The short answer
Frequency Signals
A frequency signal carries its value in how often it changes state, in pulses per second, and it is the natural output of anything that spins or oscillates: a turbine or paddlewheel flowmeter whose rotor passes a pickup, a magnetic pickup on a shaft, a vortex meter counting shedding, and some analyzers and converters. The frequency is proportional to the rate, flow or speed, through a factor the manufacturer states, and the same pulses counted over time give the total. Reading it needs an input that measures frequency directly, either a frequency input module or a high-speed counter that the controller reads at intervals, or a converter that turns frequency into 4 to 20 milliamps for an ordinary analog input. At low rates the period between pulses is measured instead of counting pulses per second, because counting a few pulses over a second gives coarse resolution. The signal is immune to the offset and drift of an analog level but vulnerable to noise pulses that add to the count, so the wiring, the threshold, and the filtering matter.
Key points
- Frequency encodes the rate; counting the same pulses over time gives the total.
- The K-factor from the manufacturer converts pulses per unit volume or per revolution into flow or speed.
- Use a frequency input or a high-speed counter; an ordinary input cannot follow more than a few pulses per second.
- At low frequency, measure the period between pulses for resolution; at high frequency, count pulses per interval.
- Noise pulses add to the count; shield, set the threshold, and filter, and verify against a known rate.
Sources
| Instrument | What produces the frequency | Typical range | Output form |
|---|---|---|---|
| Turbine flowmeter | Rotor blades passing a magnetic or inductive pickup | Tens to thousands of hertz | Sine wave from a coil, or a squared pulse from an amplifier |
| Paddlewheel flowmeter | Paddle magnets passing a sensor | A few to hundreds of hertz | Open-collector or square wave |
| Vortex flowmeter | Vortex shedding frequency | Tens to hundreds of hertz | Usually converted internally; pulse output optional |
| Magnetic pickup on a shaft | Gear teeth passing the pickup | Depends on teeth and speed | Sine wave; needs a conditioner |
| Positive displacement meter | Rotor or piston cycles | Low, often below a hertz | Reed or open-collector pulse |
| Frequency converters | A transmitter that outputs frequency proportional to its value | Defined by the device | Square wave |
From frequency to flow
Q = f × 60 ÷ K
- Q = flow rate in volume per minute
- f = frequency in pulses per second
- K = K-factor in pulses per unit volume from the meter calibration
The K-factor is on the meter calibration certificate and is entered in the input configuration or the program; a meter replaced with one of a different K-factor reads wrong until the number is changed. Turbine meters have a K-factor that varies slightly with flow, and a linearization table from the certificate improves accuracy across the range. The total is the pulse count divided by the K-factor, kept in a counter that survives power loss.
Reading it
- Frequency input module
- Measures frequency directly and reports it as a value, with a configurable range, threshold, and filter. The cleanest solution.
- High-speed counter
- Counts pulses; the program reads the count at a fixed interval and computes the frequency as the difference over the interval. Fine for high rates; coarse at low rates.
- Period measurement
- The counter timestamps each pulse and the program computes frequency as the reciprocal of the period. Necessary below a few hertz, where a one-second count window resolves only whole pulses.
- Frequency to current converter
- A device that outputs 4 to 20 milliamps proportional to frequency, for an ordinary analog input. Adds a device and a calibration; loses the pulse total unless the converter also counts.
- Ordinary discrete input
- Reads a state each scan; misses pulses faster than about half the scan rate. Suitable only for very slow pulse sources, and never for frequency measurement.
Resolution of a counting method = 1 ÷ T_window pulses per second
- T_window = the counting interval in seconds
- A one-second window resolves 1 hertz; at a 5 hertz signal that is 20 percent, which is why low rates use period measurement
Noise
A frequency input counts every edge that crosses its threshold, and a noise spike from a drive or a contactor is an edge. The symptoms are a flow reading that jumps when a motor starts, a total that creeps with the process stopped, and a frequency that reads when the meter is still. The defenses are shielded twisted pair grounded at the panel, routing away from power and drive cables, an input threshold set above the noise and below the signal, a filter or minimum pulse width in the input configuration, and, for coil pickups, a conditioner near the meter that squares the signal before it travels. A meter with a squared output and a short cable rarely has the problem.
Verifying
- A frequency generator or a calibrator with a frequency output into the input at known values across the range; the reading should match the calculation from the K-factor.
- A handheld frequency meter on the signal with the process running, compared with the controller reading.
- A volumetric or comparison check against another meter or a tank draw-down for the whole chain.
- The total against the meter register if it has one, over a day.
Frequently asked questions
- Frequency or 4 to 20 milliamps from the flowmeter?
- Frequency when the input can read it: no drift, no offset, and the total comes free. Current when the run is long and noisy, the controller has only analog inputs, or a converter is already present. Many meters offer both; use frequency for the total and current for the rate if the design wants belt and braces.
- The flow reads zero below a certain rate.
- The input cutoff or the minimum frequency setting, or the meter itself below its minimum flow. Turbine and paddlewheel meters have a minimum below which the rotor stalls or the K-factor is far off; the input should report zero there and the design should size the meter for the range.
- Why does the total drift when the pump is off?
- Noise pulses counted while the meter is still, or a rotor turning slowly in a leaking check valve. A minimum frequency cutoff, a better threshold, and shielding fix the first; a valve fixes the second.
- How fast can a counter input go?
- High-speed counter inputs handle tens of kilohertz or more; ordinary inputs a few pulses per second at most. Check the module specification against the maximum meter frequency at full flow.
Related topics
- Pulse SignalsPulse outputs for totalizing: flowmeters, energy meters, rain gauges, and feeders, output types, scaling in volume per pulse, counter inputs, pulse width and rate, debouncing reeds, totalizer retention and rollover, and verifying against the register.
- Flowmeter InstallationThe installation rules that decide whether a flowmeter reads correctly: straight run by meter type, keeping the pipe full, orientation, velocity range, grounding for magnetic meters, isolation and access, and the commissioning checks that prove the meter.
- Flow TroubleshootingA method for a flow reading that is wrong, noisy, zero, or disputed: establish the truth with an independent check, separate installation from configuration from signal chain, then the specific checks for magnetic, ultrasonic, and DP meters.
- 4-20 mA Signals from the Instrument SideHow a 4-20 mA current loop is powered and wired at the transmitter, why the loop resistance budget matters, and how to check a loop with a meter.
- PLC I/O SystemsDiscrete and analog input and output modules, sinking and sourcing, relay and transistor outputs, wiring conventions, and the module diagnostics that find field faults from the panel.
- VFD Noise on Analog SignalsAnalog readings that jump or wander when a drive runs, and speed references that make a drive hunt: how the drive output couples into signal wiring, why the input turns kilohertz noise into a slow wander, the tests that pin it on the drive, and the fixes.
Direct contact
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