Gas Detection

Question 1

Name 3 types of air quality hazards.

Answer 1

1) Toxic
2) Flammable
2) Oxygen deficient

Question 2

What is OEL?

Answer 2

Occupational Exposure Limit: maximum concentration that will cause no adverse health effects to a worker. There are different OELs for different workplace health hazards.

Question 3

What is LEL?

Answer 3

Lower Explosive Limits: minimum concentration of a gas or vapor that will explode.

Question 4

Explain the difference between portable, personal or a fixed gas detector.

Answer 4

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.

Question 5

What type of gas detector is used to test the atmosphere in a confined space?

Answer 5

Portable gas detector

Question 6

Name 4 types of toxic gas detectors.

Answer 6

1) Electrochemical
2) Solid state
3) Infrared
4) Photoionization detectors

Question 7

Explain the effect of low temperature on the output of electrochemical cells.

Answer 7

The sensor output fall to zero if the inner electrolyte freezes.

Question 8

What are the advantages of using an electrochemical toxic gas detector?

Answer 8

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

Question 9

What are the disadvantages of using electrochemical toxic gas detectors?

Answer 9

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)

Question 10

What are the advantages of using MOS (Metal Oxide Semiconductor) toxic gas sensor?

Answer 10

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

Question 11

What are the disadvantages of using MOS (Metal Oxide Semiconductor) toxic gas sensor?

Answer 11

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

Question 12

What are the 4 components of a CO2 IR sensor?

Answer 12

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

Question 13

What type of detector detects Volatile Organic Compounds (VOCs)?

Answer 13

Photoionization detectors

Question 14

What are TLVs?

Answer 14

Threshold Limit Values: represent the conditions that nearly all workers can have exposure to daily without experiencing adverse health effects.

Question 15

Describe TLV-TWA

Answer 15

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.

Question 16

Describe TLV-C.

Answer 16

Ceiling: concentration limit during any part of the working exposure.

Question 17

What happens when there is no TLV-C listed/set for a substance?

Answer 17

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.

Question 18

What is STEL?

Answer 18

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.

Question 19

What are the 2 types of combustible gas detectors?

Answer 19

1) Catalytic bead
2) Infrared

Question 20

What is the purpose of a flame arrester?

Answer 20

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.

Question 21

What types of gas molecules absorb IR light?

Answer 21

Gasses with dissimilar atoms in their molecules

Question 22

What is the difference between single point and open path IR gas detectors?

Answer 22

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)

Question 23

What is the difference between a bump test and a calibration?

Answer 23

A bump tests validates a calibration whereas a calibration requires a zero and span adjustment.

Question 24

What are the characteristics of a single point IR combustible gas detector?

Answer 24

1) Can identify leaks at their exact locations
2) Used indoors close to potential leak sites
3) Measure %LEL values at their location

Question 25

What are the characteristics of an open path IR combustible gas detector?

Answer 25

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

Question 26

What are the characteristics of Catalytic Bead gas detectors?

Answer 26

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

Question 27

What is the difference between catalyst inhibition and poisoning?

Answer 27

An inhibitor desensitizes the sensor making it more likely to fail at triggering alarms.

A poison permanently damages the sensing bead.

Question 28

How can loss of sensitivity be checked?

Answer 28

A Frequent bump test

Question 29

Why do catalytic combustion sensors need oxygen in the calibration gas?

Answer 29

To burn the combustible gas to generate heat

Question 30

What is the purpose of correction factors for combustible gasses?

Answer 30

Sensors react differently to each combustible gas

Question 31

What is meant by propane equivalent calibration gas?

Answer 31

Methane gas gives the same response as propane

Question 32

Is H2S reactive or non-reactive?

Answer 32

Chemically reactive

Question 33

What are the three installation considerations for a fixed gas detector?

Answer 33

1) Density of the gas
2) Wind direction
3) Leak location

Question 34

Combustible Hydrogen gas detector is installed near the ________.

Answer 34

Ceiling

Question 35

Lead acetate tape is used for detection of ….

Answer 35

H2S process

Question 36

What kind of detector cannot be used for gases with similar atoms?

Answer 36

IR detector