Why do regular bump tests and calibrations need to be performed on gas detectors?
What is a Bump Test? (or Function Check) A bump test is when a known concentration of gas is exposed to the gas detection to verify the gas detector’s sensors and alarms are working properly. The ‘test’ gas concentration is higher than the detector’s alarm set points. The test should trigger the detector’s alarm to go off.
A bump test also makes sure that a sensor responds to a known gas concentration in the amount of time expected. Often response time is specified as a t50 or t90– the time for the sensor to get to 50% or 90% gas concentration. A bump test does not change the sensor’s settings – it is simply a quick verification.
Why is it important to perform bump tests? Bump testing ensures the continued accuracy of the gas monitoring equipment. Additionally, it protects the health and safety of people working with hazardous gases. Over time, the accuracy of gas detection instruments can deviate from their initial calibration settings.
Test frequency? The industry-wide test frequency for portable instruments is to bump test before use. Always follow the manufacturer’s recommendation for calibration frequency. Read the manual, not the literature – ALL gas detectors have a recommended calibration frequency. For fixed instruments, follow the factory recommendation for bump testing and calibration. For both portable and fixed instruments, higher quality sensors will generally require less frequent calibration. This is due to them having less drift than lower quality sensors.
Calibration
What is a Calibration? (or Calibration Check or Full Calibration) Gas detector calibration is the process of exposing a gas detector to a known concentration of gas and adjusting the detector to read that concentration accurately in accordance to the manufacturer’s instructions, internal company policy, and [when applicable] any appropriate regulatory agency guidelines.
A calibration is typically done with two known gas concentrations – zero and a gas concentration (referred to as a span value). The gas concentration for the span value is usually 40-60% of the sensor’s range – a sensor that reads 0-100 ppm would be calibrated with a gas between 40 and 60 ppm. This “2-point” calibration sets two accurate readings for the sensor’s output – zero and the span value. Since most sensors have a linear output this 2-point calibration adjusts the sensors accuracy across it’s full range ( 0-100 ppm in the example above).
Calibration frequency? Equipment should be calibrated regularly; monthly is a common standard. Again, check the manufacturer’s manual (NOT their literature) to find the factory recommendations for bump testing and calibration.
What is the difference between bump test and calibration? A bump test is a quick functional check to ensure a gas detector’s sensors and alarms respond correctly when briefly exposed to a known concentration of gas. It confirms the instrument is working properly but does not adjust its accuracy. In contrast, a calibration is a more detailed procedure that verifies and fine-tunes the detector’s readings by comparing them to a certified calibration gas. While bump tests are typically done daily or before each use, calibrations are performed less frequently to maintain long-term accuracy and compliance.
Some of the reasons to bump test and calibrate:
Best Practices: To meet the manufacturer’s recommendations, and to make sure that instruments are maintained to the factory recommendations in case of an accident or incident.
Natural Degradation: Sensors and electronic components gradually degrade chemically and experience drift over time. A calibration resets the output of a sensor to adjust for this degradation.
Environmental Conditions: changes in temperatures, humidity, and high levels of airborne particulates can cause chronic changes in sensor performance.
Over-Range Exposure: Exposure to high concentrations of target gases and vapors can alter sensor accuracy, or even damage a sensor.
Catalytic Sensor Poisons and Inhibitors: Catalytic hot-bead LEL sensors can be affected by volatile silicones, hydride gases, halogenated hydrocarbons, and sulfide gases through chronic or acute exposure. A bump test will confirm that the sensor will respond to combustible gases. A coated or damaged catalytic sensor will read 0 when gas is present.
Electrochemical Sensor Exposure: Electrochemical toxic gas sensors can be compromised by solvent vapors and highly corrosive gases. A bump test will confirm that the sensor will respond to the target gas.
Physical Stress: Harsh storage and operating conditions, such as dropping the instrument onto a hard surface or submerging it in liquid, can impact the integrity of electronic components and circuitry. Even regular handling and jostling can create enough vibration or shock to affect the accuracy of the sensors in an instrument.
Calibration Rules
1. Follow the manufacturer’s guidelines for proper calibration.
2. The type and concentration of calibration test gas, sample tubing, flow regulators, and calibration adapters are critical elements in the calibration process.
3. Operators should conduct the testing in an environment that is the same as (or similar to) the working conditions (e.g., temperature, humidity, atmospheric pressure).
4. Only use a certified traceable test gas, and do so before its expiration date. The supplier should provide a certificate of analysis for every test-gas cylinder. Reactive gases such as hydrogen sulfide and chlorine, will only remain stable for a limited period which is why a test gas cannot be used after its expiration date.
Additional Resources: OSHA- Safety and Health Information Bulletins Calibrating and Testing Direct-Reading Portable Gas Monitors
