Aircraft maintenance requires workers to enter confined spaces that contain jet fuel vapors. As with all confined spaces, gas monitors are required to confidently decide if the atmosphere is safe for worker entry. Because of the physical qualities of jet fuel, specialized gas monitoring techniques should be considered.
This Application Note addresses the following:
• Conventional LEL (Lower Explosive Limit) sensors were designed to measure methane and lack the sensitivity to accurately measure jet fuel vapors.
• Photoionization detectors (PIDs) are accurate and reliable hydrocarbon sensors and are uniquely suited for measuring jet fuel vapors.
• PIDs should also be considered to measure the toxicity of jet fuel and the other chemicals
commonly used in aircraft maintenance.
• Aircraft Maintenance
• Jet Fuel Manufacturers
• Aircraft Manufacturers
• Military Aircraft
• Municipal Airports
Why Not Use A Conventional LEL Sensor?
While jet fuel is flammable, the LEL sensors found in virtually every confined space monitor do not have enough sensitivity to accurately measure jet fuel vapors. Workers can often see and smell jet fuel when in a wing tank, yet the meter does not detect it.
This can seriously undermine workers’ confidence in their monitor.
Catalytic LEL Sensors Designed to Measure Methane
LEL sensors were originally designed to solve the problem of measuring methane levels in coal mines. Most LEL sensors use a Wheatstone bridge to measure the heat released when a flammable gas burns on a catalyst bead. The temperature rise causes a change in resistance, which is measured and converted to % LEL.
Catalytic LEL Sensors Simplified
A Wheatstone bridge (catalytic bead) sensor is simply a tiny electric stove with two burner elements. One element has a catalyst (such as platinum) and one doesn’t. Both elements are heated to a temperature that normally would not support combustion. However, the element
with the catalyst “burns” gas at a low temperature and heats up relative to the element without the catalyst. The hotter element has more resistance and the Wheatstone bridge measures the difference in resistance between the two elements. Effectively, this sensor measures the heat released when a gas burns.
Catalytic LEL Sensor Limitations
Four main factors affect the performance of Wheatstone bridge LEL sensors in a wing tank entry environment:
1. Gases burn with different heat outputs (“hotter”).
2. Gases have different LEL values, so some gases have more molecules present than others at the same %LEL.
3. “Heavier” hydrocarbons have difficulty diffusing through a flame arrestor to reach the LEL sensor.
4. Chemicals commonly used in aircraft maintenance can poison LEL sensors.
Overall Sensor Response
The overall sensor response is a combination of the first three factors. If the gas burns relatively hot, the response will be stronger. If the gas has a high LEL concentration, more gas will be present for a given %LEL and the response will be higher. If the gas is “heavy” (high boiling point and flash point), the diffusion rate is slower and less gas gets to the sensor per unit time, causing a weak response. The metal frit flame arrestor that limits the diffusion is necessary to make the sensor intrinsically safe and prevent the hot sensor itself from igniting an explosion. It does not prevent gases like methane, propane and ethane from reaching the Wheatstone bridge. However, it severely limits the diffusion of heavy hydrocarbons like jet fuel, diesel, and some solvents.
The overall sensitivity of various gases compared to methane is listed in the table that follows. For example, ammonia has a higher response thanmethane because both are light gases, but the LEL for ammonia is higher. Jet fuel burns “hotter” than methane, but the overall response is much weaker because Jet Fuel is much heavier and has a much
lower LEL. If an LEL monitor is calibrated on methane and then is used to measure jet fuel vapors, the monitor will theoretically display less than one third of the true reading. In some practical cases, we have found even lower response with Jet fuels and found that LEL sensors could not read diesel fuel vapors at all.
LEL readings can be corrected by choosing calibration gases that are more appropriate to the gas that you are measuring. It is impossible to make a compressed gas standard for jet fuel. Therefore, it is recommended that a “surrogate” calibration method be used. The chart above shows that the LEL response of hexane is much closer to jet fuel than methane. Some manufacturers calibrate their LEL sensors to hexane for this reason. However, the response to jet fuel is just 68% of that for hexane. Therefore, when calibrated to hexane and reading
10% of LEL in a space containing jet fuel vapors, the real reading would theoretically be 16% of LEL.
Testing by independent labs like TRW have verified that Wheatstone bridge sensors do not have appropriate sensitivity for jet fuel. Therefore, even when their output is boosted to allow for the low response of jet fuel, Wheatstone bridge LEL sensors lack the sensitivity for measuring at the jet fuel levels necessary to protect workers making confined space entries.
LEL Sensors Poison Used in Aircraft Maintenance
Under the best of situations it is difficult for catalytic bead LEL sensors to measure jet fuel vapor. However, chemicals commonly used in aircraft maintenance can seriously degrade LEL sensor performance. The most serious poisons are silicon compounds. Just a few parts per million (ppm) of silicon compounds are sufficient to degrade the catalyst and sensing performance of a Wheatstone bridge LEL sensor. These compounds are used in a wide range of products, including lubricants, adhesives, silicone rubbers (including caulking and
sealant compounds), and others. Chlorinated hydrocarbons are another common group of
chemicals that degrade LEL sensor performance. They are frequently found in solvents, including degreasing and cleaning agents used in and around aircraft.
PIDs: A Better Jet Fuel Sensor
PIDs are sensitive hydrocarbon sensors originally designed to measure ppm (parts per million) levels of hydrocarbons for the environmental industry. PIDs are uniquely suited to measuring a hydrocarbon mixture like jet fuel. Recent breakthroughs in PID technology make them compact, rugged and affordable for the aircraft maintenance environment. Wing tank entries should not be made if the concentration of jet fuel in a wing tank is over 10% of LEL (or 800 ppm jet fuel vapor). Based upon the following chart, one can see that PIDs will provide the most consistent readings for a decision at 10% of LEL.
Sensor accuracy affects user confidence. At 10% of LEL, a PID is clearly the more accurate sensor.
• PID range of uncertainty: 160 ppm
• LEL Sensor range of uncertainty: 480 ppm
So a Wheatstone bridge LEL sensor has three times the range of uncertainty relative to a PID for measuring 10% of jet fuel LEL.
Measure PPM of Jet Fuel for toxicity
The ACGIH (American Conference of Government Industrial Hygienists) some time ago established an 8-hour TLV (threshold limit value) of 200 mg/m3 (approximately 35 ppm) for kerosene products. Most jet fuels are kerosene mixtures that fall under this exposure limit. In order to attain this level of protection, confined space monitors that measure jet fuel in low ppm levels are required. PIDs offer a compact, reliable solution to the problem of protecting technicians who have to work in or around jet fuel. Catalytic bead LEL sensors have a detection limit of about 1000 ppm for kerosene and cannot possibly measure in the TLV range.
PID Action Levels (at 35 ppm toxicity):
• Worker can enter wing tank without respiratory protection if PID is below low alarm (35 ppm)
• Worker can enter wing tank with respiratory protection if PID is above low alarm but
below high alarm (between 35 and 800 ppm)
• Worker cannot enter wing tank if PID displays any high alarm (above 800 PPM or
10% of LEL)
PIDs Protect Maintenance Personnel from Chemical Exposure
Many chemicals are used in aircraft maintenance, including paints, degreasers, and solvents. The PID is a total hydrocarbon analyzer that measures all of these chemical vapors. While a PID can’t differentiate among common hydrocarbons, if the PID alarm is set for the “worst” chemical, then a worker will be safe in the presence of all the other chemicals.