Pressure Sense- Shut Down

[tests]

I have an application where I need to monitor the pressure in an air plenum to ensure air flow is present in a computing device. The air plenum will have positive and negative pressure depending on which of the two blowers are being used (one blows out, one blows in). If the pressure is +2" W.C. or -2" W.C. I know that there is air flowing through the device. At some point, the direction of the air flow will change and the pressure will switch from positive to negative or vise versa. This should take 4 seconds. If the pressure is not high enough after 4 seconds, I want to shut down the unit.

I want to do this without complex circuitry or software of any kind for documentation purposes. Any ideas out there? Maybe a COTS item or using a pressure sensor with a time delayed voltage monitor that will activate a relay.

Thanks

 

[ScottyUK]

How much space have you got, and how much money?

Low differential pressure switches such as this are used in, among other things, balanced flue boilers to detect fan operation. They aren't especially large. Have you looked on Google for "differential pressure switch"? probably a good starting point.

The delay could be achieved using a hardware timer from, e.g. SAIA Burgess, Crouzet, Finder, etc if you are averse to software options.




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One day my ship will come in.
But with my luck, I'll be at the airport!

 

[tests]

Thanks Scotty,

I definately don't have a lot of space. Maybe 6" square or less. Money shouldn't be an issue if it prevents software documentation and PCB documentation.

I've found quite a few pressure sensors and switches. Most don't measure the vacuum (remember -2" W.C.).

If I have to make a PCB then my cost will go up a lot. I would like a turn key solution or piece parts that don't require a lot of assembly.

Maybe something like this only smaller:

http://www.phoenixcontrols.com/prodinfo.html#APM

 

[VEBill]

Measure the rack temperature instead. Much simpler and probably more reliable. The only down-side is that you'll lose perhaps a minute or two of warning time.

If you really want to ensure that the fans are running, use a simple swinging plate with mechanical or optical switches. The reversing time will require some sort of circuit or software.

 

[ScottyUK]

(remember -2" W.C.).

If you're using a differential switch it doesn't care - just reverse the connections. Use one switch for +ve pressure sensing and the other for -ve sensing, and parallel the switch contacts.

Have a look at the following link for an example:

http://europe.hbc.honeywell.com/downloads/EN2R9007.PDF

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One day my ship will come in.
But with my luck, I'll be at the airport!

 

[IRstuff]

Why not a anemometer?

If you want to know air flow, you should measure it directly.



TTFN

 

[jraef]

Here is a little transducer to give you a 1-6VDC output from -2.5 inH2O to +2.5.

http://www.omega.com/ppt/pptsc.asp?ref=PX160

Then run this into a simple setpoint controller like thier Dpi32.

http://www.omega.com/pptst/DPi_Series.html

Program a high alarm setpoint and a low alarm setpoint. Anything inside that band leaves your relay in normal state, anything on either side causes a change in state.

$300 total

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[plcsavvy]

If the speed of the fan is proprtional to the inches of water colum, you can use a tachogenerator.
A DC tachogenerator can output a 0-10VDC signal.
this signal can then be compared to a setpoint value to activate an analog in to relay output device to disconnect power.
below are links to a company that manufactures pressure switches both negative and positive. Thay alo provide a device that accepts a 4-20ma or 0-10vdc analog input and operates a relay when a setpoint is reached. You will also need a time delay relay since you require a 4second delay prior tpo dropping the power.
best regards, plcsavvy

http://www.allaboutcircuits.com/vol_1/chpt_9/4.html

http://www.sentinelpower.com/ACI/dp.html

http://www.sentinelpower.com/ACI/afs.html

 

[MarkRW]

I'm assuming that the reason for the airflow is cooling. Easiest method is to measure the temperature: if the device is getting hot you've got a problem. Temp sensors are cheap.

Otherwise I'd recommend using an air flow sensor instead of pressure sensors. There are a few inexpensive airflow sensors. Some use an internal heater and sense the drop in temperature due to cooling air. The one I saw were PCB mounted: looked like a TO-5 transistor pack.

M Walter

 

[waross]

I was composing a reply and then saw that ScottyUK had already said it.
A couple of extra issues. If the temperature of the mounting location is below the dew point of the flow in the duct, condensation is an issue. Mount the device and the tubes to facilitate drainage.
Years ago, I had some problems with similar devices. They were proving air flow in a recirculating direct fired kiln. This was a code requirement for the heater. The process air was hot and humid. Condensation was an ongoing problem, even with careful installation.
I suggested proving air flow by detecting rotation of the fan shaft. Our Gas licence holder checked with the AHJ and the answer came back that it would be acceptable.
The devices used were designed to detect belt slippage on conveyors in grain elevators. As I remember they included a time delay to allow the conveyor belt to come up to speed.
The point is, if you're trying to prove air-flow, you may be able to do it by proving rotation of the fan shafts.
In support of jraef, Omega has switches also. Check out their web site.
yours

 

[itsmoked]

You want simple, I agree with VE1BLL. Swinging plate. You can use prepackaged optical interrupters or put a magnet on the plate and use two reed switches. Run these to a retriggerable delay-on-break relay that leaves the output state the way you want it unless its input state shifts for more than 4 seconds.

 

[felixc]

Airflow sensors, like MarkRW describes are also worth considering. You could even make one yourself, using two thermistors and a couple of heating resistors and an opamp.

 

[kontiki99]

Maybe you can use some sort of unpowered fan mounted in your plenum to generate a signal.

It could turn either way.

Paint every other blade wite and use an opto coupled pair to reflect off the white blades and create pulses.

Then use a simple counter.

 

[amptramp]

Try Dwyer for low pressure switches:

http://www.dwyer-inst.com