Color Indicator Tubes
Color indicator tubes can be used as a quick pass/fail method to check compressed air purity for many contaminants in compressed breathing air. Depending on the testing kit design, color indicator tubes have some drawbacks: they generally cannot discover Particulate; the required hardware is not cheap, you will not have a lab repot, and the sampling process generally cannot duplicate the high flow rate conditions of SCUBA/SCBA charging or airline supplied compressed breathing air.
However, when used correctly with an appropriate sampling kit, and taking advantage of technical support from an experienced lab, the test kit can be a life saver.
Color indicator tubes are sealed glass tubes about 4 inches long and about ¼ inch in diameter (depending on the manufacturer). Each tube contains chemicals that react with a specific compound (such as toluene, moisture, carbon monoxide, and so on) to form a color or stain inside the tube. The concentration of the contaminant is determined by the length of the stain (or the intensity of a patch of color) after passing a known volume of air through it.
There are several manufacturers of indicator tubes, but the Drager, Sensidyne and Gastec tubes appear to be the most widely used. Tubes made by different manufacturers are not interchangeable because of size, volume and flow rate requirements.
The Drager tubes are ideal for compressed gas systems because they can tolerate a fast air flow, and generally require a large sample. The Sensidyne and Gastec tubes generally use a small volume at a low flow rate, thereby making them less useful for checking compressed air. However, their low sample volume requirements makes them superior for situations where available sample volume is limited. For example, we use them with the Navy's new one man deep diving suit, and for testing of their SRC's (Submarine Rescue Chamber).
Testing Compressed Air with Color Indicator Tubes
All the color tube manufacturers allow the tubes to be used with a hand pump, electric pump, or flowing gas system. Generally there are 3 approaches in using the tubes:
1. One approach is to flow the sample into a Tedlar bag, and then using a pump to suck the air through the tube. The pumps tend to be discrete-stroke (non-continuous), and are either manipulated with a special manual or electric pump (piston or bellows). The correct volume is determined by the number of strokes. The hand pump tends to be very tedious and tiring, but the electric type are expensive and not necessarily portable
2. Generally the color indicator tube directly to the compressed gas system via a flow controller. The flow controller can be a pressure regulator or set of critical orifices. The sample volume depends on time and flow rate instead of a pump stroke. Typical flow rates for the Drager tubes used to check compressed air are standardized at fixed flow rates of 0.2, 2.0 or 4 lpm. The volume of air pumped is tube dependent, and ranges from 100cc to 20 liters.
3. The Drager Corporation in the U.S. has about 6 different types of compressed air samplers, each dedicated to a specific activity. For example, the Drager Aerotest Simultan sampler for low pressure compressed pharmaceutical air can sample 7 analytes simultaneously, but the balance of their other samplers can sample only 4 specific analytes simultaneously. In contrast to Drager, the Sensidyne compressed air sampler can only sample one compound at a time.
The ACI system (our system) uses reduced pressure (but no pressure regulator) combined with a critical orifice specific to each tube. It allows us to safely sample HP air at 16 SCFM. Although our sampling devices use Drager Tubes, our sampling kits are simpler, more sophisticated, and can sample up to 10 tubes at the same time plus a test for oil mist + particulate in only 5 minutes, thereby providing a wide range of test options. In contrast neither Sensidyne or Drager can provide that capability. There are many more technical advantages with the ACI samplers.
Some Specifics: Oil Vapor Detection with Drager Tubes
The 4 main tubes used with all kits testing compressed breathing air are: CO, CO2, moisture and Oil Mist + Vapor. Although most tubes are simple and easy to interpret, that is not true for the Oil Tube. For this reason we have singled it out here. We have chosen the Drager Oil tube because (in our opinion) it is more popular and versatile than the Sensidyne Oil detection tube. The Sensidyne tube albeit simpler, cannot determine synthetic lubricants. Synthetic oils are currently the most common compressor oils in use.
1. The Oil Vapor tube does not work on stain length, but rather on a color change from white to beige or yellow change at the end of the sampling time when you break a small vial inside the tube. That vial contains sulfuric acid which reacts with captured oil mist, and produces a color ( beige to black)..
2. Unlike most color indicator tubes, you will not know if oil mist or vapor is present until after you pass the air through the Drager tube. This means that you must sample for a specific period of time to deposit enough oil to be detected. The chart gives you a choice of time depending on what minimum level of detection you are comfortable with: 0.3, 0.5, 1.0 or 5.0 mg/m3. The type of oil used in your compressor must also be known because some oils are easily discovered, while others resist reacting with the acid in the tube. For example, if you want to know if the oil mist level in your air is 0.3 mg/m3 you will use the 0.3 mg/m3 column in the chart provided to find the minimum detection level associated with the oil you use in your compressor. You must then read the column to the left which tells you how long to sample. Sampling times range from 1 minute to an hour, but the average is about 15 minutes.
3. At the end of the specified sampling time (with the air still flowing) you will bend the tube in the middle to break the acid vial which will cause the acid to travel up into the white section. If oil has been captured, a discoloration will appear, telling you that oil vapor is present in your air.
4. It is important to realize that if no color appears, that means that oil is absent at the detection level you chose based on your compressor oil. If, however, a color appears, it means that oil is present above the detection level you selected. Unfortunately, if you need to know how much (and an air analysis lab is not available) you would have to repeat the test for a shorter time using a new tube. This is easy if you always use the same oil, the same test procedure and the same person does the sampling. If you change brand of oil, you can send a sample of the new oil to Dragger Safety. They will send it to Germany for an evaluation of detectability with the Drager Oil tube.
The Advantages of Color Indicator Tubes
1. Once you own the hardware, you can perform the tests on compressor air as well as air in storage banks and SCUBA/SCBA flasks.
2. You can perform the tests any time you wish; you do not have to contact a laboratory.
3. The results are instantaneous.
4. The current cost of using these 4 tubes (if you ignore the hardware cost) is about half the cost of an annual laboratory report, but if you sample quarterly the cost is similar to that of a laboratory analysis because of discounts. On the other hand, if you add shipping, taxes and customs fees, to send your air sample to a distantly located lab, the color tube approach becomes less expensive again.
5. Color indicator tubes will satisfy OSHA compressed air testing requirements in the US, but if you are a PADI dive center, the color tubes might not satisfy PADI testing requirements.
Color Indicator Tubes: Good and Not So Good
It should be remembered that Color Tubes provide good indications of a problem or lack of a problem. They are extremely useful when time or location prevents a laboratory analysis. However, they do not provide precise data, are less trustworthy as the contaminant concentration approaches the maximum safe allowed level. (But if you are that close to failing you should already be changing filters.) Here are some other considerations. Our comments in parentheses show a solution to most weaknesses.
1. If you make Nitrox by partial pressure blending, color indicator tubes cannot qualify air as oxygen compatible. (However with our kit you can mail the membrane filter to us for an analysis of particles and oil mist.)
2. Color indicator tubes do not result in a certified report to satisfy any agency in the US, Canada, or Europe. That might be different in other countries. (For most dive shops, and all fire departments, a certificate is seldom necessary.)
3. Drager's new hydrocarbon tube (CH25401) works on many hydrocarbons but not on true gases.
4. Color tubes cannot reveal a particulate problem; this is a dangerous shortcoming. (Our membrane filter will show you if you have a particulate problem.)
5. If you rely on just the tubes, you reduce the help any lab can give you when a mystery problem pops up. There aren't very many people around that are experts in sampling and analysis of air. You might get some help from the Drager people, but we found it to be very frustrating. (But that is why we are in business; to give you adivce on technical issues.)
With all this in mind, testing compressed air with color indicator tubes alone is an important safeguard in situations where time and logistics are more important than money.