1. How do oval gear flow meters work?
Oval gear flow meters (also known as oval gear flow sensors) are positive displacement devices used for measuring the volume flow rate of clean liquids. The sensor is built around a similar concept to a lobe pump operating in reverse. The sensor housing contains two oval gears, meshed together so that neither gear can rotate independently of the other. The force of the liquid at the inlet causes the gears to rotate. As they rotate, each gear traps a precise volume of liquid between itself and housing, releasing the liquid at the outlet side of the flow meter. The meshing of the gears prevents back-flow of liquid from the outlet to the inlet. There is never an open flow path from the inlet to the outlet. With each full rotation, each gear twice traps a precise volume of liquid and releases it at the outlet side. The volume flow rate can therefore be determined by measuring the rotational speed of the oval gears. The speed of rotation is commonly measured using a Hall effect sensor which counts the number of half or full rotations of one of the gears and outputs a square wave output signal.
Oval gear flow meters are bi-directional, meaning that they can measure flow in both directions. However, a flow meter with a single Hall effect sensor cannot determine the direction of flow. To determine direction, some oval gear flow meters include a second Hall effect sensor, mounted 90° out of phase so that the two square wave outputs are also 90° out of phase. The flow direction can be determined by assessing which square wave leads the other. This is the same method used by angular encoders to determine the direction of rotation.
2. General characteristics of oval gear flow meters
Oval gear flow meters are positive displacement flow meter which use two oval gears to precisely meter liquid flow. They are highly accurate sensors, capable of measuring the flow of liquids (but not gasses) over a very large range of flow rates and viscosities, including extremely high viscosities. Oval gear flow meters are not able to measure flows of less than about 3% of their maximum allowable flow rate (i.e. a turndown ratio of about 1:30). Furthermore, their accuracy declines at low viscosity due to leakage and they are not suitable for use with liquids that contain solid particulates (it is recommended that a filter is placed at the sensor inlet to trap particulates). Oval gear flow meters are available in a variety of materials including stainless steel, aluminum and PEEK, the choice of which effects their maximum operating temperature and pressure as well as their chemical compatibility. Most oval gear flow meters output a square wave output signal whose frequency is proportional to the flow rate.
3. Input and output signals
Oval gear flow meters require a 10-30 Vdc supply voltage. The supply voltage is used to power the Hall effect sensor and built in electronics. The output signal is a square wave whose frequency is proportional to the flow rate. Some sensors provide a 4-20 mA analog output signal. Other sensors produce two square wave outputs, which doubles resolution and enables flow direction to be determined. Square wave signals normally require an external pull up or pull down resistor for preventing a floating output (see our application tips).
4. Applications of oval gear flow meters
Oval gear flow meters are commonly used flow sensors for accurate metering of medium to high viscosity liquids over a range of flow rates. Typical applications can be found within the food, chemicals, pharmaceuticals and oil industries.
5. Typical specification
|Cost||Medium to high|
|Measurement range||0.01-0.3 L/min to 3-100 m3/min)|
|Liquid temperature||10 to 120 ° C (up to 280 ° C possible)|
|Max. pressure||10-700 bar|
|Supply voltage||10-30 Vdc|
|Output signal||Single/double square wave, 4-20 mA|
|Passive / active||Active|
|Contact / non-contact||Contact|
6. Purchasing tips
- Material compatibility: For applications with corrosive liquids, it is necessary to assess the chemical compatibility of the materials from which the flow meter is made. Common housing materials include Stainless steel and Aluminum. Common gear materials include Stainless steel and PEEK. The housing also includes a liquid seal, typically made of Viton, EPDM, or FKM. The choice of materials also affects the maximum operating pressure and temperature, though these are already accounted for in the sensor’s specification sheet.
- Uni/bi-directional: Oval gear flow meters can measure liquid flow in either direction. However, most are not able to determine the direction of flow. Those that can, do so by detecting gear position at two locations, 90° out of phase with one another, and outputting two square waves, A and B. Flow direction is determined by assessing which square wave leads the other.
- Pressure drop: Oval gear flow meters cause a pressure drop within the process liquid. The magnitude of the pressure drop is highly dependent on viscosity. At low viscosity (e.g. water) the pressure drop is usually very small. However, at high viscosity (e.g. syrup), pressure drops can exceed 1 bar, particularly when operating close to the sensors maximum flow rate.
- Viscosity: The typical range of liquid viscosity for oval gear flow meters is 1-100,000 cSt, though models are available that are compatible with viscosity as low as 0.3 cSt and as high as 1,000,000 cSt (see list of liquid viscosity). Improved accuracy is obtained at high viscosity because there is less leakage of liquid past the gears.
- Output signal: Most Oval gear flow meters output a single square wave signal. There are sensors available with 2 square wave signals (improving resolution and enabling flow direction to be determined) and sensors with analog 4-20 mA outputs.
- Turndown ratio: The turndown ratio is the ratio between the minimum and maximum measurable flow rates. Oval gear flow meters are typically able to measure minimum flows of about 3% of their maximum flow rate (i.e. turndown ratio of 1:30). This is a larger turndown ratio than that of turbine flow meters.
- K factor: The K factor is the number of output pulses (i.e. rising edges of the square wave) per unit flow volume. It has units of pulses per liter or pulses per gallon and defines the maximum resolution of the flow meter. The K factor is dependent on the size of the liquid volume trapped between each gear and the housing.
- Passive versions: Oval gear flow meters are available with mechanical counters that display the cumulative flow volume. This type of sensor does not have built in electronics and does not require a voltage supply.
7. Advantages of oval gear flow meters
Oval gear flow meters:
- Provide extremely precise flow measurement.
- Are available in a very large range of flow rates and have larger turndown ratios than turbine flow meters.
- Are extremity robust and can operate at high pressures.
- Are not significantly affected by liquid viscosity and so do not require re-calibration for new liquids.
- Have a longer lifetime than other flow meters with moving parts.
- Are useful over a very wide viscosity range.
- Do not require straight sections of pipe at their inlet and outlet.
- Can operate in both directions (though not all oval gear flow meters can detect the flow direction).
8. Disadvantages of oval gear flow meters
Oval gear flow meters:
- Can not be used with gases or very low viscosity liquids.
- Accuracy decreases when used with low viscosity liquids.
- Are not suitable for use with liquids that contain solid particulates
- Cause a significant pressure drop in the liquid.
9. Application tips
- Mounting: Oval gear flow meters can be mounted either horizontally or vertically. If installed vertically, having the liquid flow upwards will aid in the removal of air bubbles. Some manufacturers recommend that if installing vertically, the flow meter should be orientated such that the gear shafts remain horizontal. Unlike turbine flow meters, oval gear flow meters do not require a straight section of pipe at their inlets and outlets. This enables them to be installed in confined spaces.
- Filters: Sold particles contained within the liquid will wear the gears. It is strongly advised to place a filter before the sensor inlet to trap solids.
- PNP/NPN transistor: The flow meter output is controlled by a transistor of PNP or NPN type. A transistor is a switch which either conducts or does not conduct electricity. A PNP type transistor conducts during the logic 0 portion of the square wave and an NPN transistor conducts during the logic 1 portion. When the transistor is not conducting, the sensors output is floating and will pick up background noise. The output must be tied to either the supply voltage (for PNP) or to ground (for NPN). This is performed using a pull-up resistor (for PNP) or a pull-down resistor (for NPN). Process controllers normally have built in pull-up or pull down-resistors. However, care must be taken to make sure that the sensor is connected to the corresponding port type on the controller. Additional information is available on the Azom and se websites.