Help please - looking for a paddle type air flow sensor

[GaryMo]

I'm looking for an air flow sensor to monitor airflow in an extraction system.

Ideally it would be a basic paddle design with either a N/O, N/C or both contacts.
I have no access into the ducting so it would be inserted into an appropriatly sized hole and fixed from outside.

Any ideas?

 

[Spark123]

Is the air being drawn from the same room? Is the air clean?
Wondering if a Differential Pressure switch would do.

 

[bernardgreen]

What size is the duct and what is the air speed ( approx ) at which the sensor should switch ? What needs to be switched

Paddle operating a micro switch.

Paddle moving a magnet to operate a reed switch with possibly a slave relay for heavier current.

Or for a solution with no moving parts to jam. If there is a significant pressure differential between two points along the duct when the air is flowing then use a differential pressure sensor to detect the difference. Either side of a bend in the duct or introduce a small constriction.

The air flow detector in some boilers use a differential pressure sensor so one of those might be an off the shelf solution

Edit Spark types to fast

 

[Spark123]

There's these too:
http://www.epvl.co.uk/category-22/CM2K.html

 

[ColJack]

for fitting the DP, you'd either put one before and one after the fan, or if it's just for a duct, then you put one in the duct and one to free air..

which one you put where depends on the need..
pressure or vacuum etc..

 

[TicklyT]

How about mounting an electrically heated thermistor in the airflow, and monitoring it's resistance as the airflow cools it

 

[GaryMo]

Thanks for the suggestions.

Basically it's two fume extraction systems which are constantly being switched off but the insurance company at this particular site want it on all the time.
The engineering manager asked us to fit a flow sensor to trigger a strobe and beacon when the airflow drops to zero.
The airflow sensor will be mounted within ducting is a hazardous area (defined as zone 1) so it'll be switching through a barrier.

I like the idea of differential pressure - that could be a good option as most of the paddle switches, including the one that Spark123 linked to are for fluids and not air.

 

[flameport]

If the problem is people switching these extractors off, why not change or remove the switches so they can't be switched off?

 

[GaryMo]

They're old EXd DOL starters local to the extraction motors fed via pyro and considering the area they're located it's not easy to modify the existing arrangement.