What K-factor is
Turbine, positive displacement, oval gear and many vortex meters give a pulse output rather than an analogue signal. Each pulse represents a fixed increment of volume, and the K-factor states how many pulses make up one unit:
K-factor is not a catalogue figure — it belongs to the individual meter and comes from its calibration certificate. Two identical meters from the same batch will have slightly different K-factors, which is why swapping a meter without updating the K-factor in the flow computer introduces an immediate error.
Worked example
A turbine meter with K = 100 pulses/L reading 50 Hz:
- Q = 50 ÷ 100 = 0.5 L/s = 30 L/min = 1.8 m³/h
- Pulse period = 1 ÷ 50 = 20 ms between pulses
- To totalise in litres, the counter needs a scale factor of 1/100 = 0.01 L per pulse
Now prove it: 4,520 pulses counted while 45.0 L passed through the prover gives K = 4,520 ÷ 45.0 = 100.44 pulses/L — the meter is producing slightly more pulses per litre than its nameplate says, so it was over-reading by about 0.44%.
K-factor versus meter factor
Both correct the same reading, but they are used differently. In custody transfer practice the K-factor is left alone and a meter factor is applied on top:
A meter factor of 0.9956 means the meter over-reads by 0.44% and every indicated volume gets multiplied by 0.9956. The reason for keeping K-factor fixed is traceability — the meter factor history becomes a record of how the meter is drifting, and a step change between provings signals wear, damage or a change in fluid conditions. Adjusting the K-factor each time would erase that history, which is the same as-found discipline that applies to any calibration record.
Field notes
- K-factor is not constant across the range. Certificates list it at several flow points; a single number is an average that is wrong at both ends. Below the meter's linear range, bearing drag makes the rotor lag and the error grows quickly.
- Viscosity shifts the curve. A meter calibrated on water and installed on oil will not hold its K-factor. Universal viscosity curves exist for this reason.
- Check the flow regime. Turbine meters need developed turbulent flow — verify with the Reynolds number calculator at minimum flow, and give the meter its specified straight run.
- Watch the counter's input speed. High K-factor meters at high flow can exceed a PLC input's counting rate; the pulse period output here tells you what the input has to keep up with.
- Pulse outputs are noise-prone. Long unshielded runs pick up spurious counts that read as flow that never happened — a totaliser that creeps at zero flow is the classic symptom.
Frequently asked questions
What is the K-factor of a flow meter?
The number of output pulses the meter produces per unit volume passed — commonly pulses per litre or per US gallon. It is a physical property of that individual meter, established by calibration, and it converts pulse frequency directly into flow rate.
How do I calculate flow rate from pulse frequency?
Flow rate equals frequency divided by K-factor. With frequency in Hz and K in pulses per litre, multiply by 60 to get litres per minute: Q = f × 60 ÷ K. A 100 pulses per litre meter reading 50 Hz is passing 30 L/min.
What is meter factor and how is it different from K-factor?
K-factor converts pulses to volume. Meter factor is a dimensionless correction applied afterwards, equal to true volume divided by indicated volume. Custody transfer systems keep the K-factor fixed and adjust the meter factor after each proving run.
Why does K-factor change with flow rate and viscosity?
Turbine meters rely on the rotor spinning proportionally to flow, and bearing drag becomes significant at low flow or high viscosity, so the rotor lags. That is why calibration certificates give K-factor across several flow points rather than one number, and why linearity is specified over a stated range.
Provided for reference and education. Custody transfer measurement must follow the applicable API MPMS or ISO procedures. See our disclaimer.