A gas turbine meter has no numbers inside it - it simply spins a rotor in the flow and produces a stream of electrical pulses as the blades pass a pickup. Turning that pulse train into a billable volume is the job of a flow computer's frequency input. This guide explains how an AGA 7 turbine meter generates pulses, how the meter's K-factor converts a pulse count into actual volume before it is corrected to base conditions, and why dual-pickup integrity checks are used to protect custody turbine measurement.
Frequency Input (Turbine) in one line: A frequency input is the flow computer channel that counts electrical pulses from a turbine meter. As gas drives the turbine rotor, a magnetic pickup produces one or more pulses per blade passage, so pulse frequency is proportional to flow rate and total pulse count is proportional to volume. The flow computer divides the pulse count by the meter's K-factor (pulses per unit volume) to get actual volume, which it then corrects for pressure, temperature, and compressibility to reach base conditions.
An AGA 7 gas turbine meter places a bladed rotor in the flowing stream, and the gas spins that rotor at a speed proportional to the flow velocity. A magnetic or modulated-carrier pickup mounted in the meter body senses each blade as it passes, generating a small electrical pulse. Because the rotor turns faster when more gas flows, the pulses arrive more frequently at high flow and less frequently at low flow, so the frequency of the pulse train directly represents the instantaneous flow rate and the accumulated pulse count represents the volume that has passed.
Many turbine meters offer both a high-frequency and a low-frequency output. The high-frequency signal comes straight off the blade-sensing pickup and produces many pulses per unit of gas, giving fine resolution and fast response that a flow computer likes for accurate custody work. The low-frequency signal is scaled down, often through the meter's mechanical register or electronics, to a smaller number of pulses per unit volume, which is convenient for simpler counters and totalizers but carries coarser resolution. Custody installations generally wire the high-frequency output to the flow computer's frequency input to preserve resolution.
A turbine meter's K-factor is the number of pulses it produces per unit of volume - for example, pulses per actual cubic foot - and it is established by flow calibration of that specific meter. To convert pulses to volume, the flow computer simply divides the accumulated pulse count by the K-factor: total pulses divided by pulses-per-cubic-foot yields cubic feet at flowing conditions. Because the K-factor is unique to the meter and can vary slightly across the flow range, better installations use a K-factor curve or linearization rather than a single number, so accuracy holds from low to high flow.
It is important to remember that the volume the K-factor produces is at flowing, or actual, conditions - it reflects the pressure and temperature of the gas as it passed through the meter, not the contract base. The frequency input and K-factor only get the flow computer to actual volume; a separate correction step then applies the measured static pressure, flowing temperature, and a compressibility factor to translate that actual volume into base-condition volume for billing. Skipping or mis-configuring the K-factor is a direct volumetric error, because every downstream number inherits it.
Because a turbine meter's entire output is a pulse train, a fault in the pickup, wiring, or a piece of debris on the rotor can silently corrupt the measurement, so custody-grade installations often use two pickups and a pulse-integrity check. With dual pickups mounted at a known angular offset, the flow computer compares the two pulse streams for the expected phase and count relationship; if they diverge beyond tolerance, it flags a fault, because a genuine flow signal should show up consistently on both channels while noise, a failing pickup, or a damaged blade will not. This guards against undetected under- or over-registration at a point where volume equals money.
Bringing these signals into a cloud SCADA platform such as Merobix lets operators watch turbine health continuously rather than discovering a problem at month-end reconciliation. The pulse frequency, flow rate, K-factor-derived volume, and the dual-pickup integrity status can all be trended together, so a technician sees when a pickup starts dropping pulses, when frequency behaves erratically at low flow, or when the two channels disagree. For a remote gathering site, that live visibility means a turbine fault becomes an alarm the same day rather than a mystery imbalance found weeks later, which is exactly when it is cheapest to fix.
The K-factor is the number of output pulses the meter produces per unit of volume, such as pulses per cubic foot, determined by flow calibration. A flow computer divides the total pulse count by the K-factor to get actual volume at flowing conditions. Because it can vary slightly across the flow range, precise installations use a K-factor curve rather than a single value.
The high-frequency output comes directly from the blade-sensing pickup and produces many pulses per unit volume, giving fine resolution suited to custody flow computers. The low-frequency output is scaled down to fewer pulses per unit volume and is used for simpler totalizers and counters. Custody measurement usually uses the high-frequency signal to keep resolution and accuracy high.
Two pickups allow a pulse-integrity check. The flow computer compares the two pulse streams for the expected count and phase relationship, and flags a fault if they diverge. This catches problems like a failing pickup, wiring noise, or a damaged rotor that could otherwise corrupt the measurement without warning, which matters because the pulse count directly determines the billed volume.
This page references the standards, specifications, and official documentation published by the organizations below. Editions, product capabilities, and documentation change over time - confirm current requirements and specifications directly with the source.
Last reviewed: July 27, 2026. Merobix is not affiliated with, endorsed by, or sponsored by these organizations; their names are used only to identify the standards and products discussed.
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