Combustible and Toxic Gas Sensors for Ethylene Oxide

For ethylene oxide (EtO) sterilization and other other operations using potentially flammable concentrations both ppm and combustible gas sensors are used. The ppm sensors are used to protect workers from the toxicity of the gas whereas the combustible gas sensors are used to protect personnel and property from the risk of explosion. 

EtO is one of the best gas sterilants available, with excellent penetrating power and it uses allows a wide range of medical and other devices to be sterilized. In order to allow its safe use in commercial sterilization applications it is necessary to use both ppm level sensors for protection against toxicity and combustible sensors for protection against explosion.

EtO is a toxic gas with an OSHA permissible exposure limit (PEL) of 1 ppm calculated as an 8 hr time weighted average (TWA) and 5 ppm calculated as a 15 minute TWA. Since EtO is carcinogenic, toxic and a primary irritant it is important to ensure that workers using EtO are protected from exposure in the event of leaks. In addition, the odor threshold for EtO is over 400 ppm, well above the OSHA PELs and so even if there was EtO at a concentration over a hundred times the PEL, workers would be unaware of its presence.

PPM levels of EtO are normally detected using electrochemical sensors which respond well to EtO, (good response time, linearity, repeatability etc.) but tend to suffer from cross sensitivity issues, i.e. they respond to carbon monoxide, alcohols and any other easily oxidizable organic vapor that comes their way. The ChemDAQ EtO monitor solves this problem with a unique chemical filter that allows EtO to pass through but removes most interferent gases and vapors. A typical EtO monitor for ppm levels has a range of 0 to 50 ppm and a resolution of 0.1 ppm. Gas chromatographs are sometimes also used to detect EtO; they have very good selectivity to EtO, but monitor periodically or for multiple points sequentially, and are more complex systems requiring much more maintenance. The ChemDAQ EtO monitor uses electrochemical sensors which provides the best all round performance.

EtO is flammable in air at concentrations greater than 3% by volume. Ethylene oxide is normally diluted with nitrogen to prevent formation of an explosive mixture, but sometimes human error and bad luck intervene and EtO sterilizer explosions have occurred.

To prevent explosions, combustible gas sensors are used to detect potentially flammable atmospheres. In commercial applications there are two main types of combustible gas sensor. The older, but still widely used technology is the pellistor which consists of two ceramic beads formed over platinum wire coils that form two arms of a Wheatstone bridge. One bead has a catalyst which heats up in the presence of a combustible gas, whereas its catalyst-free partner provides a reference for environmental changes such as temperature and humidity.

The other kind of combustible sensor uses infrared, at the absorption band for the C-H stretch in hydrocarbons. In the presence of EtO, the absorption of infrared light is measured to give the signal. Infrared sensors tend to be more expensive than pellistors but be more stable (longer calibration interval) and not subject to poisoning from sulfides and silicones. Pellistors are lower cost, and can detect any combustible gas, not just those that have a C-H absorption and so are the best option where the flammable gas does not have C-H bonds or is unknown. To be fair, in recent years, the cost of infrared sensors has come down and newer pellistors are more resistant to the poisoning. The ChemDAQ combustible gas sensor uses infrared since for EtO it offers the best performance

Both types of combustible sensor measure EtO and the monitors report the concentration in units of percent of the lower explosive limit (% LEL).  Combustible sensors are also widely called LEL sensors and the tersm are essentially interchangeable. As mentioned above, the LEL for EtO is 3% by volume in air and so an alarm at 25% LEL corresponds to 0.75 % volume EtO or 7,500 ppm.

People often ask why they need both ppm and combustible sensors. The answer is that they detect EtO for different purposes and at very different concentration ranges. If there is a small leak in the ppm range, then the ppm sensor activates to protect the workers and tell them to correct the problem, evacuate or don appropriate supplied air respirators. A combustible sensor will not detect EtO at ten times the OSHA PEL, and a ppm sensor will be over range long before the concentrations rises to even 1% LEL (300 ppm).

When the concentration is high enough for the combustible sensors to alarm, those people using positive pressure full-face respirators also need to get out since explosion is now a significant risk. In a typical setup, if the EtO concentration reaches the low alarm level, the ventilation is increased and if the concentration reaches the high alarm limit sprinklers will activate to reduce the risk of explosion. The high and low alarms are typically set at 10 and 25% LEL, though many organizations may have more conservative values.

If you are using 100% EtO on a large enough scale that there is a risk of explosion, then both ppm and combustible EtO sensors should be used. In addition, if nitrogen is being used to dilute the EtO to prevent formation of combustible mixtures, then there is a risk of asphyxiation from lack of oxygen should the nitrogen supply flood the work area. In such a situation oxygen sensors should also be employed. A well designed gas monitoring system will have a range of components designed for specific tasks that reflect the needs of the particular facility.

Since there is a risk of an explosion, gas monitoring must be intrinsically safe or explosion proof. ChemDAQ offers intrinsically safe sensors for oxygen, ppm ETO and %LEL EtO; contact ChemDAQ for more details.