VisGuard

A Closer Look

Visibility is stated in the form of an extinction coefficient that corresponds to the light attenuation caused by air pollution. It is possible to use either the level of transmitted light or scattered light as basis of the measuring system, because most of the light attenuation is caused by scatter effects.

The Sigrist VisGuard measures the scattered light intensity of a sample drawn into the instrument from the tunnel. This configuration permits extremely simple installation without time-consuming adjustment, reliable checking and correction of the zero and reference points without ambient air effects, and the ability to carry out measurement either inside or outside the tunnel cavity. An optional heater at the sample inlet effectively eliminates any troublesome mist effects.

To achieve the earliest possible detection of hot or cold fires producing smoke, a second current output or a limit contact (with a higher threshold value) can be used which is independent of measuring range 4-20mA output. This system opens up new possibilities for improving tunnel safety.

The tunnel air is extracted by a blower and carried through the flow cell. In the in-situ version the blower is integrated in the sensor. For the extractive arrangement, separate fans are used because more power is required. The blower also feeds filtered purge air to the flow cell, thus enveloping the sample in a protective shroud of clean purge air. This effectively keeps the optics clean and minimizes drift caused by fouling.

The  VisGuard is available in different versions:  In-situ, Extractive or Mini-extractive Installation

Flexible integration into your monitoring system 

Integration is simple with the following options:

• SIREL control unit with 4 – 20 mA signal output and 2 relays contacts
• SIBUS multichannel control unit to connect up to 8 instruments (also units from other manufacturers to measure wind speed, COx, NOx monitors, as required )
• SIPORT PB for direct integration into a Profibus communication network

Easy calibration using a calibration rod 

Calibration is performed very quickly, and simply with no tool or consumables:

• The VisGuard uses a highly stable and homogeneous optical calibration unit to performs the calibration automatically, and in less than 30 seconds
• Fast and reliable calibration method without any tools
• Zero point filter optionally available
• Works independent from the current air quality existing in the tunnel

Easy maintenance 

Due to the limited time available for maintenance of tunnel sensors, the VisGuard maintenance is very simple easy to ensure reliable data:

• No tools required for maintenance tasks
• Quick release enclosures for quick access to the various parts
• Maintenance interval is only once per year
• Maximum time needed is 10 minutes per sensor
• Changing of the filter is only required every 1 to 5 years (depending on the soiling)
• LED light-source means no need for lamp changing