Choosing the Best Gas Detector - Sample Draw vs. Diffusion

Sample-Draw vs. Diffusion

• Drawbacks of diffusion only designs:
  • Instrument only able to monitor the atmosphere in the immediate vicinity of sensors
  • Only way to obtain readings from remote location is to lower the instrument by rope or lanyard into the confined space
  • Not possible to use monitor for "pick hole" sampling (requires additional hand aspirator sample draw kit or motorized pump)

• Drawbacks of sample-draw only designs:
  • Sample lag time: instrument cannot detect contaminants until they reach the sensors
  • Potential for leakage in the system: critical to test system for leakage prior to use
  • Potential for pump malfunction: some instruments only operable as long as pump functions

Optimal design

• Instrument can be used in either diffusion or sample draw mod
  • "Shuttered" pump designs allow instrument to be used in diffusion mode without having to remove the motorized pump
• Motorized pump has its own power supply
  • "Parasitic" type pumps are powered via the instrument battery, reducing operation time per charge
• Motorized pump is diaphragm type design
  • Improves pump flow rate and maximum remote sampling distance

Hand-aspirated sample-draw kit

• Available for all models of diffusion type multi-gas instruments
• Make sure to squeeze the bulb the required number of times for sample to reach the sensors
• Continue to squeeze bulb until readings are stable
• Make sure to test the system for leakage prior to use:
  • Block end of the sample tubing or probe with finger
  • Squeeze the aspirator bulb
  • Bulb should stay deflated until blockage is removed