FAQ

FAQ’s for Cox Website

What is the difference between the Piro-Flowgages (Piro-Velocitygage) and the Hall Tube or the Collins Tube?

The Piro-Flowgages, including the Piro-Velocitygage are Rotameter style meters, with a sample probe located at the center of the pipe. They operate on the velocity of the flow which enters the calibrated tube and suspends a floater in the tube, from which the meter reading is taken.
The Hall Tube and the Collins-Style Tube are pitot tubes. They operate by sensing the differential pressure in the line (the “static” pressure vs. the “dynamic” or “flow” pressure). This shows up in a vertical inverted u-tube water manometer as two columns of water of differing height. The difference in the heights of the two columns is translated into a flow reading.


Are the Piro-Flowgages more or less accurate than the Hall or Collins Tubes?

It is best said that the Hall and Collins-Style tubes are more accurate across a wide variety of conditions. If the reading is taken in an “ideal” location (10 diameters downstream of an obstruction, and 4 diameters upstream), the readings of all the instruments will be virtually identical, within the stated accuracy of ±2%. But the Piro-Flowgages are a singlepoint sampling meter, while the Hall Tube and Collins-Style tube take readings across the inside diameter. In less than ideal conditions, the Hall and Collins-Style meters are more accurate.


The State of California has a pump efficiency program that rebates a part of the cost of a pump efficiency test. Are all of your meters recognized by the State of California as qualified to make these tests?

The Hall and Collins-Style tubes are recognized by the Center for Irrigation Technology (CIT) at California State University in Fresno, and by the Irrigation Training and Research Center (ITRC) at California Polytechnic State University in San Luis Obispo. These two institutions manage the state pump efficiency testing program.
Neither of these bodies recognize the Piro- Flowgages as qualified instruments. Their concern is the repeatability of the instrument in the hands of un-qualified personnel. C.W. Cox remains fully confident in the accuracy of the Piro-Flowgages, but we understand the caution of these two organizations in providing their recommendations to the State of California.


What is the accuracy of your flowmeters?

The stated accuracy of the flowmeters is ±2% of any reading on the scale. This is based on the meter being in an “ideal” location (10 diameters downstream of an obstruction, and
4 diameters upstream).


What is the accuracy of the flowmeters if the meter in not in an “ideal” location?

Because of the many and varied situations where a meter reading could be taken, there is no single answer to this question. There is a study done by the University of California at Davis that tested our meters and many others in a wide variety of locations and next to or near a variety of obstructions. Our meters were more accurate than any of the others in all cases. But the accuracy varied with each condition. This study is available from C.W. Cox on request.


How do you know the accuracy of the flowmeters?

The Hall and Collins-Style flowmeters have been tested at the Center for Irrigation Technology (CIT) at California State University in Fresno, and by the Irrigation Training and Research Center (ITRC) at California Polytechnic State University in San Luis Obispo. Both tubes were also the object of a study at the University of California at Davis. The Hall Tube was also tested at the University of Southern California. Finally, we have a test stand in-house with which we calibrate many of the instruments.


What is the difference between the Hall Tube and the Collins Tube?

The Hall tube is a self-averaging pitot tube with impact holes (openings in the tube facing the oncoming flow) all across the inside diameter of the pipe. The reading sent to the manometer is an averaged flowrate. The Hall Tube is inserted through a single drilled and tapped hole in the pipe.

The Collins-Style tube is a single point pitottube, which is positioned at key points across the inside diameter of the pipe to obtain an averaged result. The readings are usually taken at 2, 6 or 10 separate points along the inside diameter. Each of these readings is averaged with the others by the user to obtain an averaged flowrate. The Collins-Style tube is inserted through the pipe via two drilled and tapped holes in the pipe, 180° apart in the side of the pipe.


Can the Hall and Collins Tubes be inserted into pressurized lines for a “hot” test (the water pump is not turned off)?

It depends on how wet you want to get.
The Hall tube can be inserted through a valved manifold the side of the pipe. It requires a “Long” version of the tube.
The Collins-Style tube cannot be inserted into a pressurized pipe without getting quite wet.


What is the difference between a “Long” Hall Tube and a “Short” Hall Tube?

The Long Tube is used for testing pressurized pipe (the water pump is not turned off). It is physically longer than the Short Tube, in order to account for the manifold on the outside of the pipe in which the tube is inserted.


Can you test a non-pressurized line with a Long Tube?
Yes.


What do the different sizes on the Hall and Collins Tubes mean?

The Hall tubes are made to service a range of pipe sizes. The #2 tube for instance will test inside diameter sizes from 6″ to 12″. The #3 tube will test 13″ to 18″, and so on. All of the tubes use the same inverted U-Tube manometer, so a tester can own a #2 complete set and then purchase different size tubes separately to expand his test range.
The Collins-Style tubes come in18″, 24″, 36″, 44″ and 60″ long. Each tube has a range of pipe sizes it will service.
Nominally, the ranges are these:
18″    Up to 4″ pipe
24″    Up to 6″ pipe
36″    Up to 12″ pipe
44″    Up to 18″ pipe
60″    Up to 26″ pipe


If I already have a manometer for the Collins Tube, can I use it with the Hall Tube? [Or Vise-Versa]

No. The scales on the Hall Tube manometer are based on Gallons-Per-Minute-Per-in2-of-Inside-Diameter. The Scale on the Collins-Style tube is based on Gallons-Per-Minute-Per-Ft/Sec of water velocity. The two scales are completely incompatible with each other.


Can I get a manometer scaled in Inches or centimeters?

Yes. But there is an additional cost involved.


What is the scale range of the flowmeters?

The basic range on the flowmeters is from 2ft/sec to 10ft/sec.
The Hall Tube scale is based on this 10ft/sec rating, but may be increased using a manometer extension to perhaps 14 or 15ft/sec.
The Collins-Style scale has an upper limit of 11ft/sec, but also may be increased using a manometer extension to 14 or 15 ft/sec.
The Pyro-flowgages may be increased to perhaps 17 ft/sec, but this involves testing outside of our facilities and therefore involves additional costs.
Finally, the Pyro-flowgages do not work well at scales less than 2 ft/sec.



How does the Well Sounder work?
The well sounder is a simple circuit that reads current flow from one side of a battery to the other. In the Single Wire variety, one pole of the battery is attached to the metallic well casing. The other side of the battery is attached to the wire that is lowered into the well casing. When the wire touches the water, the circuit is closed and the voltmeter indicates a connection. The amount of wire that is lowered into the well indicates the depth of the well.
In the double wire variety, the battery is simply connected to each of two wires that are both lowered into the well casing. When both wires touch the water the circuit is complete and the voltmeter indicates a connection.


What is a Prandtl Tube?
A Prandle tube (also called an “Overhung” tube) is an L-shaped pitot tube that works the same basic way as the Collins-Style tube, except that it is inserted into the pipe through a single drilled and tapped hole in the pipe.
The Prandtl tube uses the same manometer and scale as the Collins-Style tube with a multiplier added into the equation.


What is an Overhung Tube?
See the question “What is a Prandtl Tube”.


Can your flowmeters be used in open channel flow?
No.


Can your flowmeters be used in pipe that is not flowing full (partially full pipes)?
No.


The manometer for my Hall Tube won’t balance in the “Bypass” mode.
There are a couple of reasons for that.
1. There is air trapped in the hoses that connect the Hall Tube to the manometer. This needs to be purged out. Refer to the directions for use for the Hall Tube.
2. The Hall tube isn’t turned 90° to the flow.
3. The Valves on the Hall Tube Head are not in the right positions. Refer to the directions for use of the Hall Tube.
4. There is a leak in the inside tube of the Hall Tube. This must be repaired at our offices.


My Pyro-Flowgage (Piro-Velocitygage) all of a sudden started reading much lower than expected.
This is most likely dirt getting logged in the calibration tube. This is the small hole that connects the tapered hole and the straight hole near the top of the instrument. This can be cleaned out by the user but whatever you do, DO NOT try to adjust the calibration screw. This will render your flowmeter useless.
Remove the top bleed valve. Remove the floater assembly. With a small wire, rod out the end of the calibration tube. The use compressed air to blow out anything left in the tube. Replace the floater assembly and the bleed valve. DO NOT tighted the bleed valve too much. When the o-ring sets (is visible on the underside of the plastic body), stop tightening. Observe the floater assembly. If it begins to bow, back off until the bow goes away.


What size fitting do your meters fit into?

The Piro-Flowgages and the #1 Hall Tube are all made to fit into a 1/2″ Female Iron Pipe fitting.
The Hall tube sized #2 through #6 fit into a 3/4″ Male Iron Pipe fitting. Sized above #6 fit into a 1″ Female Iron Pipe fitting.
The Collins-Style tube fits through a 1/4″ Female Iron Pipe fitting.
The Overhung (Prandtl) tube fits through a 3/8″ Female Iron Pipe Fitting.


What is a Corporation Stop (Corporation Cock)?

A Corporation Stop is a male end, threaded 1/4-turn, full port plug valve. A full port ball valve may be used in the place of a corporation stop. A standard port ball valve will not work.


Can a Piro-Flowgage meter for Vertical Pipe be used on the side of a horizontal pipe?

Yes, but the flow hole has to point upstream. If this is the application you want, it is best to sketch a picture of the application so we can customize the position of the flowhole.


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