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VisGuard Visibility Monitor for Road Tunnels

Visibility, CO concentration, and sometimes also NO concentration are the measurable criteria for assessing air quality in road tunnels. These parameters are therefore used for controlling the ventilation system in order to minimize energy consumption and also for closing down the tunnel whenever pre-established limits are exceeded. 

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 scattered light intensity is multiplied by a factor to obtain the extinction coefficient.

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.

The VisGuard employs a variation of the long-proven SIGRIST dual-beam measuring method. It determines the relation between the light scattered at a 30°angle and the directly transmitted light. This system elegantly eliminates the effects of any light source fluctuations as well as ageing or temperature effects of the electronics.

To achieve the earliest possible detection of hot or cold fires producing smoke, a second current output or a limit contact with a much higher threshold value can be set entirely independent of measuring range used for visibility. By detecting fires early, 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

  • Regardless of the required mounting, there’s always a solution available

  • In case of an existing extractive version (replacement of old K- and C-type instruments) it is only necessary to change the sensor and control unit. The existing suction pipes and ventilators can still be used

  • The  mini-extractive version allows an installation where space is very limited (e.g. in a SOS-niche)

VisGuard In situ 1 

VisGuard In-Situ


VisGuard Extractive


Compact design

The instrument needs only little space and has a low weight:

  • Installation possible also when space is limited
  • Instrument can be mounted and dismantled by one person
  • Simple mounting using the integrated bracket
  • An optical alignment – as necessary for instruments based on absorption measurement – is not required. Therefore, installation is much easier. 

Flexible system integration

For the integration in new or existing tunnels there are various communication options available, allowing a simple system integration of the VisGuard:

  • 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 like e.g. wind speed, CO-, NO-monitors etc.)
  • SIPORT PB for direct integration into a Profibus communication network

Protected optics

 The measuring chamber is continuously purged with clean air. Purge air is produced inside the instrument (VisGuard in-situ):

  • Effective prevention of dirt deposits in the measuring chamber and on the optical parts
  • Guarantee for a drift-free and therefore accurate measurement
  • Extended maintenance intervals

visguard protected optics

Easy calibration using a calibration rod

Since decades, SIGRIST offers so called secondary standards for checking and adjusting the calibration:

  • Fast and reliable calibration without any tools
  • The VisGuard recognizes the checking rod and performs the calibration automatically within less than 30 seconds
  • Zero point filter optionally available
  • Works independent from the current air quality existing in the tunnel

VisGuard Extractive with inserted checking rod sml

LED light source

SIGRIST uses the well-proven LED-technology since 1998. LED’s have the following advantages:

  • Very low power consumption = low operating cost
  • Very long life span, typically > 10 years. Therefore, no regular change of the light source is required
  • Very stable light intensity over the whole life span, therefore, literally no drifts. One calibration check  per year is sufficient

Easy maintenance

There’s limited time available for the maintenance of tunnel sensors. Maintenance of the VisGuard is very easy and quick:

  • No tools required for maintenance tasks
  • Quick release closures allow a quick access to the various parts
  • Maintenance interval is only once per year
  • Maximum time needed is 10 minutes per sensor

Low maintenance cost

SIGRIST instruments are known for a low total cost of ownership. The VisGuard is no exception:

  • Practically no parts subject to wear and tear
  • Changing of the filter is only required every 1 to 5 years (depending on the soiling)
  • No need for lamp changing



Measuring principle:

30° scattered light measurement

Measuring span:

0 .. 100 PLA

Measuring ranges:

0 .. 15 · 10-3 E/m; 7 additional freely configurable ranges


< ± 0.25 % of full scale value

Measuring wavelength:

880 nm


Enclosure material:

stainless steel 1.4571

Sample temperature:

–20 °C .. +50 °C


5 l/min (In-situ) / 25 .. 30 l/min (extractive)

Ambient temperature:

–20 °C .. +50 °C

Ambient pressure:

±3000 Pa (±30 mbar)

Protection type:



6.5 kg (In-situ) / 5.0 kg (extractive)

Depth from wall:

235 mm

Sample heater (optional):

230 VAC; 25 W

Connection Box - SIPORT PB

Power supply:

85 .. 264 VAC / 47 .. 440 Hz

Power consumption:

25 W

Current output:

0/4 .. 20 mA; burden max. 600 Ω


2 separately configurable relay contacts 250 VAC, 4 A

Protection type:



1.5 kg

Connection to VisGuard:

4-core cable, up to 100 m

Power supply:

85 .. 264 VAC / 47 .. 63 Hz

Power consumption:

10 W / 25 W (with heater)

Bus (version PB):

Profibus DP

Degree of protection:



Polyester, glass-fiber reinforced


0.85 kg

Connection to VisGuard:

4-core cable, up to 100 m

Sampling System (0-30m) 

Blower type:


Power supply:

230 VAC / 50 hz

Power consumption:

90 W

Ambient temperature:

–20 °C .. +40 °C

Degree of protection:



7 kg

Sample heater (optional):

230 VAC; 42 W (extractive)

Sampling System (30-500m) & Multiple Sampling

Blower type:



Power supply:

230 VAC / 50 or 60 hz

3x 400 VAC / 50 or 60 hz



3x 440 VAC / 50 or 60 hz

Power consumption:

1 - 1.5 kW

950 W - 1.6 kW

Ambient temperature:

–20 °C .. +40 °C


Degree of protection:




22 kg


Sample heater (optional):

230 VAC; 42 W (extractive)



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