Maximum RVRs?
 

Can IRVR measurements exceed the length of the runway beside which the transmissometers are placed?

I would say yes; the length of a RWY has little (or nothing) to do with the RVR required to see the RWY (or APP lights/touchdown zone). If you go to a 1200m RWY with a high minimum or NPA you might need 2000m RVR to actually see the RWY at your mimimums. Don't think RVR values of >1500m are ever given but I know of plenty RWY's shorter than that...

Most airport RVRs are given as actual, ie. 650/400/300 until the RVR improves to more than 1500m when it is given as >1500. A few airports will give RVRs up to 2000m.
Self explanitory I think.
Hope this helps.

Thanks, I realise the required RVR is independent of runway length but wasn't sure if physical separation between transmissometers had any bearing on what RVR values they can measure, evidently not then.

Are you a pilot? How could you have missed these if almost every RWY has 3 sets of them? Here's some pictures to keep a lookout for:
http://farm4.static.flickr.com/3195/...8d7509.jpg?v=0
http://www.keesfloor.nl/weerkunde/7z...wayvisual2.jpg
As you can see they're probably not even 100m apart...

A bit off-topic, but what is the lowest RVR ever recorded? 0?

It would appear from the following, that the maximum range would be defined per the specific installation...

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Runway Visual Range (RVR) is a term used in aviation meteorology to define the distance over which a pilot of an aircraft on the centreline of the runway can see the runway surface markings delineating the runway or identifying its centre line. RVR is normally expressed in feet or metres.
RVR is used as one of the main criteria for minima on instrument approaches, as in most cases a pilot must obtain visual reference of the runway to land an aircraft. The maximum RVR reading is 2,000 metres or 6,500 feet, above which it is not significant and thus does not need to be reported. RVRs are provided in METARs and are transmitted by air traffic controllers to aircraft making approaches to allow pilots to assess whether it is prudent and legal to make an approach.
Originally RVR was measured by a person, either by viewing the runway lights from the top of a vehicle parked on the runway threshold, or by viewing special angled runway lights from a tower at one side of the runway. The number of lights visible could then be converted to a distance to give the RVR. This is known as the human observer method and can still be used as a fall-back.
Today most airports use Instrumented Runway Visual Range or IRVR, which is measured by devices called transmissometers which are installed at one side of a runway relatively close to its edge. Normally three are provided, one at each end of the runway and one at the mid-point.



A transmissometer is an instrument for measuring the extinction coefficient of the atmosphere, and for the determination of visual range. It operates by sending a narrow, collimated beam of energy (usually a laser) through the propagation medium. A narrow field of view receiver at the designated measurement distance determines how much energy is arriving at the detector, and determines the path transmission and/or extinction coefficient. Atmospheric extenction is wavelength dependent phenomenon, but the most common wavelength in use for transmissometers is 550 nm, which is right in the middle of the visible waveband, and allows a good approximation of visual range.
Transmissometers are sometimes referred to as telephotometers, transmittance meters, or hazemeters.