Flow Meter Output Types
Deciding what output best connects to a particular type of flow meter is typically not a problem for an experienced engineer. However for the non-specialist or occasional flow meter user this decision may pose a problem as there are a wide array of flow meter output pulse types and each has its advantages and disadvantages. In essence these are all on/off switches in various different guises.
The input circuitries of flow indicators often have configuration routines to accommodate various device outputs. Correctly configuring these connections is crucial for efficient system operation.
Today flow meters are offered with output pulse types including Reed Switches, Transistors (NPN and PNP junctions), Logic devices, Namur sensors (current pulse) and Magnetic coils.
Here are some tips to help you make an informed decision when selecting the optimum output pulse type for your flow meter.
1. Reed switch
This is the simplest form of pulse output. It is literally a magnetically operated switch in a glass capsule. Typically Reed switches are inexpensive and easy to understand but limited to lower frequencies. They can be used in hazardous areas providing the site engineer is happy with simple apparatus as there is no danger of extraneous voltages or currents being produced. The device merely switches the voltage offered to it and if this is suitably protected the reed switch is perfect. The logging flow meter “sees” the voltage change when it closes. However there are a few limitations with using Reed Switches. They are mechanical switches and have a finite life as fatigue and breakage is likely after around 109 operations under ideal conditions. The contacts should be protected by a current limiting component to prevent a supply being directly switched causing them to weld together. For hazardous areas this extra protection can stop an intrinsically safe instrument reading the contact closures. There is also a potential problem with contact bounce which causes some very high frequency pulses which can be seen by many modern electronic instruments. There are two solutions to this. First is to use a slow input to the connected equipment. This may be a “slow speed” input on a PLC or a small capacitor and resistor may be used to prevent the bounce being seen by the instrument. A second method is to utilise the software to ignore fast pulses, there are several well established methods of doing this.
Transistors are simple solid state switches which are usually configured in Open Collectors. This simply means that you have to add some external circuitry, such as a resistive load configured for NPN or PNP.
a. NPN. An Open Collector NPN output is configured like a switch to the 0V supply of the flow meter. It requires a ‘Pull-Up’ resistor to a positive voltage. The transistor is then said to ‘sink’ current from the external resistor back to 0V when it is activated. The advantage is that this voltage might not necessarily be the supply voltage of the sensor as shown below but can be any chosen (within specified limits) to suit the rest of the instrumentation network e.g. 24 volts for safer transmission over longer distances. The exact value of the resistor is not critical, but needs to be chosen to be large enough to limit the current (within the specified limit) at the chosen voltage, but small enough to provide enough current to activate the switch. Typical pull up resistor values are between 1K and 50kOhm.
b. PNP. An Open Collector PNP output is the mirror image of the NPN counterpart, providing a switch to the sensor supply voltage. It has to be connected externally via a ‘Pull Down’ resistor to 0V, this output switch is then said to ‘source’ current to the external resistor. Although more popular with many PLCs; in practice the PNP output is less flexible than that of the NPN as its voltage range is predetermined. Typical pull down resistor values are also between 1K and 50kOhm.
3. Logic output
A logic output is an output which switches (conditions described as logic 0 and logic 1) between predetermined voltage levels. The most common logic output devices are TTL and CMOS. TTL is defined as a logic 0 (output below 0.4V) and a logic 1 (above 2.4V). Usually a 0 is a few mV and a 1 is close to 5V. CMOS is defined relative to an internal supply voltage (usually 3.3V or 5V) as having a 0 at below 33% of the supply voltage and a 1 at above 66%. In practice CMOS output will be have a 0 output at virtually 0V and a 1 output close to the internal supply.
4. Namur Sensor (EN 60947-5-6)
A Namur Sensor is a two wire sensor which is supplied with a constant voltage. With a Namur sensor, as the flow though the flowmeter varies the resistance of the sensor changes. Typically the current cycles from 2.1 to 1.2mA as the target passes the sensor. These devices are normally used with specialist flow meters in hazardous areas as the power consumption is very low and the change in resistance/current can be easily monitored remotely. Converters to standard outputs are available.
5. Magnetic coil
If you move a magnet in front of a coil, you get a voltage. Implemented in a flow meter the magnet and the coil are stationary in the sensor and a magnetic turbine blade changes the magnetic coupling sufficient to induce a voltage swing. Though induced voltages are typically low levels (c. 10millivolts) they are cyclical and quite easy to detect. If the environment in which the sensor operates is electrically “noisy” the signal should be amplified or converted to a more robust level prior to transmission. As indicated earlier in the NPN section low voltage pulses do not travel well over long distances.
The table below provides a useful summary to guide your decision on the optimum output for your flowmeter.