Deciding What Output Works Best with a Particular Flow Meter

An experienced engineer can easily make a decision on what output works best with a particular type of flow meter. However, the non-specialist or occasional flow meter user may find it difficult to make such a decision due to the myriad of flow meter output pulse types available with their own advantages and drawbacks.

Configuration routines are often available to the input circuitries of flow indicators to accommodate different device outputs. These connections need to be appropriately configured to achieve an efficient system operation.

Flow meters are supplied with output pulse types such as magnetic coils, Namur sensors (current pulse), logic devices, transistors (NPN and PNP junctions), and reed switches. This article provides some useful tips to make an informed decision on the selection of an output pulse type appropriate for a particular flow meter.

Reed switch

The simplest form of pulse output is reed switches, which are magnetically operated switches in a glass capsule. Reed switches are cost effective and easily understandable but limited to lower frequencies. They can be used in hazardous condition providing the site engineer is happy using simple apparatus as there is no risk of extraneous voltages or currents being generated.

The role of reed switches is simply to switch the voltage offered to it. The voltage change is detected by the logging flow meter when it closes. However, reed switches have their own limitations. Since they are mechanical switches, they experience fatigue and breakage after roughly 109 operations under ideal conditions. A classic reed switch operation is shown in Figure 1.

Classic reed switch operation

Figure 1. Classic reed switch operation

Reed switches must have a current limiting component to prevent the contacts from welding together due to direct switching of the voltage supply. In adverse conditions, this additional protection can prevent an inherently safe device reading the contact closures. Contact bounce is likely to occur, causing some very high frequency pulses as experienced by most modern electronic instruments.

One solution is using a slow input to the connected device. A “slow speed” input may be used on a PLC or a small resistor and capacitor may be employed to avoid the bounce being experienced by the device. A alternate method is to utilise the software to ignore fast pulses.

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Transistors

As simple solid state switches, transistors are typically configured in open collectors by adding some external circuitry like a resistive load that is configured for PNP or NPN. The configuration of an open collector NPN is similar to a switch to the 0V supply of the flow meter. A ‘Pull-Up’ resistor is required for a positive voltage (Figure 2).

NPN transistor

Figure 2. NPN transistor

The transistor is then informed to ‘sink’ current from the external resistor back to 0V during its activation. This voltage does not need to be the supply voltage of the sensor but is user-selectable within the specified limits to meet the requirement of the rest of the instrumentation network, for instance, 24V to transmit safely over longer distances.

Although the exact resistor value is insignificant, it must be large enough for limiting the current at the selected voltage but small enough to supply enough current for the switch activation. Pull up resistor values are typically between 1K and 50kOhm.

An open collector PNP output is simply the mirror image of the NPN output. It provides a switch to the supply voltage of the sensor and needs to be coupled externally by means of a ‘Pull Down’ resistor to 0V. This output switch is then informed to ‘source’ current to the external resistor. The PNP output is well accepted in many PLCs, but is less flexible when compared to the NPN counterpart because it has a predetermined voltage range. Pull down resistor values are typically between 1K and 50kOhm.

PNP transistor

Figure 3. PNP transistor

Logic Output

A logic output is an output switching between predetermined levels of voltage with conditions described as logic 0 and logic 1. TTL and CMOS are the most widely used logic output devices. TTL is described as a logic 0 (output below 0.4V) and a logic 1 (above 2.4V). Typically, a ‘0’ represents a few mV and a ‘1’ is close to 5V (Figure 4).

TTL signal

Figure 4. TTL signal

CMOS is described corresponding to an internal supply voltage (typically 3.3 or 5V), having a ‘0’ at below 33% and a ‘1’ at above 66% of the supply voltage. Practically, CMOS output will have a 0 output at nearly 0V and a 1 output at nearly the internal supply (Figure 5).

Typical CMOS

Figure 5. Typical CMOS

Namur Sensor

A Namur sensor is supplied with a stable voltage. The resistance of the two-wire sensor changes in response to varying flow through the flow meter. The current cycles are typically from 2.1 to 1.2mA when the sensor passed by the target. Specialist flow meters used in hazardous locations normally feature Namur sensors due to very low power consumption and convenient remote monitoring of fluctuations in current/resistance. Converters to standard outputs are available.

Magnetic Coil

When a magnet is moved in front of a coil, a voltage is generated. In a flow meter, the coil and the magnet are stationary components in the sensor. The change in the magnetic coupling caused by a magnetic turbine blade is large enough to induce a voltage swing. These induced voltages are generally low levels but are cyclical and easily detectable.

If the operating environment of the sensor is electrically “noisy,” it is necessary to amplify or convert the signal to a more robust level before transmission. Transmission of low voltage pulses is difficult over long distances. The following table helps users to make an informed decision on the optimum output for their flow meters:

Reed switch Transistor NPN Transistor PNP Logic level Namur Magnetic
Low cost meters Hall sensor
Older PLCs
Modern PLCs
Computer
Electrically noisy events The higher the pulse voltage the better Typically 24V With a local amplifier
Low speed sensing
Hazardous areas Simple apparatus Some modern electronic flowmeters offer a variety of outputs for hazardous areas With suitable converter

This information has been sourced, reviewed and adapted from materials provided by Titan Enterprises Ltd.

For more information on this source, please visit Titan Enterprises Ltd.

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