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Displacement Type Flow Meter: Positive Displacement Volumetric Chamber Senses Fixed Fluid Volumes
Quick Answer: A positive displacement flow meter with a volumetric chamber senses fixed fluid volumes directly. It does not depend on velocity profile or turbulence. For diesel, lube oil, chemicals, and food liquids, this design gives stable batch totals even when viscosity changes.
As a displacement type flow meter, this instrument counts physical cavities. Each cavity has a known volume. The rotating element moves only when liquid passes through. That direct counting is the core advantage. There is no need for long straight pipe runs.
How a Positive Displacement Volumetric Chamber Works
Inside the meter body, a rotating element forms a moving seal against the chamber wall. The element can be an oval gear, a gear pair, a rotary piston, or a nutating disc. Each rotation sweeps a set volume from the inlet to the outlet. A sensor on the rotor detects each pass. The transmitter converts the pulse train into flow rate and total flow.
The positive displacement volumetric chamber senses actual volume, not velocity. A chamber of 50 mL per revolution still moves 50 mL per revolution whether the liquid is water or heavy fuel oil. Temperature and pressure change the liquid density. The chamber volume remains fixed. The meter reports volume, not mass.
Why This Matters for Viscosity and Low Flow
Turbine meters and vortex meters lose accuracy at low flow. Magnetic flow meters require conductive liquid, often above 5 µS/cm, and full pipe conditions. A positive displacement flow meter does not need a fully developed velocity profile. Low Reynolds numbers do not break the reading. Slip across the clearances is the main error source. Thin liquids such as solvents can slip more at low speed. Thick liquids such as resins, molasses, and lube oil reduce slip and often improve accuracy.
We have seen this on customer sites many times. A paint manufacturer in Vietnam ran batch viscosity from 80 cP to 400 cP. Their turbine meter error drifted to 3 percent. An oval gear PD meter held 0.5 percent across the same range. The customer then removed three flow conditioners from the skid. The pipe became shorter and easier to clean.
Common Applications and Materials
Positive displacement flow meters are common on diesel transfer in mining fleets in Chile and Peru. Marine fuel oil measurement in Singapore and the UAE also uses PD meters because heavy fuel oil viscosity can reach 380 cSt at 50 °C. Palm oil batching in Malaysia and Indonesia uses stainless steel oval gear meters with Tri-Clamp connections. Chemical dosing in Saudi Arabia water plants uses PTFE lined chambers for corrosive additives.
Silver Automation Instruments (Silver Instruments) supplies positive displacement flow meters from DN10 to DN200. Body materials include cast iron, 316L stainless steel, aluminum, and PPS. Seal materials include PTFE, FFKM, and Viton. Process connections include threaded BSP, NPT, DIN flang
Key Specifications to Specify
Do not send us a one line request. We need this data to select the right positive displacement flow meter. Fluid name, flow range, operating pressure in bar, operating temperature in °C, pipe size in DN, viscosity in cP or cSt, and connection standard. For diesel, typical data is 20 to 200 L/h, 3 to 6 cP at 25 °C, DN15, PN16. For heavy fuel oil, the data might be 500 to 5000 L/h, 180 cSt at 50 °C, DN50, PN40.
Standard outputs include pulse, 4-20 mA HART, and RS485 Modbus RTU. A PT100 temperature sensor can be built in. For hazardous areas, we can supply ATEX Zone 1 or IECEx versions. Accuracy is typically 0.5 percent of reading. Repeatability is 0.1 percent or better. If you need mass flow in kg/h, send us density at operating temperature. The transmitter can calculate mass from volume when density is stable.
FAQ: Positive Displacement Flow Meter Selection
Q: What is the minimum flow for a DN15 positive displacement flow meter?
A: Minimum flow depends on viscosity and accuracy class. A DN15 oval gear meter can often read 2 to 20 L/h for diesel. Give us the viscosity in cP and we will confirm the lower limit from the factory curve.
Q: Can I use a PD flow meter for diesel with 4-20 mA HART?
A: Yes. Silver Automation Instruments supplies diesel oval gear meters with pulse plus 4-20 mA HART. We also add a local LCD totalizer if you need operator reading on the skid.
Q: Is straight pipe required before the meter?
A: No. Positive displacement flow meters do not require long straight pipe runs. You can install them after elbows, valves, or strainers. Keep the line full and install a filter upstream to protect the chamber from particles.
Q: Which material is best for food grade palm oil?
A: 316L stainless steel body with PTFE seals and Tri-Clamp connections. This material set suits palm oil, edible oil, and other food liquids. Avoid aluminum and cast iron for direct food contact.
Q: How does viscosity affect calibration?
A: PD meters are usually calibrated on water or oil. High viscosity reduces slip and can improve low flow performance. Very high viscosity increases pressure drop. We need the operating cP value so the chamber clearance and rotor type match the application.
Get a Specific Quote from Silver Automation Instruments
Send us your pressure in bar, temperature in °C, pipe size in DN, flow range, and viscosity in cP. Also include the connection type and output signal. We will reply with a model recommendation, price, and delivery time. Our team answers technical questions on Whatsapp and WeChat for Southeast Asia, the Middle East, Latin America, Africa, and Oceania.
Tel: +86-25-68650347
Whatsapp: +86-25-52155837
WeChat: +86 15365082610
Product pages: flow-meter.com.au
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