Flow sensors are an essential part of the systems that make up our modern way of life. They are at work in your vehicle's fuel system, gas and water system distribution infrastructures, and countless other manufacturing systems. Several varieties of flow sensors exist, but we'll focus on the two most famous types of flow sensors and how they work.
What is a Flow Sensor?
A flow sensor is a component that measures the flow of a fluid such as a gas or liquid. Flow sensors utilize both mechanical and electrical subsystems to measure changes in the fluid's physical attributes and calculate its flow. Measuring these physical attributes depends on the fluid's physical attributes. Gaseous, liquid, and non-Newtonian fluids behave vastly different from one another, so the methods we use to measure their flow must be different as well.
Flow Sensor Types
There are several types of flow sensors. Here, we'll explain two of the most popular types: differential pressure and thermal mass flow sensors.
Type 1: Differential Pressure Flow Sensors
A fluid's velocity is directly related to the fluid's pressure, as shown by the Bernoulli equation:
dp=(density*velocity^2)/2
Differential pressure flow sensors calculate the pressure of the fluid. The sensors measure pressure at two varying velocities while preserving the fluid density. In most differential pressure flow sensors, a single pressure sensing node ― like a pitot tube ― sits on the edge of the fluid flow path. A nozzle or change in orifice diameter accelerates the fluid, which is measured again at a higher velocity. Once the sensor determines the difference in pressure, we can use the Bernoulli equation to find the fluid velocity. Finally, we can use that fluid velocity to determine the overall flow through the pipe.
This process requires controlling several variables to calculate the fluid's velocity accurately. Therefore, differential pressure-based flow sensors are usually assemblies that "interrupt" the piping, rather than adapting to an existing pipe. Several different sensors utilize the differential pressure methodology, such as:
- Venturi tubes
- Rotameters
- Orifice plates
- Pitot tube arrays
All these sensors have varying degrees of accuracy and system pressure loss. Differential pressure flow sensors are the most popular type of flow sensors due to their performance and versatility.
Type 2: Thermal Mass Flow Sensors
Commonly used in gaseous, low-flow, high-accuracy applications like semiconductor manufacturing, thermal mass flow sensors utilize a fluid's thermal properties to measure the flow through a system. A thermal mass flow system has one of two basic configurations, but both rely on a fluid's propensity to absorb thermal energy and measure the amount of energy in the fluid. Here's a brief guide to the two configurations:>
- Method one: A heating element and a thermal sensing element work together to measure the amount of energy the fluid absorbs as it flows across the heating element and the thermal sensor. First, the fluid absorbs the energy from the heating element. Next, the sensor measures the fluid to determine how much energy it absorbed.
- Method two: A single heating element works to keep the temperature at a fixed level. As the fluid absorbs energy, it cools the heating element, which requires more energy in the system to maintain the temperature. We calculate mass flow by determining the amount of energy that the heating element uses to maintain itself at a constant temperature.
In both methods, the fluid's velocity is directly related to how much energy it can absorb. The slower the fluid flows, the more time the energy has to transfer from the heating element to the fluid. The faster it flows, the less time it has to be absorbed by the fluid. One critical component of thermal mass flow sensors is that they must be calibrated to the specific fluid in the thermal mass flow system. Thermal mass flow sensors are fine-tuned to support varying gas purity levels, such as pharmaceutical-grade nitrogen versus industrial, manufacturing-grade nitrogen. If properly calibrated, these sensors can be extremely precise and reliable, which is why they are so popular in manufacturing settings.
Flow Sensor Applications
A variety of flow sensors are available to satisfy niche requirements and applications. The most common parameters that will impact the type of flow sensor you need are:
- Volumetric ranges of the flow sensor
- Material
- Pressure
- Required accuracy
A flow sensor may be the perfect tool to help you understand and control a mechanical or chemical subsystem in your application. Check out Arrow.com to shop flow sensors from top manufacturers like Honeywell, OMRON, Sensirion, and TE Connectivity.
