FAQ Sonic Choke

The Eight Most Common Questions asked about Fox Sonic Chokes or Critical Flow Venturis

1. Why should I care if there is a shock wave at the throat of the venturi?

Because once that shock is established, nothing that happens downstream can influence the flow pattern going into the venturi. (No downstream perturbations can propagate upstream). That means that once you have locked the venturi into one flow rate, it is completely unaffected by anything downstream, unlike ΔP devices. Flow rate is now fixed, stable, and repeatable and is solely a function of upstream pressure, not ΔP!

2. Downstream pressure fluctuations have absolutely no effect on flow?

Sorry. As long as downstream pressures stay below 88% of upstream pressure, they can fluctuate wildly, while flow rates remain constant, stable, repeatable. This is what is meant by Recovery – the maximum ratio of downstream to upstream pressures (in psia) where flow through the nozzle remains choked.

3. So critical flow Venturis convert a pressure regulator into a flow regulator?

Correct. Once you know the characteristic curve of the nozzle, you can crank in any flow you want simply by setting the inlet pressure to the nozzle. Downstream conditions are irrelevant.

4. How accurately can they control flows?

Fox can guarantee theoretical flow rates through our nozzles to ±2% , although overall system accuracy will depend upon your pressure controls and instrumentation. NBS-traceable calibrations are available and are accurate to ±1/2%.

5. But I thought I really wanted to meter flow?

Many of our callers are looking to meter flow so they can use a control valve to set, or control, a predetermined flow rate. They really want to regulate flow, and use a control valve with a flowmeter to do so. How about simply regulating flow rate with such high accuracy that it doesn’t need to be metered?

6. Why don’t we hear more about them in the control and instrumentation publications?

Chokes have been around since Bernoulli so they are not new. And they’re inexpensive. You don’t need a lot of feedback and controls to generate a stable flow rate. Its as simple as you can get. Maybe that’s the reason.

7. How long has Fox Valve been selling them?

Since 1961. That’s a long time.

8. Are they available from stock?

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