Captor Hoods can be an effective control, in particular for welding applications, where they are typically installed as flexible arm (moveable) hoods.

Why are they then often ineffective in practice in controlling the fumes?

Because the operator often welds (works) outside the Control Zone (the Captor Distance)!

How can we determine the extent of the Control Zone?

We can use Fletcher’s equations – or those of Dallavalle/Silverman – but I prefer Fletcher’s equations as I believe they give a more accurate result for a wider range of Captor Hood shapes and sizes.

Fletcher’s equations will, for:-

  • a given hood size and shape,
  • the Captor Velocity required
  • the distance the operator needs to work away from the hood (Capture Zone/Captor Distance),

– provide the required entry Face Velocity (in m/s) in order to achieve adequate control.

Fletcher originally produced a Nomogram to simplify the maths and a copy of the Nomogram and instructions on how to use it can be downloaded here:-

Fletchers Nomogram Instructions

Running through several options and iterations can be fiddly on paper and in this post I’ve put Fletcher’s Equations into a spreadsheet for free download (see below). This simplifies the process and even with very limited maths – everyone should be able to ascertain very quickly and accurately the effective Captor Distance and/or the required Face Velocity for a given hood and work arrangement.

However we often want to use Fletcher in reverse (and although I love maths – for the life of me I cannot reverse his equations :-().

In other words – in this case – if you know the actual entry Face Velocity (by site measurement) and the hood size and shape – then you can use the downloadable spreadsheet – by trial and error (ie putting different distances in to the equations) to get the Control Zone/Captor Distance.

You simply keep feeding distances in at the first line until it provides the Face Velocity you got on site by measurement.

Fletchers Nomogram_V1.8

Footnote:  Do remember that in the UK, HSE advises that the Captor Velocities for welding should be 0.5m/s – 1.0m/s.

Use the lower end of the range for small scale mild steel welding in a draft-free work area and the upper end of the range (ie 1.0m/s) for large scale welding, or for toxic metals (stainless, galvanised etc), or for work in a drafty environment.

The results from Fletcher will interest you, instruct you, amaze you, stun you, horrify you – depending on your your involvement with Captor Hoods (user, manager, designer, supplier etc).  You have been warned!

COMING SOON – Watch this space for our forthcoming short training video clip which will be on the subject of Captor Hoods, Captor Zones and Fletcher!