Required response time for variable air volume fume hood controllers

This paper describes results from tests made with the aim of investigating how quickly the exhaust air flow rate through fume hoods needs to be contro lled in order to present contaminants from leaking out of the fume hood and putting the safety of the laboratory personnel at risk. The measurements a ere made on a laboratory fume hood in a chemical laboratory. There,were no other fume hoods in the laboratory, and the measurements were made without interference from persons entering or leaving the laboratory or talking ab out in it. A tracer gas method was used with the concentration of dinitroge n oxide (N2O) being recorded bg a Foxboro Miran 101 infra-red gas analyser. In parallel with the tracer gas measurements, the air,velocity through the face opening,ras also measured, as was the control signal to the damper co ntrolling the air dow rate. The measurements show an increased outward leak age of tracer gas from the fume hood if the air how rate is not re-establis hed within 1-2 s after the sash is opened, If the delay exceeds 3 s the saf ety function is temporarily defeated, The measurements were made under, vir tually ideal conditions. Under more typical conditions, the fume hood could be exposed to, various other external perturbations,,which means that the control system should re-establish the correct exhaust flow more quickly th an indicated by the measurement results obtained under these almost ideal c onditions, (C) 2000 British Occupational Hygiene Society. Published by Else vier Science Ltd. All rights reserved.