Infrared Gas Detectors

Whilst open path infra red gas detectors are popular and finding wide application offshore, your Author has learned the hard way that there are important 'golden rules' with regard to using these devices offshore. If any one of these rules is broken, the installed system will probably be unreliable and unsatisfactory. The golden rules are as follows:

1

Open path gas detectors can be configured in two modes: a direct beam from transmitter to receiver or using a retro-reflector. The direct beam method has proved to be more reliable offshore.

2

The source technology used with these units can be either a filament lamp or a flash tube. The flash tube method has proven to be more penetrating offshore in poor weather conditions.

3

Limit the beam length ideally to no more than 20 metres, with 30 metres as a maximum and always choose a mid-range unit, for example, choose the 15 to 60 metre option for a range of 30 metres not the 10 to 40 metre version.

4

It is essential that both transmitter and receiver units are rigidly mounted. That can be typified as using a maximum of a one-metre length of 3-inch box section steel welded to primary steel.

5

It is essential that the optics are kept clean and a routine maintenance schedule should be in place to wash optical surfaces with detergent in warm water, followed by a rinse with clean water at regular intervals, dependent upon local atmosphere conditions (and diesel fumes are frequently the source of contamination).

6

Position the transceiver or receiver so that the sun cannot shine directly into the lens or be bounced off the sea. This is often difficult to achieve and the use of existing structures located behind the transmitter can, on occasions provide effective screening. If the location of the receiver is such that it 'looks' towards a large bluff surface - your Author remembers using a Test Separator on one occasion - then as the sun rises or sets, the bluff surface prevents the suns rays penetrating an on-axis receiver; a solid floor is also needed to prevent sunlight bouncing from the sea, In addition, some manufacturers of open path IR detectors provide a matt black cowling to fit to the front of the receiver to act as a 'blinker', hence reducing the ingress of slightly off-axis sunlight.

7

Although a number of detectors claim to be totally unaffected by sunlight, including some of the models mentioned below, the physics upon which these detectors depend cause your Author to have considerable doubts and I retain this requirement as desirable for most models and to date, make only one exception. Perhaps now, and in the future, I should place less emphasis on this previous weakness as the technology improves!

Four models that have found a wide application are:

• PLMS model GD 4024 - these comprise 2 separate transmitter - receiver units - now 'old' in design
  
• The Spectrex type marketed by MSA - there are a large number of variants available - now 'old' in design
  
• The Draeger Pulsar - I have had good results from this model.
  
• Zellweger Searchline Excel - modern and believed to be resistant to sun interference