4

• Canary in the coal mine – first direct reading “instrument”!
• 9 Characteristics of Direct Reading Instruments

  • Specificity
  • Sensitivity
  • Response Time
  • Calibration Requirements
  • Accuracy and Precision
  • Portability
  • Grab sample or continuous
  • Intrinsic safety
  • ease of use
• Applications of direct reading instruments

  Identify the source of exposure

  Provide immediate assessment

  Can be used as continuous monitors
• Advantages of DRI

  1.Immediate estimation of contaminant concentration ➢ Can be compound-specific or general screening

  2. Audio and/or visual cues when concentration exceeds a certain threshold

  3. No need for laboratory analyses and delay in getting results

  4. Generally, lower costs for assessments (over time)

  5. Data logging (timed elapsed history, STEL’s, TWA’s)

  6. Personal, Area or Fixed
• Disadvantages

  1. High initial costs of instrumentation
  2. Requires frequent calibration and regular maintenance
  3. May not have appropriate calibration facilities
  4. Not always accepted for compliance monitoring purposes 5Some instruments lack portability
  5. Ergonomic issues e.g. weight
  6. Growing, but limited number of compounds able to be sampled
• Before Sampling with a direct-reading instrument

  • Calibrate instrument
  • Understand instrument limitations
  • Determine sensitivity of instrument
  • Determine accuracy - should not exceed plus or minus 25%
• Bump testing is the process of briefly exposing the installed sensors to an expected concentration of calibration gas that is greater than the low alarm set point. The only way to verify proper sensor and alarm operation is to perform a bump test. The bump test checks for sensor and alarm functionality but does not measure sensor accuracy and no adjustments are made to the instrument during a bump test.
• From a hygiene perspective, inhalation is the most common route of exposure
• The choice of an instrument will be based upon:

  PURPOSE OF SAMPLING – identification of immediate hazards, monitoring hazards long-term, or use as a survey tool

  CONTAMINANTS – known or unknown

  CHEMICAL CLASS – organic vs. inorganic

  CONCENTRATION – percentage levels, ppm or ppb; peak vs. constant level exposure

  INSTRUMENT PHYSICAL CLASSIFICATION – personal, portable, transportable, or fixed

  SAMPLING STRATEGY DEVELOPMENT – define areas to be sampled and number of samples to take with direct reading instruments; follow-up with conventional methods (e.g., air sampling pump + charcoal tubes)
• LEL/LFL - Lower Explosive Limit/ Lower Flammable Limit (synonymous) UEL/UFL - Upper Explosive Limit/Upper Flammable Limit (synonymous)
• VOC - Volatile Organic Compounds

  PID - Photo Ionization Detection
• LTEL - Long Term Exposure Limit (synonymous with TWA)
• At a concentration in air lower than the LEL, gas mixtures are "too lean" to burn. Methane gas has an LEL of 5.0%. If the atmosphere has less than 5.0% methane, an explosion cannot occur even if a source of ignition is present.
• IP - Ionization Potential
• IS rating - Intrinsic Safety Rating
• RFI/EMI - Radio Frequency Interference/Electro-Magnetic Field Interference
• T90 - Refers to sensor response time -Time sensor takes to reach 90% of full response
• IDLH - Immediately Dangerous to Life and Health toxic gas concentration
• Type of Gas Detectors

   Diffusion  Pumped  Infrared  Single/Multi Gas
• Oxygen sensor performance – Sensor generates electrical current proportional to the O2 concentration – Sensor used up over time (last approximately two years depending on use)
• Catalytic Bead Sensor Characteristics

  Inhibitors: -Halogenated hydrocarbons (Freons , trichloroethylene, methylene chloride, etc.)
• Catalytic Bead Sensor Characteristics

  Oxygen Deficiency: -If O2 concentration less than 10% O2, LEL sensor will not read properly. Safest approach is to include an oxygen sensor with any combustible sensor.
• Catalytic Bead Sensor Characteristics

  Over Exposure: -sensor may be damaged by exposure to higher than 100% LEL concentrations (To prevent damage, sensor is switched OFF and instead of the LEL reading OL = {Over Limit} is displayed).
• Catalytic Bead Sensor Characteristics

  Non-Specific: -Cannot determine the species of gas being detected -Some combustible gases are toxic at concentrations well below the detection capabilities of the catalytic bead sensor -Not capable of detecting some combustible gas hazards
• Example: find how many ppm are in 1.7%.  x(ppm) = 10000 ⋅ 1.7% = 17000 ppm
• Capable of detecting hundreds of compounds – the PID sensor readings reflect the Total of all detectable Volatile Organic Compounds (VOCs) in the atmosphere being monitored
• PID - Limitations:  non-specific sensing technology; the detector cannot identify the species of gas being detected  some routine maintenance recommended  be aware of the detector’s lowest detectable limit and human exposure limits for the target substance  Moisture build up in the sensor can create false positive readings
• Common Gases Detectable by PID

  •Hydrocarbons with chemical names ending in –ane; - ene; -yne; EXCEPT methane and ethane

  •Alcohols – compound with chemical names ending in – ol

  •Aldehydes – compounds with chemical names ending in –aldehyde •Ketones – compounds with chemical names ending in –one

  •Esters – compounds with chemical names ending in - ate
•  NOT Detectable by PID

   Radiation

   Air (N2; O2; CO2)

   Natural Gas – CH4; C2H6

   Acid Gases – HCl; HF

   Compounds not in gas phase, eg: SulphruicAcid (H2SO4); Nitric Acid (HNO3)
• Colorimetric devices

   Most commonly used for exposure measurement are glass tubes containing solid reagent chemicals
• Colourimetric devices

   Advantages

  Simple to use

  Rapid results

  Convenient
• Colourmetric Devices

  Disadvantages

  Subjective to interpretation

  Many tubes have associated interferences

  Poor accuracy in some cases
• EVM series

  Portable-area monitoring instrument with a laserphotometer that measures, displays and stores concentration levels of airborne-dust