An Introduction to Pulse Output Flowmeter Devices
Within the flowmeter industry, the use of pulse output devices to signal flow is commonplace, and we often assume all our customers are also familiar with them. However, across the diverse application areas we serve, this is not always the case.
All of the flowmeters of Titan Enterprises we are supplying are designed with a square wave pulse output as below.
Titan Enterprises produce five main types of pulse generating flowmeters, each of which output in proportion to the flow from the meter. These comprise Hall Effect transistors, Reed switches, Namur sensor, and an optical detector, all of which are activated by a rotating turbine or gear from our mechanical meters.
Also, we offer a range of ultrasonic meters, which also provide a pulse output. The scope of this article is to cover the most commonly utilized pulse types in standard industrial use.
Hall Effect Sensors
Hall Effect sensor devices offer fantastic stability and lifespan, as well as being able to operate at high frequency. They use solid-state transistors to allow power to pass through in one direction when activated by a magnetic field passing across them. There are two types of Hall Effect sensors. Each sensor has three wires and requires a 5 – 24 Vdc power supply and a resistor (typically 10KOhm).
They operate as NPN “sinking” or PNP “sourcing.” An NPN “sinking” sensor requires a “Pull up” resistor across the positive power and the pulse output wire. When the NPN transistor is inactive, the energy flows to the pulse output wire via the resistor. When activated by a magnet in the rotating turbine or gear, the NPN sinks to the zero volt wire, and the signal output value drops. As flow passes through the meter, the sensor is continually switched on and off in proportion, giving the pulse output. PNP or “sourcing” Hall Effect sensors operate in reverse and the Resistor (Pull Down), is placed between the Pulse output wire and the zero volts. When activated, the power switches from the zero volts to the pulse output wire.
Reed switches are activated in the same fashion as Hall Effect devices, with a magnet passing across them. As simple physical switching devices, connecting and disconnecting an input/output connection, Reed switches only require two wires and can operate on much lower power than a Hall Effect sensor. This is advantageous where you wish to use battery units or operate in hazardous (ATEX) environments.
As Reed Switches are mechanical devices, they are not as stable at high frequencies (200Hz max) and do have a more limited lifespan of operation than a Hall Effect sensor. In particular, Reed Switch contacts can literally ‘bounce’ when opening and closing, and care needs to be taken to avoid several pulses being mistakenly counted when a single open or close action occurs. As a result – flow meters often have ‘debounced’ reed inputs specifically designed to do this.
Optical sensors are occasionally used on turbine flow meters, as they can offer high low flow performance for applications involving clear fluids. The infrared optical sensor systems employed by Titan are transistor-based, similar to the Hall Effect NPN sinking devices. With these optical sensors — the output from a light source is broken by the rotating turbine blade causing the signals to the output to pulsate. As the turbine does not require magnets mounted on it to activate the pulse, a much lighter design can be used, so improving the low flow performance.
Ultrasonic flow meters are different from other types of flow devices in that they measure flow without using any moving parts. They have no rotating or repeating aspect, which can create pulses directly. The flow is determined by an algorithm producing a number that exactly represents the flow at that instant. This number is used to synthesize a pulse output (square wave) based on a parameter (often expressed as “pulses per unit” or something similar, usually controlled by the user), which in turn is converted to an NPN or PNP type output as described previously.
One of the most common problems experienced by flowmeter users is the unfamiliarity of the termination requirements of their control/indicating equipment. To utilize the different types of pulse output, you must be able to connect to a system that can read it, such as a display or a signal processing device. However, flow meter users often need to integrate directly with their control systems. A PLC device is typical, but recently we have been getting inquiries for connecting to Arduino or similar electronic interfaces as well. Whatever the device, without the correct wiring, it will not operate, and sensors can even be damaged, so it is always important to refer to the manual and understand the electronic configuration requirements before installation.
For more information on any of our products, contact us today.