Gas Detection Flashcards
Name 3 types of air quality hazards.
1) Toxic
2) Flammable
2) Oxygen deficient
What is OEL?
Occupational Exposure Limit: maximum concentration that will cause no adverse health effects to a worker. There are different OELs for different workplace health hazards.
What is LEL?
Lower Explosive Limits: minimum concentration of a gas or vapor that will explode.
Explain the difference between portable, personal or a fixed gas detector.
Portable detectors: sample interior of buildings or vessel via a test hole. The probe is attached to the gas detector by a hose.
Personal detectors: contain sensors, controllers, and an alarm indicator all in one unit and is worn by workers.
Fixed gas detectors: a gas detector mounted in an area with high potential of a gas leak. They continuously monitor the air.
What type of gas detector is used to test the atmosphere in a confined space?
Portable gas detector
Name 4 types of toxic gas detectors.
1) Electrochemical
2) Solid state
3) Infrared
4) Photoionization detectors
Explain the effect of low temperature on the output of electrochemical cells.
The sensor output fall to zero if the inner electrolyte freezes.
What are the advantages of using an electrochemical toxic gas detector?
1) Small and lightweight for portable use
2) The output current is linear with gas concentration
3) Able to detect toxic gasses in the ppm range
What are the disadvantages of using electrochemical toxic gas detectors?
1) Have a limited operating and storage temperature range
2) Finite operating and storage lifespan
3) Output is dependent on temperature
4) Sensors can respond to other gasses giving false readings (interference)
What are the advantages of using MOS (Metal Oxide Semiconductor) toxic gas sensor?
1) Fast response time
2) Can measure both low and high concentrations of gas
3) Can operate at higher temperatures than electrochemical gas sensors
4) Longer lifespan than electrochemical gas sensors
What are the disadvantages of using MOS (Metal Oxide Semiconductor) toxic gas sensor?
1) Sensor resistance change is non-linear with concentration
2) Sensitive to changes in humidity
3) Older MOS sensors lack in response if they haven’t been exposed to gas regularly
4) Interference from other gasses may cause false alarms
What are the 4 components of a CO2 IR sensor?
1) Infrared lamp
2) Measure detector that selects only the IR light that the CO2 absorbs
3) Reference detector that selects a wavelength that is unaffected by CO2
4) Sample chamber
What type of detector detects Volatile Organic Compounds (VOCs)?
Photoionization detectors
What are TLVs?
Threshold Limit Values: represent the conditions that nearly all workers can have exposure to daily without experiencing adverse health effects.
Describe TLV-TWA
Time Weighted Average: concentration for an 8 hour work day and a 40 hour work week. Refers to the amount of time that a worker can have exposure to a substance repeatedly without an adverse effect.
To calculate TWA, multiply concentration by the amount of time each day and divide by the hours worked every shift.
Describe TLV-C.
Ceiling: concentration limit during any part of the working exposure.
What happens when there is no TLV-C listed/set for a substance?
Worker exposure can not exceed the 8 hour TWA and the 3x and 5x rule applies. Worker exposure can not exceed 3 times the 8 hour TWA for more than a total of 30 minutes during a continuous 24 hour period and 5 times the 8 hour TWA at any time.
What is STEL?
Short Term Exposure Limit: 15 minute TWA that workers must not exceed at any time during the workday even if the 8 hour TWA is within TLV.
What are the 2 types of combustible gas detectors?
1) Catalytic bead
2) Infrared
What is the purpose of a flame arrester?
Should ignition take place at a bead the explosion is contained and can not pass through the flame arrester. The flame arrester cools the flame to a level below ignition so outside gasses are not affected.
What types of gas molecules absorb IR light?
Gasses with dissimilar atoms in their molecules
What is the difference between single point and open path IR gas detectors?
Single point IR gas detectors sample the atmosphere in a fixed short path of a few inches. (% LEL)
Open path IR gas detectors measure gas over an area (LEL.m)
What is the difference between a bump test and a calibration?
A bump tests validates a calibration whereas a calibration requires a zero and span adjustment.
What are the characteristics of a single point IR combustible gas detector?
1) Can identify leaks at their exact locations
2) Used indoors close to potential leak sites
3) Measure %LEL values at their location
What are the characteristics of an open path IR combustible gas detector?
1) Used outdoors to cover large areas
2) Can identify the general area of a gas leak
3) Reads in LEL.m
4) Measure the total combustibles passing through the IR beam
What are the characteristics of Catalytic Bead gas detectors?
1) Must have O2 present to oxidize the gas
2) Catalytic beads may become poisoned
3) If a bead fails a bump test must be performed to detect a loss in sensitivity
4) Prolonged heating can damage the bead sensors
5) Respond to Hydrogen
What is the difference between catalyst inhibition and poisoning?
An inhibitor desensitizes the sensor making it more likely to fail at triggering alarms.
A poison permanently damages the sensing bead.
How can loss of sensitivity be checked?
A Frequent bump test
Why do catalytic combustion sensors need oxygen in the calibration gas?
To burn the combustible gas to generate heat
What is the purpose of correction factors for combustible gasses?
Sensors react differently to each combustible gas
What is meant by propane equivalent calibration gas?
Methane gas gives the same response as propane
Is H2S reactive or non-reactive?
Chemically reactive
What are the three installation considerations for a fixed gas detector?
1) Density of the gas
2) Wind direction
3) Leak location
Combustible Hydrogen gas detector is installed near the ________.
Ceiling
Lead acetate tape is used for detection of ….
H2S process
What kind of detector cannot be used for gases with similar atoms?
IR detector
