Video – “How to Measure Captor Hood Control Distance?”

Measuring the Real Control Effectiveness of Captor Hoods

This OXYL8 training video explores one of the most frequently misunderstood aspects of captor hood performance: determining the effective control distance (or control zone) of a captor hood.

Captor hoods are widely used throughout industry for controlling welding fume, solder fume, dusts, vapours and other airborne contaminants. However, their effectiveness depends heavily upon the operator working within the hood’s effective capture zone.

The video examines traditional methods of assessing captor hood performance and considers whether smoke testing alone provides sufficient information to determine the true control distance of a hood. It also discusses the use of Fletcher’s equations and captor velocity calculations as alternative methods of assessing performance.

Topics covered include:

Captor hood control zones
Effective range of captor hoods
Smoke visualisation techniques
Fletcher’s equations
Captor velocity calculations
Commissioning considerations
Thorough Examination and Test (TExT) considerations
Welding fume control
Hood positioning

This resource is particularly relevant to:

P601 TExT Engineers/P602 LEV Designers
Occupational Hygienists
LEV Commissioners
Duty Holders
Health & Safety Professionals

Source Document

Source: OXYL8 Ltd
Presenter: Bill Cassells
Topic: Captor Hood Control Distance
Video Series: OXYL8 LEV Training Videos

Status: Current Resource
Last reviewed by LEVCentral: June 2026

Using the OXYL8 LEV Calculation App

The calculations discussed within this video, including captor hood velocity and control distance assessments, can be carried out using the free OXYL8 LEV Calculation App.

The app includes a range of practical LEV tools used by designers, commissioners and TExT engineers, including:

Fletcher’s Captor Hood Calculations
Hood Face Velocity Calculations
Duct Velocity Calculations
Airflow Calculations
Benchmarking Tools
LVHV Calculations
General LEV Engineering Formulae

These tools allow engineers to quickly assess hood performance and compare measured results against design expectations whilst working on site.

The OXYL8 LEV Calculation App is available free of charge for both Apple and Android devices.

Learn More About the OXYL8 LEV App

LEVCentral Expert Commentary

Many LEV engineers are taught to assess captor hood performance using smoke.

A common approach involves releasing smoke on the hood centreline and progressively moving away from the hood until capture is lost. The point at which the first visible escape occurs is often used as a practical indication of the hood’s control distance.

Whilst this approach can be useful, the video highlights several important questions:

Does visible smoke accurately represent contaminant behaviour?
Does the method provide repeatable results?
Is the resulting distance sufficient for highly hazardous contaminants?
Should calculated capture velocities also be considered?

These questions become particularly important where the contaminant being controlled presents significant health risks, such as:

Stainless steel welding fume
Hexavalent chromium
Isocyanates
Solvent vapours
Fine respirable dusts

The video therefore encourages engineers to think beyond simple smoke observations and consider whether additional assessment methods may be appropriate.

Key Learning Points

Captor Hoods Have a Limited Effective Range

Unlike enclosing hoods, captor hoods rely on generating sufficient air movement to draw contaminants towards the hood opening. Their effectiveness reduces rapidly with distance from the hood face.

Smoke Testing Has Limitations

Smoke visualisation can provide valuable information but may not always represent the behaviour of all airborne contaminants under real working conditions.

Operator Position Is Critical

Many captor hood failures occur because the process or operator works outside the effective control zone rather than because of insufficient airflow.

Fletcher’s Equations Provide Another Approach

Captor velocity calculations can be used to estimate the velocity field in front of a hood and help assess whether sufficient control is likely to be achieved at a given distance.

Commissioning Should Demonstrate Effective Control

The objective of commissioning is not simply to achieve airflow values but to demonstrate that the hood effectively controls the contaminant.

Why This Matters During Commissioning

Establishing the Control Zone

One of the key objectives during commissioning is determining how close the contaminant source must be to the hood for effective capture to occur.

Benchmarking Future Performance

Control distance measurements can provide useful benchmark information for future TExT examinations.

Supporting User Training

Operators need to understand the practical limits of hood performance and where work should be positioned relative to the hood.

Improving Hood Selection

Understanding control distance helps designers determine whether a captor hood is appropriate or whether a more enclosing design is required.

Relationship to HSG258

HSG258 emphasises that the hood is often the most important component of an LEV system because it determines whether contaminants are successfully captured before reaching the worker’s breathing zone. Successful control depends on matching the hood design to the process and the way work is actually undertaken.

This video provides a practical demonstration of these principles and highlights why understanding hood performance is fundamental to effective LEV design and testing.

Related LEVCentral Resources

Recommended Learning

Thought Leadership

Defensible LEV Commissioning
Demonstrating Effective Control
Beyond Airflow Measurements
Understanding Captor Hood Performance

LEVCentral Observation

Captor hoods remain one of the most widely used LEV hood types, yet they are also among the most frequently misunderstood.

Engineers often focus on airflow measurements whilst overlooking the practical question that really matters: can the hood effectively capture contaminants where the work is actually taking place?

This video challenges conventional assumptions and encourages a more thoughtful approach to determining control effectiveness.

Ultimately, successful LEV systems are judged not by airflow alone, but by their ability to prevent contaminants entering the worker’s breathing zone.