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Introduction to Flow Meters and Flow Sensors
Note: Before moving on to the topic of flow meters and flow sensors, you may wish to familiarize yourself with the science of fluid flow by reading this article.
Many industrial applications require a measurement of flow. Sometimes, the flow measurement must be very accurate -- such as in the measurement of material that is used in processes or manufacturing -- so that total amount of raw material can be determined. These types of flow measurements are important when the amount of material being metered is for recipe batches, or when material is loaded or transported and must be measured so it can be paid for. In other applications, such as hydraulic systems on large machines, it's important to determine the amount of fluid flowing in specific parts of a system in order to determine if the valves are working correctly of if a failure has occurred and fluid is being bypassed to the reservoir tank. In some applications the exact amount of flow is not important, and a more general measurement will be enough to indicate if a pump is working, or if a drain line is functioning properly.
A flow sensor or flow meter can be classified as either intrusive or nonintrusive. If a flow meter or flow sensor disturbs the flow of fluid it's measuring, it's classified to be intrusive. If a flow meter or flow sensor does not disturb the flow of fluid it's measuring, it's classified to be nonintrusive.
A flow sensor or flow meter can also be classified by the manner in which it measures flow. e.g., a positive-displacement flow meter is very accurate because it physically measures every drop of fluid, whereas a bypass meter is not as accurate because it measures a small amount of fluid flow and calculates the remainder. Differential flow meters place an obstruction in the path of the fluid flow, such as an orifice plate, and the amount of flow is determined from a calculation based on the amount of pressure drop the fluid has as it moves through the orifice plate. A velocity flow meter measures the velocity of the fluid as it passes the turbine. The fourth way to measure fluid flow is to use a mass flow sensor that calculates the total volume of material that passes through the sensor and determines the flow from this number.
Another way to classify flow meters is to determine how the signal from
the sensor will be used. e.g., the signal can be used to determine
the amount of flow at any given point in a system and indicate it on
a meter or display. Or it can be used to mathematically sum the flow
in order to determine the volume of fluid that has passed the sensor
in a given time. The flow
signal can also be sent to a recording device or a transmitter where it's sent to a control system. In the control
system, the sensor signal can be sent to a controller (such as a PLC
computer) where it can be compared to a set point.
The controller can increase or decrease its output signal based on this
comparison. The flow sensor signal can be used to control the system
or report the flow for an alarm package that can be set to indicate when
the flow is too high or low.
Types of flow meters
Below you will find our top book recommendations for learning the fundamentals of Flow Control in maintenance, engineering and scientific applications.
Control: Passive, Active, and Reactive Flow Management
From the Publisher
Excerpted from Flow Control Devices by Y.Dvir. Copyright © 1997.
Reprinted by permission. All rights reserved.
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