Definition of Flow Sensor
Flow sensors are an important part of the systems that shape our modern way of life. They work in your vehicle’s fuel system, gas and water system distribution infrastructure, and countless other manufacturing systems. There are several types of flow sensors, but we will focus on the two most well-known types of flow sensors and how they work.
Flow sensors are components that measure the flow of liquids such as gases or liquids. Flow sensors utilize mechanical and electrical subsystems to measure changes in fluid physical attributes and calculate their flow. Measuring these physical attributes depends on the physical attributes of the fluid. Gaseous, liquid, and non-Newtonian fluids behave very differently from each other, so the methods we use to measure their flow must also be different.
Flow sensors can be divided into two groups: Contact Sensor and Non-contact Sensor. Contact flow sensors are used in applications where the measured liquid or gas is not expected to get clogged inside the pipe when it comes into contact with the moving part of the sensor. In contrast, non-contact flow sensors have no moving parts, and are commonly used when a liquid or gas (generally a food product) monitored is contaminated or physically altered by coming into contact with a moving part.
Types of Flow Sensor
Differential Pressure Flow Sensors
Differential Pressure Flow Sensors calculate fluid pressure. The sensor measures pressure at two different speeds while maintaining fluid density. In most differential pressure flow sensors, a single pressure sensing node such as a pitot tube is at the edge of the fluid flow path.
Nozzles or changes in the diameter of the hole accelerate the fluid, which is measured again at higher speeds. Once the sensor determines the pressure difference, we can use Bernoulli’s equation to find the velocity of the fluid. Finally, we can use the speed of that fluid to determine the overall flow through the pipe.
This process requires controlling several variables to accurately calculate the velocity of the fluid. Therefore, differential pressure-based flow sensors are usually assemblies that “disrupt” piping, rather than adapting to existing pipes. Several different sensors use differential pressure methodologies, such as:
- Venturi Tube
- Rotameter
- Hole plate
- Arrangement of pitot tubes
All of these sensors have varying degrees of accuracy and system pressure loss. Differential pressure flow sensors are the most popular type of flow sensor because of their performance and versatility.
Thermal Mass Flow Sensor
Generally Thermal Mass Flow Sensors are used in gas applications, low flow, high accuracy such as semiconductor manufacturing, thermal mass flow sensors utilize the thermal properties of fluids to measure flow through the system.
Thermal mass flow systems have one of two basic configurations, but both rely on the fluid’s tendency to absorb heat energy and measure the amount of energy in the fluid. Here’s a quick guide to both configurations:
First Method: Heating elements and thermal sensing elements work together to measure the amount of energy a fluid absorbs as it flows across heating elements and thermal sensors. First, the fluid absorbs energy from the heating element. Next, the sensor measures the liquid to determine how much energy it absorbs.
Second Method: One heating element serves to keep the temperature at a fixed level. When the fluid absorbs energy, it cools the heating element, which requires more energy in the system to maintain temperature. We calculate the mass flow by determining the amount of energy that the heating element uses to maintain itself at a constant temperature.
In both methods, the velocity of the fluid is directly related to how much energy it can absorb. The slower the flow of fluid, the more time energy has to move from the heating element to the fluid. The faster it flows, the less time the liquid has to absorb.
One important component of thermal mass flow sensors is that they must be calibrated to a particular fluid in a thermal mass flow system. Thermal mass flow sensors are finely tuned to support different levels of gas purity, such as pharmaceutical-grade nitrogen versus manufacturing-grade nitrogen for industry. If calibrated correctly, these sensors can be very precise and reliable, which is why these sensors are so popular in the settings of manufacturing.
Vortex Flow Sensor
Vortex flow sensors consist of small hooks (known as “buff bodies”) that flex backwards and forwards when in contact with flowing liquids or gases. The difference in pressure (i.e. vorticity) produced by the hook is measured to determine the rate of flow. Mechanical flow sensors use propellers that rotate at a rate directly proportional to the flow rate. Mechanical flow sensors can also be controlled to cause the flow rate to increase or decrease.
Ultrasonic Flow Sensor
Ultrasonic flow sensors are the most popular type of non-contact flow sensor. Ultrasonic flow sensors transmit high-frequency sound pulses across a flowing liquid medium or gas. These sensors measure the time between sound emissions and their contact with the sensor receiver to determine the rate of flow of a gas or liquid.
Flow Sensor application
A variety of flow sensors are available to meet specific requirements and applications. The most common parameters that will affect the type of flow sensor you need are:
- Flow sensor volumetric range
- Materials or Materials
- Pressure
- It’s necessary to drain it.
Flow sensors may be the perfect tool to help you understand and control mechanical or chemical subsystems in your application.