The word PASS is an acronym for Personal Alert Safety System. Its purpose is to sound a loud, highly discernible audio alarm if a distress condition occurs. The device will sense an absence of motion if the wearer becomes immobilized for 25 seconds. Its alarm must function in the manual or automatic mode to assist rescue crews in locating the firefighter in distress.

    PASS devices must be highly reliable and easy to operate. NFPA Standard 1982 requires an alarm sound output of at least 95 dBA, measured at ten feet. Two important parameters of sound that must be considered are sound intensity (loudness) and sound discernibility (the ability to recognize a particular sound in a high background noise environment). Some of the earlier PASS devices had a loud sound, but were difficult to distinguish above the noise associated with the typical fireground. Present day PASS devices have overcome the problem of locating the source of the sound signal by modulating a pure tone, or generating a sound that consists of several intermittent tones. Another, and possibly the most desirable audio sound is that of a sweep frequency. This type of sound will generate multiple tones that sweep from 2,000 cycles thru 6,000 cycles. It is not easily masked by background noise.

    The sensor that permits a PASS device to operate in the automatic mode is called the motion sensor transducer. This is the heart of the PASS device. If the sensor is not sensitive enough to sense random motion, the device will constantly go into pre-alert, becoming a nuisance to firefighters that may discourage activation. The ideal sensor is one that only requires normal motion to keep the PASS inhibited, yet is sensitive enough to immediately sense a lack of motion when a firefighter is immobilized.

    There are a number of methods of sensing motion. Some PASS manufacturers use mechanical devices, such as a small metal ball to sense motion. The random motion of the ball is converted into an electrical signal as long as motion exists. Another popular method to sense motion is accomplished by the closing of a mercury filled switch. Still another method uses a ball inside an infrared light chamber. The motion of the ball interrupts the light signal and is sensed as motion. Perhaps the most progressive method involves a solid-state accelerometer device that can sense a broad range of motion, and is not position sensitive.

    Most PASS manufacturers use a custom microchip or a microprocessor to process the signal. Some chip functions are sensing low battery, motion, and sound generation. A quartz crystal may be used to ensure accurate timing. Switching is another aspect of PASS alarms. NFPA 1982 (1998 Edition) require that PASS alarms be automatically switched from the 'storage mode' to the 'sensing mode', without independent action by the wearer. Many manufacturers use a mechanical switch to activate their PASS devices. A recent improvement is all-electronic switching. These switches must be reliable and easy to operate, even with a gloved hand. If a PASS is used properly, it WILL SAVE LIVES!