Positive displacement Oval gear flow meter working and installation and troubleshooting

Principle of operation

Oval gear meters are positive displacement-type volume meters that can transfer specified incremental volumes of fluids between individual measuring chambers.

The measuring element is two high precision toothed oval gears, that are driven by the fluid flow and mesh with each other. Thus, a defined volume is transported for each rotation of the oval gears. The number of rotations gives the total amount of fluid that has passed through the meter which is detected by a sensor element.

Permanent magnets in the rotating gears can transmit a signal to an electric reed switch or current transducer for flow measurement.

Advantages of oval gear flow meter

  • Positive displacement meter for volumetric flow rate or total flow measurement

  • No inlet or outlet section required

  • High-quality construction for long service life and high reliability

  • Long-term stability

  • High measurement accuracy and repeatability

  • Easy installation

Application of oval gear flow meter

Lubrication oils, mineral oils, hydraulic oils, fuels, liquified gases, Solvents, Chemicals, Additives etc

Characteristics of oval gear flow meter

  • Sensor with pulse output signal, no local display

  • Flow rate or total flow indication by local or remote display

  • Individual calibration

  • Various versions of local displays are available: battery powered (lifetime approx. 3 years) or externally powered version with analogue and pulse output

  • Female threaded or flanged process connection

  • O-ring material FKM, EPDM or FEP

Oval Gear Flow Sensor Installation

Points to be noted before installation


Filters are fitted for a clear homogeneous fluid flow. Filters with Y or Basket type mesh strainer around 80 microns are used. A dummy section of pipe inserted and flushed before installing the meter thus eliminating debris. a strainer installed should be regularly inspected and cleaned. Failure to keep the strainer clean will drastically effect flow meter performance.


Fluid to be used should be compatible with the meter. Refering to industry fluid compatibility charts is mandatory before installation


Reverse flow should be eliminated so the pipe line is designed accordingly. High fluid flow during start-ups causes damage to the system. To avoid pressure on meter by the pipe, isolation valves or bypass valves are provided. Slow fluid flow is recommended initially as air present in the pipe will damage the meter. Cavitation should also be avoided when possible.


Oval gear flow sensors working using the low differential pressure generated across the body to drive a pair of oval gears. The reed switch can cause inaccurate rate counts at high speeds so they can be used only for low speed counters. For high speeds rotation can be detected by either a TTL (NPN transistor) or contact closure pulse output.

The performance improves with increasing viscosity achieving an accuracy of ±0.1% of reading with higher viscosities. The standard meters can be used up to 1000 cP; above this viscosity specially profiled gears must be used.

Installation procedure


  1. A bypass line included in the design facilitates for a meter to be removed for maintenance without interrupting production.
  2. Thread sealant used ,to eliminate leakage and introduction of air, on all pipe threads.
  3. Pipe work and Meter should have the working pressure rating to match the pressure output of the pump.
  4. Install a wire mesh strainer, Y or basket type as close as possible to the inlet side of the meter. (Meter 1/4” - 74 micron / 200 mesh; Meter 1” - 250 micron / 60 mesh)
  5. Meter installed in any orientation as long as the meter shafts are in a horizontal plane. Incorrect installation can cause premature wear of meter


  1. Do not over tighten meter connections
  2. Line filled slowly after initial installation to prevent high speed air purge.
  3. Test for leakage
  4. Check the strainer for swarf or foreign material, after the first 200 litres check periodically, particularly if the flow rate is decreasing.

Maintenance Procedures


The fluid supply to the meter is disconnected, and the line pressure is released before disassembly. For maintenance of the LC Display or PCB, the meter is isolated from the pipe, else it is not removed. The LC display may removed by loosening the 4 mounting screws and be orientated as required.

**image **

Item No Part Description
1 Meter body
2 Meter Cap (PCB)
3 Legend Plate
4 Meter Cap Mounting Screws
5 & 6 Rotors
7 0-Ring
  1. Pulse Caps Models: Undo the conduit connector, remove pulse cap (item 9) and remove the wires from the pulse terminal board (item 5)
  2. Standard LC Display: Mark the display orientation with a marking pen, unscrew the four large screws on top of the LC Display. Carefully separate the LC Display from the plastic housing and disconnect the wires from the pulse terminal block. Remove the mounting adaptor plate and gasket.
  3. Loosen the cap head screws (Item 7) that hold down the meter cap (Item 4), remove the screws, washers and lift off the cap.


Item No. Part Description
1 Meter Body
2 0-Ring
3 Rotors
4 Meter Cap
5 Printed Circuit Board
6 PCB Mounting Screws
7 Meter Cap Screws
8 Pulser Cap Gasket
9 Pulser Cap
10 Pulser Cap Screws
  1. Remove the o-ring (Assembly Item 2) from the oring groove in the meter cap (Assembly Item 4).
  2. Remove rotors (Item 3).


  1. Before reassembling the condition of the Rotors is checked and replaced if necessary,

  2. Check that the smooth side of the rotors (not the plug side) is facing you when inserting the rotors, the smooth side of the rotor is the magnet side.There is no difference between rotor one or rotor two.

  3. Replace the rotors (Item 3) onto the shafts at 90 degrees to each other and check their operation by turning either of the rotors. If the rotors are not in mesh correctly or do not move freely, remove one of the rotors and replace correctly at 90 degrees to the other rotor.


  1. Re-check the operation of the rotors

  2. Replace the o’ring (Item 2) into groove in the meter cap, if the o’ring has grown or is damaged in any way replace it with a new part.

  3. Replace the meter cap making sure that the locating pin in the body lines up with the hole in the meter cap.

  4. Insert the cap head screws (Item 7) and tighten in a diagonal sequence 1, 3, 2, 4, etc

  5. The replacement of cables and connectors are a reversal of the disassembly procedure, replace conduit fitting if required. When replacing the Standard LC Display confirm the orientation marks made on disassembly are aligned then screw the register into place.

  6. Test the meter by turning the rotors with a finger or by applying very low air pressure (no more than a good breath) to one end of the meter, before returning the meter to the line.

Troubleshooting Guide

Problem Cause Remedy
Fluid will not flowthrough meter a) Foreign matter blocking rotors b) Line strainer blocked c) Damaged rotors d) Meter connections over tightened e) Fluid is too viscous a) Dismantle meter, clean rotors (strainer must be fitted in line) b) Clean strainer c) Replace rotors (Strainer must be fitted in line) d) Re-adjust connections e) See specifications for maximum viscosity
Reduced flow through meter a) Strainer is partially blocked b) Fluid is too viscous a) Clean strainer b) See specifications for maximum viscosity
Meter reading inaccurate a) Fluid flow rate is too high or too low b) Air in fluid c) Excess wear caused by incorrect installation a) See specifications for minimum and maximum flow rates b) Bleed air from system c) Check meter body and rotors. Replace as required.
Meter not giving a pulse signal a) Faulty hall effect sensor b) Faulty reed switch c) Magnets failed a) Replace PCB Board b) Replace PCB Board c) Replace magnets
LCD register not working a) Battery not connected properly b) Battery flat c) Faulty wiring connections d) Faulty LC Display e) Faulty connection from LC Display a) Check battery connections b) Replace battery c) Check wiring for loose or faulty connections d) Replace LC Display e) Check wiring connections