small volume flow monitoring

[curious125]

I need to find a flow meter that will allow me to program a low and high setpoint window, and turn on an output if the flow is not within the setpoint window.

The flow is quite small. Maybe about 3 CFM
The cycle time is about 15 seconds.

And did I mention that the flow to monitor is vacuum?


Any Ideas?

 

[felixc]

Are you monitoring air? or anything corrosive?

Search on the net for "mass flow sensors", you will find interesting stuff. These are the type of sensors like those used in cars for the correct air-gas mixture.

 

[jon1]

You have a lot of things to work out before you can measure.
You talk about a vacuum. How much of a vacuum? What are the residual gases? What is the allowable composition of residual gases? You are likely to be looking at an ionisation flow meter. What ions are allowable in your residual gas composition? Don't look at this problem as a simple component selection process. Look at it as an applied physicist. Every measurement effects everything else. What can you live with?

 

[jraef]

The trick to me would be in the transducer. The set-point tripping can be accomplished in a variety of ways, such as setpoint relays like Acromag or Action Instruments, a small PLC, even the new "Smart Relays" sold by Moeller, Telemechanique, Siemens etc. are capable of that now.

What you need to do is serach for a flow transducer that will work in your environment and give you a 4-20ma or 0-10VDC output in the range you need to measure. The fact that is is in a vacuum is important only as an environmental consideration. It will still be the movement of air (or some other gas?) through the transducer that you want to measure right?

"Venditori de oleum-vipera non vigere excordis populi"

 

[curious125]

Sorry, I guess I was just a tad vague.

The vacuum source is a vacuum pump with a storage tank. Before the vacuum flows it would be at a level of about 20"Hg. the "gas" is just plain air (and what ever is floating in it) The vacuum is drawn through some channels, but is basicly drawing from open atmosphere.

All I'm really concerned about is tracking wether or not the flow falls between the Hi and Low setpoint values by having an NO or NC contact actuate if flow is outside of the setpoint window.

 

[jraef]

So you are monitoring air flow in a vacuum then? You mention "stuff floating in it". What kind of "stuff"? Dust, metal shavings, dental drill residue, elephant excrement from the wilds of East India? It would make a difference. For instance, I have used simple propeller type flow switches on dental vacuum systems because the suspended particulates are usually not going to clog the system since larger particles don't make it past the suction tube in the first place. But if you are pulling in a corrosive or abrasive dust such as silicon carbide from a sand blaster, a propeller meter would be toast in a few hours. Why not just say exactly what you want to do?

"Venditori de oleum-vipera non vigere excordis populi"

 

[curious125]

I basicly want to monitor vacuum during the moulding process on several of our moulds.

We use vacuum to help reduce the gas burns within the mould while plasticizing. I'm mainly interested in making sure that if there is no flow, or very little flow that there there would be some kind of warning or alarm to indicate that the vacuum isn't working properly.

 

[jraef]

I used these people in the past. I didn't see anything specific as to what to use in a vacuum environment like yours, but it gives you someone to contact anyway.

http://www.koboldusa.com/product_types.asp?tmplt=4&pgID=1

"Venditori de oleum-vipera non vigere excordis populi"

 

[joboyd]

Have you considered a sensitive thermistor placed next
to a 1/4 watt resistor. The reistor would be placed outside the air flow path but close enough to the thermistor to
increase its sensitivity. As air flowed past the thermistor,
its resistance would change. The output from a bridge could
be fed to a window comparator.
Some thermistors are so sensitive they can detect very
small amounts of current flowing thru a resistor.

 

[BJC]

Look up Micro Motion flow meters. They will measure very low mass flows. But probably not a vacuume as it would have no mass.

 

[nbucska]

joboyd:
use self-heating termistor

<nbucska@pcperipherals DOT com> subj: eng-tips
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