The PIU is fitted with two different pressure gauges. One of the gauges shows the differential pressure across the CBRN Filter – which gives an indication of the health of the particulate element, while the other gauge shows the vehicle overpressure by comparing ambient pressure with that in the vehicle.
In addition, the PIU can include a differential pressure switch and sounder, with the differential pressure switch also monitoring the vehicle overpressure. Should the vehicle overpressure drop below a pre-set value, the switch will allow the operation of the sounder to give an audible alarm signal. A visual alarm could be provided as alternative to, or in addition to, an audible alarm
Electronic versions of the PIU are also available, using differential pressure transducers to measure the vehicle overpressure and filter differential pressure – with the outputs from the transducers made available to the onboard vehicle data bus, or indicated on a display mounted in the PIU housing.