Despite the advantages of electronic detectors, there are many applications where GASTEC detector tubes continue to provide a suitable alternative. When GASTEC tubes are used in conjunction with electronic detectors, they can provide a more accurate and complete picture of the gases present and better understanding of the risks.
INTRODUCTION
There are challenges with electronic gas detectors that sometimes limit their utility. These may include cross-sensitivity of sensors with interfering gases, temperature or humidity limitations, gas concentrations beyond the range of the sensor, the unavailability of appropriate sensors, the cost of exotic sensors, and the need to regularly bump and calibrate the sensors.
However, many industries continue to use detector tubes despite electronic alternatives. Detector tubes for H2S remain in wide use throughout the oil and gas industries. This is due to a number of other advantages of detector tubes as well as some limitations on the electronic detects which we will explore.
Today, GASTEC tubes are used in more than 500 applications including being recognised as the industry-standard method in Fire and Hazmat response, Marine and Shipping, Oil Refining and Chemical processing, Air and Water Quality, and Laboratories.
CONFINED SPACE ENTRY
Entering enclosed spaces such as sewers, tanks, silos etc for maintenance work generally involves monitoring;
> Combustible gases (LEL)
> Oxygen (O2)
> Hydrogen sulphide (H2S)
> Carbon monoxide (CO)
Sensors for these gases are readily available for modern electronic gas detectors and can be substituted with a range of other sensors including;
> Nitrogen oxides (NO, NO2)
> Ammonia (NH3)
> Sulphur dioxide (SO2)
> Carbon dioxide (CO2)
> Chlorine (CI2)
> Chlorine dioxide (ClO2)
> Hydrogen cyanide (HCN)
> Phosphine (PH3)
> Hydrogen (H2), and a few other gases.
Disadvantages of gas detectors – These exotic gas sensors can significantly add to the operating cost of your gas detectors. They tend to be more expensive to purchase and require separate calibration gases and regulators as well.
Advantage for detector tubes – GASTEC Tubes for nitrogen oxides, ammonia, chlorine and chlorine dioxide offer an attractive, easy to use, calibration-free alternative.
The answer to measuring organic compounds
Electronic gas detector manufacturers have made great strides in developing sensors for measuring inorganic compounds. For the vast number of organic compounds (such as alkanes, alkenes, amines, aromatics, alcohols, or ethers) there are still no chemical-specific sensors. In these instances, GASTEC tubes can be used to determine the gas concentration.
Testing vertical spaces in confined spaces can be a challenge. In the case of a tank, manhole, or pit, an extension hose can be used to acquire a sample away from the pump. Multiple lengths are available. A sampling pole is also available for horizontal remote sampling, which allows sampling up to 9.2 feet (2.8m).
HAZARD ASSESSMENTS
Hazard assessments are required for each individual confined space or other potentially dangerous environments to ensure that all gas hazards are identified and appropriate gas detection and personal protective equipment is used.
Assessments will commonly identify several gas hazards including combustible gases, oxygen deficiency, and other possible toxic substances that present chronic or acute threats. Where specific hazards are identified that cannot be addressed with an electronic gas detector, GASTEC tubes can often be used to test before entry and periodically during the work progression.
Working Together
A great example is the use of trichloroethylene (C2HCl3) at a dry cleaning facility. Where oxygen deficiency and combustible gases can be present in conjunction with a toxic organic, then a gas detector could be used to continuously monitor for flammability and oxygen deficiency, while the organic may be tested directly with a GASTEC detector tube.
PHOTOIONISATION DETECTORS (PIDs)
In recent years, Photoionisation Detectors (PIDs) have been incorporated into electronic gas detectors to detect a broad range of organic gases and volatile organic compounds (VOC’s).
The use of the PID as a broadband sensor for Volatile Organic Compounds (VOCs) is enormously useful and allows for the fast, accurate detection and quantification of a wide range of organic hazards.
Disadvantage for PIDs – A PID cannot, however, be used to identify what the hazard being measured actually is.
Advantage for detector tubes – Substance specific GASTEC tubes are often used for this purpose.
Working Together – Detector tube = to identify the substance, PID = to quantify the substance
CALIBRATION
Bumping a gas detector (sometimes referred to as a ‘function test’) involves exposing a detector to a known concentration of gas and ensuring the instrument gives the proper reading.
Regardless of why a sensor fails a bump test, it needs to be calibrated. In the case of a defective or consumed sensor, the calibration will fail and the sensor would need to be replaced and the detector again calibrated.
In order to bump and calibrate electronic detectors, it is necessary to have calibration gas, a regulator with an appropriate flow rate, and some tubing to connect them together. Calibration gases also need to be kept up to date as they have a shelf life. Expiry dates for various gases differ and are typically stamped on the cylinder label.
In contrast, GASTEC tubes are calibrated during the manufacturing process. This calibration is tube specific and is usually good for three years, though there are some examples of shorter shelf life. The expiry date is stamped on the tube box so all the user needs to do is look at the date on the box to ensure that the detector tube has not expired.
Advantage for detector tubes – For a field technician operating at a distance from a supporting facility, this is a big advantage. With no calibration issues, the need for any supporting equipment for field use is eliminated. Especially where only spot measurements are required, the ability to deploy and maintain a detector tube system in the field often makes it a preferred choice over an electronic alternative.
VERSATILITY
For many gases, there are a number of different detector tube ranges available.
If the concentration over-ranges the tube, either a new tube is inserted and a smaller volume is taken, or a higher range tube can be used. If we use Hydrogen Sulphide (H2S) as an example, the typical sensor found in a handheld electronic gas detector would have a range of 0 – 100 ppm or 0 – 500 ppm. This is not sufficient for many gas detection applications.
There are 14 different Gastec Tubes for the detection of H2S with ranges from 0.05 ppm to 40,000 ppm (40% by volume). This allows the most suitable range of tube to be selected for the application at hand.
TIME WEIGHTED AVERAGE (TWA) MEASUREMENTS
Passive dosimeter tubes are used for monitoring time-weighted average (TWA) concentrations over time. Using a tube holder, they are typically attached to a pocket or lapel and will measure the TWA for up to 10 hours.
The target gas is passively sampled (as opposed to using a pump) and the colour stain increases over time. To read the tube, the stain reading (ppm.hrs) is simply divided by the number of hours it has been worn. This is not the same thing as measuring the real-time gas concentration, but it is a useful measurement for compliance purposes since it references the TWA compliance level and lets the worker know when they are in danger.
SUMMARY
Like most technologies, electronic sensors and colourimetric tubes come with both strengths and weaknesses.
It’s critical to know your application and any potential hazards in order to determine how to use each technology most effectively to obtain accurate results. Used together, colourimetric tubes can be used to strengthen your gas detection program. Tubes can be used to confirm that sensors on the gas detector are measuring the correct gas and are giving accurate readings. In turn, the gas detectors can then be used to provide continuous monitoring, and the user can have greater confidence that they are accurately monitoring the gas hazards.
In cases where hazards are suspected but cannot be monitored with existing gas detectors because there are no gas specific sensors available, colourimetric detector tubes can be used to expand your measuring capability.
When deciding which measuring technology is most suitable for a given application, the answer may be both. For more information on detector tubes and the GASTEC detection system, please visit our website
