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A PID is similar to other continuous, real time gas monitors. It indicates a gas concentration. It also will provide an audible, visual (flashing light) and vibrating alarm when a high concentration is present. PIDs usually have two high alarms to provide an early first warning and a second higher action needed alarm.
Also, 15 minute STEL (short term exposure level) and eight hour TWA (time weighted average) alarms can be provided. The exact capabilities and functions of any particular instrument will vary from model to model.
Because a PID measures numerous different compounds it is important to gather as much information regards the probable gases that are to be measured. Only then can you set the alarm levels with any degree of confidence.
If you know the VOC that is to be measured you can simply configure the STEL and TWA alarm levels in accordance with the HSE’s document, EH40. Because EH40 does not provide instantaneous high alarm limits the setting of these alarms are at the discretion of the user.