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Phone: (352) 687-0351
    Fax: (352) 687-8925
 
General Information on P.D. Meters

We offer several designs of PD meters:

Oscillating Piston, Nutating Disk and Oval Gear designs.

Meter accuracy

For a discussion of the differences between accuracy statements expressed as % of Full Scale and % of Rate, please see our glossary section.

People think that a flow meter is accurate when they install it. Maybe, but maybe not! It would be extremely unusual for you to install a meter and get exactly the same accuracy results that the factory gave you on the calibration sheet, unless you are running the same fluid and have the same piping arrangement as exists on the manufacturer's test stand.

Lets take a typical large industrial grade positive displacement flowmeter. This meter is calibrated on water, you are running molasses. From experience the fellow at the factory doing the calibration knows that you are running a higher viscosity fluid so he "estimates". He gets his calibration data on water and says "the molasses is going to affect me about 2%, so I'll change my register reduction gearing ratio to match". This change offsets the correct reading (on water) by his estimate. His estimate could be wrong.

The meter is calibrated in a test stand, facing upward, on water or Stoddard Solvent, typically with a circulating pump putting some pulsation's in the flow stream. (Which improves low end performance). Your installation may have a very long pipe between the pump and the flowmeter which dampens the pulsation's out, the meter may be installed in a vertical line, etc. If you can set up a volumetric or gravimetric (weighscale) test, you will get different results from what appears on the calibration card which arrived with the meter.

If you want an accurate meter ... install the meter and then do a volumetric or gravimeteric test. In other words, run fluid for at least 2 minutes at the desired flow calibration into a calibrated tank or into a tank on a weighscale. Correct the meter to match the weighscale reading.

Small meter size designation

You call and say that you want a 3/4" meter for water service. If the person you are talking to knows meters and wants to give you the best possible selection, he will say "Do you want a 5/8 x 3/4 or a 3/4 x 3/4?" The normal reaction is "Why would I want a meter with a 5/8" inlet and a 3/4" outlet?" Fortunately, that is not what these fractions mean.

At one time, before design improvements which have come about in the early '90's, we would tell people:

The first fraction is the chamber capacity:

    5/8 meant a 25 GPM capacity with a 13 psig pressure drop
    3/4 meant a 30 GPM capacity with a 13 psig pressure drop
    1 meant a 50 GPM capacity with a 13 psig pressure drop (seldom used)
The second fraction is the line size the meter couples into, when used with the provided end couplings.

Note that the second fraction is not the meter end connection, but the line size the meter will install into when the coupling connections are used. The cases on meters which comply with AWWA C700 (American Water Works Assoc) have straight threads, which are 1 line size over the size pipe. Here are the actual threads:
    5/8 x 1/2 meter Actual end threads are 3/4 x 14 TPI, NPSM (male)
    5/8 x 3/4 meter Actual end threads are 1" x 11 TPI, NPSM (male)
    3/4 x 3/4 meter Actual end threads are 1" x 11 TPI, NPSM (male)
    1" meter Actual end threads are 1.5 x 11.5 TPI NPSM (male)
Why this straight thread? The meter is designed to be periodically removed and tested. If the meter ends had tapered threads, the connecting pipe coupling rides over the meter thread (interference fit). In order to remove the meter, the connecting pipe must be pushed backwards to disengage the NPT threads. But the pipe is buried under tons of dirt and will not move. By using straight threads and couplings which utilize a gasket to seal the line, one can loosen the coupling nut, spring the line slightly and pull the meter out.

If you try to screw a tapered coupling (FIP, NPT) to the meter, it will leak. Use the couplings offered with the meters.


Metering low viscosity, low lubricity fluids such as mineral spirits, toluene, isopropyl alcohol, acetone, etc.

Typically you will want to use mechanical meters because these fluids are hazardous and you would like to avoid the problems of using explosion proof enclosures on an electronic flow meter.

Viscosity considerations and the lack of capillary sealing:

Mechanical meters are tried and true devices, but they need to be applied with some concern given to the fluid characteristics. A mechanical meter which has an oscillating piston, nutating disc (wobble plate) or similar measuring element is termed a positive displacement meter. However, what makes it "positive" is capillary sealing between the measuring element and the side wall or the meshing components. If you are dealing with a low viscosity fluid, i.e. alcohol, toluene, MEK, etc. you lack the viscosity to develop a good capillary seal. This will manifest itself in leakage around the measuring element. This leakage can be substantial. In general, the larger the meter, the greater the mass of the measuring element and the greater the leakage.

Low lubricity and pressure drop:

Positive displacement meters have surfaces in the measuring chamber which touch each other. In the nutating disc (wobble plate) the center ball rides in a shaped cup, in oscillating piston meters the center pin is in contact with the roller and the piston diaphragm rides on a hub. This means that a non-lubricating fluid allows these parts to drag on each other which increases the frictional loading. This loading creates higher pressure drops.

There is a intermediate product ... The oval gear meter is a positive displacement device using dual oval shaped rotors. The rotors inherently provide better sealing because their gear design traps a pocket of fluid right at the capillary sealing point, plus the gear ends are large and have a small gap of about 0.001" to the meter body. Small oval gear meters do not develop enough torque to drive a mechanical register and require electronic displays, however, larger units ranged to 40 GPM and higher can be equipped with mechanical registers. Oval gear meters have lower flow ranges on a per line size basis and higher costs than a turbine meter.


Controlling pressure drop - derating meters for high viscosity fluids

There are many ways to measure viscosity and consequently many ways to express it. Most flowmeters use centiPoise (cP) or centiStokes (cStks) to define pressure drops. If you need help in converting viscosity numbers provide us with the specific gravity or density (English).

Crane (the valve people) publishes the ASTM equivalents of Kinematic (cStks), Saybolt Universal, Saybolt Furol and Absolute (cP) in a nomograph. This, along with a fine collection of associated data, is published in the Engineering section of their sales manual which is widely distributed.

PD meters will work over wide viscosity ranges, with minimal accuracy shifts. It is a rare PD meter which will show greater than a +1.5% accuracy shift with increasing viscosity. However, the pressure drop rises quickly as viscosity goes up. To reduce the pressure drop you choose a larger meter. To get the proper size meter you must derate the maximum recommended flow statement in the literature. Many derating curves are in the literature, but as a rule of thumb you can:

Take the maximum continuous flow rating and multiply it by:
    500 cP ............ multiply by 0.7
    1000 cP .......... multiply by 0.55
    2000 cP .......... multiply by 0.4
    4000 cP .......... multiply by 0.3
    10000 cP ........ multiply by 0.2

Volumetric compensation


Let's say that you unload 2,000 gallons of a hydrocarbon base oil that comes off the tanker at 190 deg F. The meter says it saw 2000 gallons come in. But the next day, when the oil has cooled down to 140 deg F, the tank level gauge says that you only added 1950 gallons -> you've lost 50 Gallons someplace, the meter must be off.

Both the meter and the tank level gauge are correct. The oil has a expansion factor of about 5 parts in 10,000 per degree F. When your temperature dropped the oil contracted.

We can provide a temperature sensor and mass flow display which will correct for the temperature variations, always providing a reading to some base (reference) temperature.


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Controls Warehouse  •  356 Cypress Road  •  Ocala, FL  •  34472 •  Phone: (352) 687-0351