Accurate Gas Testing with Dräger Tubes
—Dräger Tubes are the ‘special agents’ among gas detecting devices, capable of detecting over 500 hazardous substances with precise and reliable measurements. Follow these tips to ensure accurate use of Draeger Tubes.
Measuring Strategy: What Factors Are Important?
Testing tubes may be easy to use technically, but developing a measuring strategy and interpreting the results are a job for trained experts. In order to get a correct measurement result it is important to take the sample in the correct place and minimize the time between the measurement and the test.
Using a Hose in Difficult-to-Reach Places
You can use an extension hose to inspect the air quality in sewers, shafts, tanks, and other difficult-to-reach places prior to entry. Connect one end to the pump and the other end to the tube holder. This enables you to place the tube at the measuring location, such as at the bottom of a tank.
For the Dräger X-act® 5000, Dräger offers hoses made of fuel-resistant, synthetic natural rubber in the following lengths:
- 3.28 ft./1 m
- 9.84 ft./3 m
- 32.8 ft./10 m
- 98.42 ft./30 m
Humidity and Temperature Can Change the Measurement Result
For each tube, specific conditions of use must be observed. Temperature and humidity are both key factors in the measurement process. For example, high humidity at high temperatures can drive the measurement result up. You can find important hints and tips in the operating instructions or in the Dräger Tube Handbook.
Compressed Gas Monitoring
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Aerotest Alpha Instructional Video
The video describes the basic set-up and operation of the low pressure (43.5 to 290 psi) Aerotest Alpha for the measurement of condensed oil, CO2, CO and H2O in compressed air.
Aerotest MultiTest med. Int. Instructional Video
This video describes the basic set-up and operation of the low pressure (43.5 to 290 psi) Aerotest MultiTest med. Int for the measurement of condensed oil, CO2, SO2, H2S, NOx (NO + NO2), CO and H2O in compressed gases (air, CO2, N2, N2O and O2).
Aerotest High Pressure Instructional Video
This video describes the basic set-up and operation of the high pressure (maximum 4,500 psi) Aerotest HP for the measurement of condensed oil, CO2, CO and H2O in compressed air.
Too Hot? Too Cold? No Problem with the Right Accessories
—If the temperature range of the location is higher or lower than that indicated in the operating manual (normally up to 104°F/40°C), the accuracy of the measurement can no longer be guaranteed. A higher temperature changes the volume of the extracted air – which, for example, normally amounts to 100 mL (3.3814 oz.) for one stroke at 68°F/20°C for a Dräger gas detection pump. Dräger has therefore developed a special hot air probe for measuring hot gases. The design of the probe cools the hot gases to a temperature that allows them to be measured directly with the Dräger-Tube measuring system. In case of a gas temperature such as 752°F/ 400°C, the probe enables the gas to cool down to temperatures below 104°F/40°C.
Dräger also offers a tube heater for locations where the temperature thresholds drop below those specified in the operating manual, allowing precise measurements at temperatures down to -4°F/-20°C. This handy device – which generates heat by crystallizing a sodium acetate solution – does not require an electrical power supply and may be used in EX areas.
Hot-air probe for measuring hot gases
Tubes heater for extreme low temperatures
Tubes and Pumps Need to Match Each Other
One of the greatest benefits of testing tubes is that they are already calibrated. However, the scale display can only give an exact indication of the occurring gas concentration if the tube has been given the specified quantity of air as a sample. For this reason, the volume conveyed by the pump and the temporal process of the volume flow – the so-called ‘suction characteristic’ – need to be matched with the tube. These requirements are specified by both international and national testing tube standards and norms. The recommendation is: use testing tubes with a corresponding gas detection pump from the same manufacturer.
Every Stroke Counts: Pumping Correctly
—Quantity matters. In order to achieve correct measuring results, always observe the number of strokes indicated in the operating manual. For hand-operated bellows pumps, make sure that the pump components are pressed together completely with each stroke. To ensure that the air sample taken is correct, check short-term pumps in terms of sealing and suction power prior to each use.
Quick test to check bellows pump for leaks
Insert an un-opened Dräger-Tube and squeeze the pump completely. After releasing, the position of the pump body should not change within one minute.
Quick test to evaluate tightness using the Dräger accuro as an example
Squeeze the pump completely. After releasing, the pump must open instantly.
For Clean Results, Flush the Pump
You should flush testing tube pumps after each measurement by performing several empty strokes, i.e., without the testing tube inserted. This removes any possible residue that may have entered the pump bellows due to the tube reaction.
Look Carefully and Read Thoroughly
—The color changes on the tube indicate the concentration level of the hazardous substance. A reliable result requires both accurate measuring and correct reading:
- Observe the tube continuously during measurement for changes so that you will be able to recognize any complete color changes that occur. In the case of high concentrations, this can happen suddenly during the first stroke.
- To see color changes well, good lighting is important. Bright daylight is ideal.
- Avoid the long-term effects of direct sunlight. UV radiation can influence the color change. For this reason, testing tubes should normally be read directly after measuring.
- Look at the tube against a bright, neutral-colored background during reading, such as against a piece of white paper.
- If you are measuring in the dark, place the tube on the reflector of a lit flashlight.
- Compare your measuring tube with an unused tube in order to identify the exact change in color (before/after effect).
Interpreting measurement results correctly at 'tricky' times
When evaluating scale tubes, three different scenarios are possible:
Example 1
If the color display runs at right angles to the tube's longitudinal axis, then you can read the concentration directly on the scale.
Example 2
If the color display is distorted, meaning that it runs at an angle to the tube's longitudinal axis, then a long and a short color change may be detected. The median value of these displays indicates the concentration.
Example 3
If the color display does not appear consistent (diffuse gradient), a consistent end point in the color change is not clearly recognizable. Read the end point of the color change at the point where a weak color change is still visible.
Take Cross-Sensitivities into Account
If the target substance is a single gas, then the result generally only depends on the measuring range or the dominant environmental conditions. However, if the atmosphere contains other substances, it is necessary to check the degree to which these might influence the measuring results and whether the selected tube type can deliver an accurate measurement result. Help is at hand in the form of the hazardous substances database Dräger VOICE®, a free online research tool that helps you in just a few clicks to find the appropriate tube for a particular hazardous substance.
Dräger Tubes Handbook
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Tried and tested a million times: Worldwide, the Dräger Short-term Tubes have proven to be a very cost-effective and reliable method for the measurement of gases. Get the latest edition of the Dräger Tubes handbook with over 450 pages of highly relevant and useful information. You will not find a better resource when you need to test for gases in the atmosphere.
Get in touch with Dräger
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