In hydraulics, flow refers to the amount of liquid that flows per unit of time. In some industrial technologies, the total amount of liquid that flows over a period of time is referred to as the cumulative flow, and correspondingly, the flow rate is referred to as the instantaneous flow. It should be noted that the dimensions of the two are different and are not the same physical quantities at all.
 

Since the flow rate is the amount of liquid that flows per unit of time, it always takes a certain amount of time to measure the amount of liquid directly, so there is a problem of response time. There is a distinction between volume flow and mass flow, but in hydraulic technology, mass flow is hardly used, and volume flow is simply the flow rate. It is important to note that hydraulic oil can also be compressed. When the pressure increases from 0 to 20MPa, the volume is roughly reduced by 2%~3%, so the measured volume flow rate will also decrease accordingly. Because the flow meter must be connected in series to the pipeline for flow measurement, it is generally necessary to change the existing pipeline, which is more troublesome for the pressure measurement.

The flow meters are classified as follows:
 

There are many types of flowmeters, such as roots flowmeter, scraper flowmeter, drum flowmeter, nozzle flowmeter, venturi flowmeter, laminar flow meter, piston area flowmeter, hot wire and hot film flowmeter, etc., but they are not commonly used in hydraulic technology. Commonly used flow meters are as follows:
 

1. Turbine flowmeter: The turbine rotates by using the dynamic pressure of the liquid flow, and the flow rate is calculated according to the pulse triggered by the blade when it rotates. The turbine inertia is small because the response speed is faster. If the geometric symmetry of the turbine blades is done well, calculating the flow rate based on the time interval of the pulses, a measurement can be obtained in just over ten milliseconds.
 

2. Gear motor flowmeter: It is a volumetric flowmeter, which is similar to the working principle of hydraulic motor. The space between the adjacent two teeth and the plates on both sides constitutes the metering volume. A probe is mounted on the side plate, and when the gear rotates as the flow rate is pushed, the teeth passing through the probe trigger an electrical pulse that calculates the flow rate based on the frequency of the pulse.
 

3. Differential pressure flowmeter: the flow rate through the thin-walled hole is proportional to the square root of the pressure difference on both sides of the hole, and is basically not affected by viscosity. According to the principle, the pressure (or differential pressure) sensor is used to measure the pressure difference between the two sides of the orifice plate, so as to calculate the flow rate through the orifice plate according to the calibrated flow coefficient.
 

4. Float flowmeter: The float flowmeter is composed of a conical tube and a float. The higher the conical tube, the larger the inner diameter. Therefore, the higher the float position, the larger the annular area between the float and the conical tube, i.e., the flow area. The liquid flows from the bottom to the top and flows through the float, resulting in differential pressure. When stabilized, the weight of the float is balanced with the differential pressure on the upper and lower sides of the float, and the position of the float reflects the flow rate passing through.
 

5. Elliptical gear flowmeter: the cavity formed by meshing and rotating two elliptical gears with each other is used for measurement. The amount of liquid passing through is proportional to the number of turns of the gear (4 metering volumes per revolution), but not proportional to the angle of turn. Therefore, it is necessary to wait for the gear to pass many revolutions before giving the amount of liquid that will pass. It takes tens of seconds, or even minutes, to produce a piece of data. The response is too slow, and it is okay to measure the cumulative flow, which is not suitable for flow measurement in general in hydraulic technology. Generally also not pressure-tolerant.