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Pulsed Flow Meter: Electronic Transducer Pulse Frequency Sizing Laws
Quick Answer: A pulsed flow meter sends a square wave, transistor or reed switch output. The frequency is proportional to flow rate. You size the electronic transducer by matching the maximum frequency to the PLC counter or RTU input limit and by confirming the signal voltage.
Silver Automation Instruments supplies pulse output flow meters for diesel, fuel oil, water, chemicals, steam and compressed air. We provide electromagnetic, vortex, turbine, oval gear and Coriolis meters with pulse, frequency or transistor outputs.
What Is a Pulsed Flow Meter
A pulsed flow meter converts flow into a series of electrical pulses. Each pulse represents a fixed volume or mass. The total number of pulses gives total flow. The pulse rate gives instantaneous flow.
Common pulse output types include open collector NPN, PNP, reed switch and isolated transistor. The pulse width and amplitude change from meter to meter. A typical open collector output runs at 5 to 30 VDC with a current limit of 50 to 100 mA. In practice, most engineers skip this part and then wonder why the PLC counter drops pulses.
Electronic Transducer Pulse Frequency Sizing Law
The sizing law is direct. The maximum pulse frequency must be lower than the input limit of the receiving device. Most PLC high speed counters accept 10 kHz, 30 kHz or 100 kHz. A general digital input often accepts only 20 to 100 Hz. If your flow meter sends 800 Hz into a normal DC input, the input will miss pulses and totalizer values will drift.
Calculate maximum frequency with this formula.
Frequency in Hz equals K-factor in pulses per litre multiplied by maximum flow in litres per minute divided by 60.
Example: a DN25 oval gear flow meter for diesel has a K-factor of 450 pulses per litre. Maximum flow is 80 L/min. The output frequency is 450 times 80 divided by 60, which equals 600 Hz. A standard PLC high speed counter rated for 10 kHz handles this with room to spare.
For gas or steam vortex meters, the calculation uses K-factor in pulses per cubic metre and maximum flow in m3/h. Divide by 3600. A DN50 vortex steam meter with a K-factor of 12000 pulses per m3 and a maximum flow of 120 m3/h gives 400 Hz. Add a PT100 and pressure sensor for temperature and pressure compensation if the steam line pressure varies.
K-Factor and Transducer Input Matching
K-factor is the number of pulses per engineering unit. It changes with meter type, pipe size, and sometimes flow profile. Oval gear and turbine meters have a mechanical K-factor. Electromagnetic and Coriolis meters generate a simulated pulse frequency from measured flow. For these meters, you can often set the pulse per unit value in the electronics.
We have seen this on customer sites many times. A food plant in Indonesia set a Coriolis mass meter to 1000 pulses per kg. The control system input card had a maximum of 5 kHz. At a batch flow of 1800 kg/h the actual frequency stayed near 500 Hz. The system worked fine. Later they changed the pulse value to 10 pulses per kg and the frequency dropped to 5 Hz, which was too slow for accurate batching.
Match the pulse output voltage and type too. NPN open collector works well with a pull up resistor. PNP
Application Examples
A seawater desalination plant in the Middle East uses a DN100 electromagnetic flow meter with pulse output for brine discharge. The PLC counts pulses for daily volume. The K-factor is set to 1 pulse per 100 litres. Maximum flow of 180 m3/h produces 0.5 Hz. That is low but acceptable for daily totals.
A paint manufacturer in Southeast Asia uses DN15 oval gear flow meters for solvent batching. The pulse output goes to a batch controller. Each 5 litre batch needs repeatability within 1 percent. The meter K-factor is 2200 pulses per litre. The high pulse resolution gives the batch controller enough counts to stop the valve on time.
Last year a customer in Vietnam asked us for a fuel oil flow meter with pulse output for a generator set. We supplied a DN20 oval gear meter with NPN output and a 10 kHz counter. Fuel oil viscosity was 15 cP. The meter K-factor was 520 pulses per litre. Maximum flow of 30 L/min gave 260 Hz. The generator control system recorded fuel consumption without any pulse loss.
Silver Automation Instruments Pulse Flow Meter Range
We offer pulse output across most meter families. Oval gear flow meters cover diesel, lube oil, fuel oil and solvents from DN2 to DN80. Electromagnetic flow meters with pulse or frequency output cover conductive liquids from DN15 to DN800. Vortex flow meters cover steam, gas and liquids with pulse output. Turbine flow meters cover clean water and fuel. Coriolis mass flow meters provide mass pulse output for oil, chemical and food applications.
For hazardous areas, we can supply ATEX Zone 1 or Zone 2 versions. For remote sites, battery powered electromagnetic meters with pulse output and RS485 are available. Some metering electronics provide 4-20 mA HART plus pulse output on the same device. Electromagnetic flow meters need liquid conductivity above 5 µS/cm.
Send us your flow range, pipe size, fluid name, temperature and power supply. We will confirm the K-factor, maximum frequency and wiring type. Tel: +86-25-68650347. WhatsApp: +86-25-52155837. WeChat: +86 15365082610.
Pulsed Flow Meter FAQ
What is a K-factor
K-factor is the number of pulses per unit volume or mass. A K-factor of 450 pulses per litre means every litre produces 450 pulses.
How do I calculate pulse frequency
Multiply the K-factor by the flow rate in the same time unit. For litres per minute, divide by 60 to get Hz. For cubic metres per hour, divide by 3600.
What is the difference between pulse output and 4-20 mA output
Pulse output is best for total flow and batching. A 4-20 mA output is best for continuous flow rate to a PLC. Many Silver Instruments meters can provide both outputs at the same time.
Can a pulsed flow meter connect to a PLC
Yes. Use a high speed counter input for high frequency signals. Use a standard digital input only when the frequency stays below the input limit.
Which Silver Instruments flow meter is best for diesel
An oval gear flow meter with pulse output works well for diesel transfer and generator monitoring. We supply models from DN2 to DN80 with NPN or reed switch outputs.