changes color according to the concentration of the target substance. The concentration can be read directly from the tube using a calibrated scale.
Simultaneous test
To determine air pollutants, a non-specifically reacting test tube can first be used to determine by discoloration whether there is any pollutant in the ambient air. In the following, a series of tests is carried out with various test tubes to determine which pollutant is present. For this purpose, well over 160 different short-term test tubes are offered by different manufacturers, so that a wide range of different pollutants can be detected.
Simultaneous tests are used to enable the user to detect several pollutants at the same time.
In the fire service , simultaneous test procedures are used to get a quick overview of possible pollutants released in the event of a fire :
- Lead substance test for the detection of: carbon monoxide , hydrocyanic acid , hydrochloric acid , nitrous gases , formaldehyde
- Inorganic fire gases I for the detection of: Acid gases , hydrogen cyanide , carbon monoxide , basic gases , nitrous gases
- Inorganic combustion gases II for the detection of: sulfur dioxide , chlorine , hydrogen sulfide , carbon dioxide , phosgene
- Organic combustion gases for the detection of: acetone , toluene , methanol , hexane , perchlorethylene
There are other simultaneous tests for other areas of application, for example for measuring chemical warfare agents.
Passive collector
Passive collectors are used to determine the pollutant concentration in the workplace or in living spaces. The tubes are z. B. by means of a bracket at the level of the breathing area of an employee and detect the exposure of the employee over a period of 1 to 10 hours .
The measurement takes place via diffusion. The average concentration is determined by the relationship between the reading and the actual sampling time.
Chip based systems
Through further development and miniaturization it is now possible to arrange several test tubes on one chip . This chip is inserted into a special analysis device that also contains the gas detection pump. The air is sucked in and then passed over the measuring chip. Photo electronics recognize the pollutant values and show them on the display.
Alternative measurement methods
Alternative measurement methods for instrumental analysis are, for example, photoionization detectors or ion mobility spectrometers or gas chromatographs , especially those that are coupled with thermal desorption systems.
literature
- Manual for Dräger Tubes and Micro Tubes , 19th edition, Ed .: Dräger Safety AG & Co. KGaA, Lübeck 2019, ISBN 3-926762-05-5 .