FAQ ΔP Flowmeters

The Ten Most Commonly Asked Questions About
ΔP Venturi Flowmeters

1. What ΔP will the venturi create?

Fox ΔP Venturies are built to specification. You can therefore request whatever inlet-to-throat pressure differential best suits your application. If unrecoverable pressure loss must be minimized, you can specify a low maximum inlet-to-throat ΔP, such as 25” H2O. To match the range of standard ΔP transducers, max ΔP’s like 50”,100”, or 200” WC or – 5 psid are commonly specified. For high pressure systems, ΔP’s can be as high as 25 – 50 psid.

2. What is the unrecoverable pressure loss through the ΔP Venturi Flowmeter?

Typically, this is about 12% – 18% of the measured ΔP, depending on line size (smaller venturies – below about 1”- have low end of this range.) Therefore, if your maximum inlet-to-throat ΔP is 50” WC, your max. unrecoveable (inlet-to-outlet) loss is only 8 – 9” WC – very low!

3. What is the working range?

We consider a normal working range at about 5:1. This is determined not by the venturi, but by the ΔP instrumentation. Since flow is a function of √ΔP, to achieve a 5:1 range in flow requires a 25:1 range of ΔP. A transducer with a max ΔP of 100” WC and accurate to ±1/2% full scale will record, at 1/5th of max flow, a ΔP of 4” H2O ±0.5”. That is not a very accurate reading. Venturi flow range can usually only be extended by use of two ΔP transducers.

4. Can Fox provide instrumentation with the ΔP Venturi?

We can, but we usually prefer to let your control people buy the transducer, signal conditioner, etc.

5. What accuracy can we get with a ΔP Venturi?

Fox can calibrate these flowmeters with water or air to about 1/2%, and traceable to NIST. This test data can then theoretically be converted to your liquid or gas and actual design conditions.

6. What special installation constraints should we consider in the piping layout? How much straight pipe is needed?

The fluid entering a ΔP venturi flowmeter must be well-conditioned – that is, with no turbulence so the pressure taps can provide meaningful, consistent data. Standard practice is to have 7 – 10 diameters of straight pipe upstream, and 3 – 5 downstream. Small throat diameters, where β<0.3 (β, or Beta-ratio, is the ratio of throat diameter to pipe ID) are less sensitive than larger diameters. If these lengths of straight pipe are not available, then flow straighteners can be installed to condition fluid flow before the venturi.

7. What type of end connections can I get?

Whatever you need. Fox can supply whatever end connections permit easy installation in your system. This includes flanges, NPT threads, AN ends, Naval fittings, Grayloc hubs, butt weld, socket weld, etc.

8. Can we use ΔP flowmeters with cryogenic fluids, such as LH2, LHe, LOx, etc.?

This is one of the most common applications for Fox Venturi Flowmeters!

9. We have a control loop and wish to use the flowmeter to measure flow, which sends a signal to a computer which actuates a control valve, so we get the flow we need. Is this a good application?

A flowmeter will work in this application, but a cavitating venturi may work even better. With a fixed upstream pressure, cavitating venturies (liquids) or sonic chokes (gasses) will deliver an accurately controlled flow rate independent of any downstream fluctations.

10. Are they available from stock?

No. Every ΔP Venturi Flowmeter is built to order, with a delivery of about 4 – 6 weeks. Available materials of construction are stainless, brass, monel, and titanium. Standard styles include flange insert, tube insert, flanged, butt weld, and socket weld.