CAUTIONOne additional factor to take into account with the use of colorimetric tube systems is the perishability of the tubes themselves. The tubes not only have the propensity to be easily broken due to the nature of their construction, they also have a definitive shelf life.
Colorimetric tubes are glass tubes that are filled with a reagent that is known to react with a chemical or family of chemicals. Air is pulled through the tube by a pump — which can be of the manual or powered type, depending on the design at hand — and the reagent in the tube then tells us qualitatively if the product is present through a color change in the reagent. The reaction within the tube also can serve a quantitative purpose in that a graduated scale is imprinted on most colorimetric tubes so that the length of the discoloration of the reagent is read against the scale to display the concentration of the product encountered. For the purposes of this discussion we will refer to colorimetric tubes in a generic sense, as we are not advocating the use of any specific brand of equipment.
Such pieces of equipment have a rich history in the realm of air monitoring. The first colorimetric tube in the U.S. was patented in 1919 and contained a mixture that provided a visible reaction — qualitative only — when exposed to carbon monoxide. The design was a direct replacement for the live canaries used in coal mines to warn miners of the presence of carbon monoxide. The basic design has not changed drastically through the years, although the quantitative function of colorimetric tubes appeared at a later time. When used in a quantitative manner the integral scale normally is demarcated in either parts per million or percent volume.
Colorimetric tubes offer a simplistic reliability due to their design, in that when a manual pump is utilized no external power source is needed, no mechanical parts are present other than the pump itself, and no electronic components are used. Manual pumps usually take the form of either a bellows type or piston type pump. A bellows type pump is comprised of two rigid “shells” that surround a bellows mechanism. The pump halves are compressed and then released, which then draws an air sample through the tube inserted into the pump. A mechanical counter records the number of strokes completed. Piston type pumps resemble a smaller version of old-fashioned tire pumps in which a piston connected to a T-shaped handle is actuated to draw an air sample through the associated tube. A dot indictor displays the completion of each intake stroke.
To utilize the colorimetric tube itself, both ends of the glass tube are broken off so that the air sample can be drawn through. A fitting is normally provided on the pump to accomplish this task, and the user should break off the ends of the tube facing away from him or her to preclude any pieces of glass entering the eyes or striking the user. The tube is then inserted into the pump — the direction of airflow is indicated on the tube — and the pump is activated through a given number of full strokes. The number of strokes varies according to the type of tube utilized, is printed on the tube itself and is also indicated in the accompanying tube literature. The user should use the proper tube designed to indicate the chemical or product that is being monitored.
When to Use This System and Why
We can use colorimetric tubes not only to indicate the presence of a substance, but just as importantly to also indicate when a substance is not present by using the colorimetric tube system as a “classifier” to narrow down our possibilities. As we have previously discussed, the systems can also be used to indicate the concentration of a substance. The user does have to bear in mind that the quantitative function is of a more relative “ballpark” nature than results obtained from higher resolution digital meters due to the inherent characteristics of the process.
When the change in color of the reagent is at a right angle to the graduated scale, all is well and good. Oftentimes, however, the color change of the reagent lies at an angle to the scale. In those instances, the user should take the “average” measure at the midpoint between where the oblique angle starts and ends. Another challenge that sometimes arises is that when the change in color of the reagent is not a readily discernible line, but rather fades gradually. In those instances, the user should use the final edge of the color change as the correct value.
There are also five general guidelines for the reading of colorimetric tubes, as follows:
- Observe the tube during the entirety of the sampling process in order to notice any color change.
- Obtain the reading of the tube immediately following the sampling process to avoid any diffusion or fading of the color change.
- Ensure appropriate lighting is available for reading the tube results.
- Use a light background behind the tube during the reading process to assist with visibility.
- Compare the sample tube with an unused tube of the same type to highlight any color change.
As hazmat specialists and technicians in an emergency response setting we normally utilize colorimetric tubes designed for short-term use — normally defined as being utilized for measurement time frames of from 10 seconds to 15 minutes. Such tubes may employ one single indication layer, may have a pre-layer or layers in the flow upstream of the indication layer to provide the proper reaction, may have a reagent contained in an ampule upstream of the indication layer for the same purpose, or may even employ multiple tubes for simultaneous measurement capabilities.
One additional factor to take into account with the use of colorimetric tube systems is the perishability of the tubes themselves. The tubes not only have the propensity to be easily broken due to the nature of their construction, they also have a definitive shelf life. Hazmat teams that do not adequately prepare for the replacement of tubes as they reach the end of their shelf life or for the array of diverse tube types that oftentimes are required for the possible chemical hazards in their jurisdiction can be easily shocked by the budgetary impacts of the aforementioned characteristics.
In conclusion, colorimetric tube systems are reliable and robust systems that can provide real-time qualitative and quantitative air monitoring capabilities. We should not overlook these tools in our air monitoring “toolbox” when they are the right equipment for the job, as sometimes “old school” is the right tool. In closing, stay safe out there and be sure to visit the new North Carolina Association of Hazardous Materials Responders web page at www.nchazmat.com.
Glenn Clapp is Past President of the North Carolina Association of Hazardous Materials Responders and is a Fire Training Commander (Special Operations) for the High Point Fire Department. He is a Technician-Level Hazmat Instructor, a Law Enforcement Hazmat Instructor, and is a Certified Hazardous Materials Manager and Certified Fire Protection Specialist.