Date: Thu, 19 Nov 2009 21:45:36 -0500
Reply-To: DCHAS-L Discussion List <DCHAS-L**At_Symbol_Here**LIST.UVM.EDU>
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From: Peter Zavon <pzavon**At_Symbol_Here**ROCHESTER.RR.COM>
Subject: Re: Hoods
In-Reply-To: <4B056E830200002900015FD1**At_Symbol_Here**>

> From: DCHAS-L Discussion List [mailto:DCHAS-L**At_Symbol_Here**] 
> On Behalf Of Edward Senkbeil
> Sent: Thursday, November 19, 2009 4:13 PM
> To: DCHAS-L**At_Symbol_Here**LIST.UVM.EDU
> Subject: [DCHAS-L] Hoods
> The hoods (about 5 years old) in all our undergraduate 
> chemistry labs are equipped with alarms which supposedly 
> should go off with improper air flow.  However many will 
> continually alarm if the sash is pulled up by more than one 
> third the way (below where sash has a normal catch about half way up).

This sounds like normal operation as designed.  The idea is that the exhaust
volume creates an appropriate face velocity when the sash is at its normal
operating position.  So if the sash is raised above that point, during
normal operations, an alarm will sound to notify the user.  (Who is going to
be watching the flow gauges at such times?  They are intended to be
indicators to be used when you walk up to a hood, and perhaps occasionally
while working in a hood if you think its performance has become

The level of protection provided by designed face velocity with normal sash
position is thought not to be necessary during the set up or break down of
apparatus.  Therefore there ought to be some form of temporary silence
control on the alarm, used to silence it (for 15 or 30 minutes or so) in
those circumstances.  It is not a permanent off-switch because of the
potential for abuse.

> Is there any law / regulation which states that we must have 
> the alarms working if they are part of the original 
> equipment?  

I am not aware of any such law or regulation.  However, doing that does mean
you are throwing away the money and effort that was used to install these
systems in the first place.

> All the hoods have flow gauges on them which are 
> functioning, but we have been unable to get all the alarms to 
> work properly.  We are told it would be very expensive to get 
> them all functioning.  They become a problem in large student 
> labs since they continually go off, and are distracting both 
> faculty and students.
> We have considered disarming the alarm, but are concerned 
> about any regulations we might be violating.
> Students are beginning to not pay attention to the flow 
> gauges, but just automatically hit the mute button on the 
> alarms.  We believe the flow gauges are a more accurate 
> reflection of any problems.

You need to teach the students, and perhaps the lab instructors, what the
sash position is for, why there is a stop about 33% of way up, when it is
and is not appropriate to raise the sash above that point, and when it is
appropriate to mute the alarm.

Now, the selection of a 33% set point for the stop and alarm position was
probably a resource allocation decision, allowing the hood exhaust systems
to be designed for a far smaller exhaust volume.  Unfortunately, it came
with the requirement to perform your chemistry with the hood sash raised no
higher than that. 

Some 30 years ago, I was on the EHS staff of a well known university in the
eastern US and was involved in the development of sash stop with alarm and
temporary mute systems at a time when the hood manufacturers professed that
they had never heard of such a thing and saw no value in it.   At the time I
believed I had independently come up with the idea.  That may or may not be
the case, but I was certainly involved in an early roll-out of the concept.
At that time we looked at what hood users were doing and what they said they
could live with, and settled on a "normal sash position" of 26 inches open.
This was about 66% open, or perhaps a bit more, with most bench hoods.  Over
the years, energy conservation efforts have applied pressure to drop the
normal sash position to lower and lower points. 

My primary suggestion is that you review the performance intentions
associated with the original design as well as the actual current exhaust
flow performance of your hoods.  It may be that they are working as designed
but that some constraints, agreements, or intended functions have been
forgotten or overlooked.

Peter Zavon, CIH
Penfield, NY


> Any comments or suggestions welcome.
> Ed Senkbeil, Ph.D.
> Chemistry Department
> Salisbury University

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