A Review of Your Flow Sensing OptionsOctober 1, 2006 By: William Hennessy, BMT Scientific Marine Services Inc. Sensors
Of the nine flow sensor technologies outlined here, one is certain to satisfy your application.
Flow sensors are used to measure both gas and liquid flows in many monitoring and control applications. Flow can be variously defined (e.g., mass, volume, laminar, turbulent). The amount of a substance flowing (mass flow) is usually the one of interest, and if the fluid's density is constant, a volume flow measurement is generally the easiest to perform. Some technologies work for both gas and liquid flow; others are specific to what they are measuring.
Flow rate is typically obtained by first measuring the velocity of a fluid in a pipe, duct, or other structure and then multiplying by the known cross-sectional area at the point of measurement. This article will examine nine of the most commonly used technologies and devices used to measure gas and/or liquid flow.
1. Thermal Anemometers
Thermal (or "hot wire") anemometers operate on the principle that the amount of heat removed from a heated temperature sensor by a flowing fluid can be related to that fluid's velocity. These sensors typically use a second, unheated temperature sensor to compensate for variations in the air temperature. Hot wire sensors are available as single-point instruments for test purposes, or in multipoint arrays for fixed installation. These sensors are better than differential pressure types for low airflow measurements, and are commonly applied to air velocities from 50 to 12,000 fpm.
2. Differential Pressure Sensors
Differential pressure flowmeters are the most common type of unit in use, particularly for liquids. Their operation is based on the concept that the pressure drop across the meter is proportional to the square of the flow rate. The flow rate is found by measuring the pressure differential and taking the square root.
These devices, as do most flowmeters, have two elements. The primary element causes a change in the kinetic energy, creating the differential pressure in the pipe. The unit must be correctly matched to the pipe size, flow conditions, and the properties of the liquid being measured. In addition, the element's measurement accuracy must be good over a reasonable range. The secondary element measures the differential pressure and outputs a signal that is converted to the actual flow value.
Figure 1. The pitot tube
A pitot tube consists of two tubes that measure pressure at different locations within a pipe. One tube measures static pressure, usually at the pipe wall; the other measures impact pressure (static pressure plus velocity head). The faster the flow rate, the larger the impact pressure. Pitot tubes use the difference between impact and static pressure to calculate flow rate. Pitot tubes are low-cost, but their drawback is that they measure flow only at a single point and must be installed at the point of maximum flow. Changes in velocity profile can cause major errors. They are also prone to clogging. Averaging pitot tubes have several ports for measuring flow at multiple locations, which makes it possible to take changing velocity profiles into account (Figure 2).
Figure 2. Velocity pressure measurement with a U-shaped tube manometer
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