In a chemical plant, 0.6 tons of hydrogen chloride (HCl) escaped during a 10-minute period from all furnaces and vents within the potassium sulphate workshop while cleaning the HCl circuits. An employee living adjacent to the site notified the guard house of the presence of a cloud originating from the plant. The emergency sprinkling system connected to the washer was turned on to stop these emissions. Poor calibration of one of the two devices used to measure gas pressure at the furnace outlet (not directly related to the ongoing works), causing the control valve on the gas evacuation circuit to close, was responsible for this incident: since gases were no longer being drawn, they escaped from the furnaces. The lack of an alarm on this control parameter slowed personnel response, and the absence of any means for comparing the 2 pressure measurements prevented the detection of sensor drift. To reduce the probability of repeat occurrence, an alarm was installed to detect deviations between the 2 pressure readings; also, a procedure laying out the most sensitive steps, in particular those requiring a supervisor’s presence, was issued.