How to Bump Test a Fixed Gas Detector
A bump test is the fast field check that proves a fixed gas detector still responds to gas and still drives its alarm, and it is the routine most detectors see far more often than a full calibration. This procedure is for the technician performing a scheduled functional bump on an installed combustible or toxic point without adjusting it. It walks applying the certified challenge gas, reading the response against a pass threshold, confirming the alarm activates, and logging a clean result. The key distinction throughout is that a bump confirms function while a calibration adjusts accuracy, and mixing them up is a common field error.
Field Bump Test a Gas Detector in one line: To bump test a fixed gas detector, apply a certified challenge gas of known concentration through the calibration cap at the specified flow, watch the reading rise, and confirm it reaches a response within the pass window the site defines, typically enough to activate the low alarm. Confirm the detector annunciates and clears cleanly when the gas is removed, then log the as-found response as a pass or fail. A bump test proves the point still responds and alarms, but does not adjust the reading, which is what a full calibration does.
Gather the Gas, Cap, and Test Window
A bump test needs the right challenge gas and a defined pass criterion before you climb to the head. Bring the certified challenge gas the detector is characterized for, at the concentration the site procedure specifies, along with the regulator and calibration cap or flow housing that fits the head. Confirm the cylinder is in date and matches the target gas, because a bump with the wrong gas or an expired cylinder proves nothing. Know the pass window in advance: the site defines how much response counts as a pass, and often the intent is enough gas to drive the point into low alarm so the alarm path is exercised too. The difference between this functional check and a reading adjustment is spelled out in the guide to a gas detector bump test versus calibration.
Coordinate before you apply gas, because a bump can trip real alarms. Tell the control room which point you are testing so a genuine alarm during the test is understood as a test, and confirm whether any executive action tied to the point needs to be inhibited or is safe to exercise. Note the as-found background reading at the panel before you start, so you have the baseline the challenge is measured against, and confirm the point is healthy with no active fault, since bumping a faulted point tests nothing useful.
Apply the Challenge Gas and Read the Response
Fit the calibration cap squarely on the head and apply the challenge gas at the manufacturer's specified flow, then watch the reading climb. A bump is looking for a timely, adequate response, not a precise value, so what matters is that the reading rises promptly and reaches or exceeds the pass window the site set. Give the point enough time to respond fully, because a slow-responding sensor that eventually reaches the target may be aging and is worth noting even if it technically passes. The speed of that rise relates to the sensor's response characteristic covered in the note on gas detector T90 response time.
Read the peak response and compare it to the pass criterion. For a combustible point this is a percentage of LEL; for a toxic point it is a concentration in ppm. If the response reaches the window, the point passes the functional check. If it responds low, responds sluggishly, or does not respond at all, the point fails the bump and needs attention, most often a full calibration or a sensor investigation rather than being left in service. Do not adjust the reading during a bump, because adjusting turns a functional check into an undocumented partial calibration and loses the as-found record.
Confirm the Alarm Activates and Clears
The value of a bump is doubled when it also proves the alarm path, so where the pass window drives the point into alarm, confirm the alarm actually annunciates. Watch the control system register the low alarm as the reading crosses the setpoint, confirming the point still talks to the panel and the setpoint is still where it should be. This turns a simple sensor-response check into a partial proof of the alarm chain, which is why many sites set the bump gas concentration to guarantee the low alarm trips.
Remove the challenge gas and confirm the point recovers. The reading should fall back toward its background value and any alarm it raised should clear when the concentration drops below the setpoint. A point that stays in alarm after the gas is gone, or that will not return to its background reading, has a problem worth chasing, whether a stuck reading, a slow-clearing cell, or a residual gas trapped under the cap. Confirm any inhibit you applied to executive action is removed before you leave.
Log the Result and Decide Pass or Fail
A bump is only complete when it is recorded, because the record is what proves the point was functionally checked and is the trend that flags a slowly weakening sensor. Log the as-found response against the challenge gas, whether it passed the window, whether the alarm activated and cleared, and the date so the next bump interval can be set from it. A point that passed but responded lower or slower than last time is telling you it is aging, and that note is the early warning that turns a future failure into a planned calibration or sensor swap.
Decide the disposition honestly. A clean pass returns the point to normal service. A fail, a marginal response, or an alarm that did not activate takes the point out of the passed state and into a full calibration or investigation, and where the point is a life-safety detector its failure to pass a bump is not something to defer. When the response results and the point's health status are trended in a monitoring platform such as Merobix, a declining bump response across successive tests at a remote or unmanned site is visible as a trend, so a weakening sensor is caught on evidence rather than discovered failed during a real release.
Frequently Asked Questions
What is the difference between a bump test and a calibration?
A bump test is a functional check that applies challenge gas and confirms the detector responds and alarms within a pass window, without adjusting anything. A calibration applies certified gas and adjusts the reading so it matches the known concentration accurately. A bump answers whether the point still works; a calibration corrects how accurately it reads. Bumps are done far more often because they are fast, and a failed bump usually sends the point on to a full calibration.
What counts as a passing bump test?
A response that rises promptly and reaches the pass window the site procedure defines, which is often enough gas to drive the point into its low alarm so the alarm path is exercised too. The point should respond in a timely way, reach or exceed the threshold, and, where the bump trips the alarm, annunciate and then clear cleanly when the gas is removed. A low, slow, or absent response fails the bump and sends the point on to calibration or investigation.
Should I adjust the reading during a bump test?
No. Adjusting the reading turns a functional check into an undocumented partial calibration and destroys the as-found record that a bump exists to capture. A bump only confirms the point responds and alarms; if the response is off enough to warrant correction, that is a signal to perform a full documented calibration, not to trim during the bump. Keep the bump purely a pass or fail functional test and record the as-found response either way.
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