Dual Mode Measurement improves flexibility
The Monitair Chevron airflow indicator uses a low-power electronic microcontroller and custom display hardware to automatically monitor the efficiency of LEV systems. This is achieved by measuring the static pressure in the extract duct, which is indicative of the transport velocity in that duct. This usually entails positioning the Chevron’s sampling point as close to the point of air intake as possible, with no restrictions to airflow before this point.
(diagram reproduced and modified courtesy of HSE website)
When the above configuration is used, with the sampling point at A, any downstream impediment to safe extraction can be detected using just one alarm criterion – that a drop in duct suction (or an increase from the calibrated negative static pressure) will indicate one or more system faults. These could include a blockage, a leaking joint, a closed blast-gate or a drop in fan capacity, to name but a few.
This simple model of an LEV system is indicative of the majority of systems in use in industrial envronments today, but there are variations from this.
One of the most common variations is the spray booth. Most spray booths comprise a contained, or semi-contained area, often with flexible curtains through which operators enter or leave the booth. One common configuration involves the use of pleated filters mounted in the wall of the booth, with a plenum behind, which has an extract fan mounted directly onto it. Since the static pressure in the booth is very low – in the order of tens of Pascals at most – the only sensible place to sample the pressure is behind the filter in the plenum. This causes a problem when the above single alarm criterion is used, because as the filter gets blocked, the negative pressure in the plenum increases, which would normally be interpreted as a good condition, so a blocked filter will not result in an alarm.
As it became apparent that Chevrons could be used in spray booths, a second mode of operation has been added to the device’s software – a ‘dual alarm’ mode. This mode adds another criterion to the ‘low pressure’ situation described above. This second criterion has an additional ‘high pressure’ fail mode. If the negative static pressure rises to 50% above the calibration point, this is interpreted as a ‘filter blocked’ mode, and the moving chevron changes to a cross flashing at approximately twice the rate of the ‘low pressure’ rate.
All Chevrons with a serial number greater than 1248 ( June 2016) can be configured as ‘dual alarm’ units.
If you believe the Chevron is intended to be used in a spray booth or similar type of configuration, please let us know when you place your order, and the ‘dual mode’ configuration will be set before dispatch. If you already have Chevrons which you would like to use in this configuration, you can re-configure your unit(s) fairly easily. This note is included with new units.
Longer Battery Life
A second enhancement has been implemented in Chevrons from July 2017 (Ser No 2778 onward) that improves battery life. This mod introduces a low-power mode, which does not flash the LED or take any measurements until the unit is calibrated for the first time. Whilst in this low-power mode, the current consumption is less than 35uA – so the unit could run like this for over 6 years. The reason behind this mode is to reduce the battery draw before the unit is used in anger. Sometimes Chevrons are kept in store before they can be installed, and the low power mode means that the effect on battery life will be greatly reduced.
The Chevron is an evolving product and, if there are further enhancements you would like to see, please email me at email@example.com
Thank you for your continued interest in the Monitair Chevron