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Positive Displacement Meters and PD Meters: High-Viscosity Tolerant Mechanical Gear Technology

Quick Answer: Positive displacement meters, also called PD meters, trap fixed fluid volumes between rotating mechanical parts and count each pocket. Oval gear PD meters tolerate high viscosity because the sealing path stays short and the gears move at a speed tied directly to flow volume. Silver Instruments supplies oval gear PD meters from DN10 to DN200 for fuels, oils, syrups, resins, and chemical batching with 4-20 mA HART output and local totalizers.

How Positive Displacement Meters Actually Measure Flow

A positive displacement meter does not need flow profile conditioning. A pair of oval shaped gears rotates inside a precision chamber. Each rotation moves a known volume. A sensor counts rotations through a pickup and converts the count into flow rate and total. In a DN25 meter, one full rotation may pass about 0.05 litres. The output is linear for volumetric flow, so calibration is often a single factor.

Here is the thing. The measurement is not based on velocity. The reading is based on volume. That is why PD meters remain stable on thick fluids where electromagnetic or ultrasonic signals struggle. Most engineers choose a PD meter when the fluid is too viscous for a turbine meter or when pipe straight run is limited.

Why PD Meters Handle High Viscosity

High viscosity fluids create two problems for velocity based meters. The velocity profile becomes flat or irregular, and the meter may slip internally. The high viscosity tolerant mechanical gear technology in an oval gear PD meter comes from tight clearances and short sealing paths. The fluid cannot pass without rotating the gears. With a 1000 cP oil or a 5000 cP syrup, the oval gear design still registers volume with an accuracy around plus or minus 0.5 percent of reading in many cases.

We have seen this on customer sites many times. A diesel flow meter with turbine blades may read high when fuel is cold and thick. An oval gear PD meter on the same line holds a flat error curve from 20 cP to 5000 cP if the bearings and gear material are selected correctly.

Last year a customer in Vietnam asked us to replace a turbine meter on alkyd resin at 780 cP. The turbine meter read low by 6 percent. A DN25 oval gear meter with PPS gears brought the batch error back within plus or minus 0.5 percent.

Mechanical Gear Technology in Oval Gear PD Meters

Oval gear meters use a pair of matched gears. The gears can be made from PPS, aluminum, cast iron, or stainless steel. PPS gears suit solvents and low temperature fuels. Cast iron gears suit lubricating oils. Stainless steel gears suit food grade liquids and corrosive chemicals. Bearing choice matters. Ball bearings fit clean oils. Sleeve bearings handle low lubricity liquids but require lower pressure drops.

Pulse output is common. Many Silver Instruments PD meters also support 4-20 mA HART, RS485 Modbus, and local batch control. For hazardous areas, we supply ATEX Zone 1 configurations with Ex d or Ex ia pickups. The mechanical register is still an option where no power is available.

Typical Applications by Industry

Fuel oil measurement for generators and boilers is one major use. A mining site in Western Australia ordered DN40 oval gear meters to measure diesel consumption from day tanks. A palm oil processor in Indonesia uses DN50 cast iron PD meters for crude palm oil transfer at 80 degrees Celsius. A food factory in Thailand measures glucose syrup with stainless steel oval gear meters cleaned by hot water flush.

Chemical dosing is another field. Because PD meters output a volume pulse, the meters work well with batch controllers. A small additive skid can use a DN10 PD meter for 5 to 50 kg/h of corrosion inhibitor. The meter sends a pulse every 0.01 litre and the PLC stops the pump at the target.