During the early days of sensor development, manufactures recommended that the instruments be fully calibrated before each use. This was very cumbersome for industry and compliance was difficult. As a result, industry began to lobby the manufacturers to relax the recommendation or offer some suitable compromise. The manufactures original requirement was based on the knowledge that sensors slowly fail over time, and fail at different rates depending on the environment in which they are used. In order to satisfy their customers and maintain the integrity of the instruments, the manufactures began recommending a function test or “bump test” option.

Bump tests are a quick and simple way to determine if a sensor still functions. The basic premise is to subject the sensor to the substance that it is designed to detect at a concentration slightly greater than the lower alarm limit. If the sensor alarmed, then the user has assurance that the sensor will offer protection, if it failed then the user could replace it with a functioning unit. This is a good compromise which, when performed correctly, will protect people from harm.

The trouble is, sometimes it’s impossible to determine if employees are actually performing the bump tests or if they truly understand the importance of the procedure. Every year, faulty sensors give a false sense of security to employees which could, and too often does, lead to disaster.

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During a recent study* of over 4.7 million bump test records by a major manufacturer of gas detection equipment, it was found that on average, 3 instruments out of 1000 will fail to respond properly. When the bump test interval is extended to 20 days, the failure rate doubles. A further study combined this test data with an analysis of how frequently gas detectors are exposed to hazardous, alarming conditions. This study found that, on average, one out of every 100 gas detectors not bump tested before use will fail to respond and alarm properly to an actual gas alarm event every 25 days.

Sometimes monitors are on the shelf for months before they are used. When this happens and the period of time between bump tests are as much as 6 months, failure rates have been as high as 50%. Therefore, half of the detectors in service may actually lull employees into believing that they are safe when they are actually at high risk for exposure. All monitors, not simply personal monitors, should be tested before use. Multi-gas detectors have multiple sensors that can fail, which gives them an even greater chance of failure. A quick bump test can give employees the confidence that they will actually be protected by the equipment.

The exact time of failure for a single sensor is, in large part, impossible to determine. Sensor life depends upon, not only the quality of manufacture but also on the environment in which the instrument is used. Sensors are very sensitive to shock from drops, humidity, and exposure to the gasses that they are meant to detect. Generally, the more rigorous and demanding the environment is in which they are used, the shorter the life of the sensors will be. The only way to detect these failures is to perform a daily bump test of the instrument. Without the bump test, employees may be rolling the dice and hoping that they are not within the 3% that will not make it out alive. Take a minute to perform this quick and simple check. It might save your life!

_*Industrial Scientific_

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