Liquid Turbine Flowmeter with Pulse Output – K-Factor Explained
JLC International is selling the well-known liquid trubine flowmeters of the series 800, 900 and 1000 for many years and for all kinds of applications; from dozing chemicals in agriculture spraying machines to measuring the few ounces of a liquour shot in a casino.
The flowmeter has a precision turbine at its core. The turbine is rotating almost frictionless in robust sapphire bearings. The embedded ceramic magnets in the turbine blades are encapsulated and with every turn detected through the chamber wall by a Hall Effect detector. The Hall Effect detector creates a NPN pulse with a height slightly less than the power supply, which can be between 4.5 to 24 Vdc.
Of course, the number of pulses per second is called the frequency, expressed in 1/sec or Hertz (Hz) and is proportional to the flow. When the flowmeter is calibrated at the factory, the meter produces a certain number of pulses per unit of volume called the K-factor, e.g. 7500 pulses per gallon, depending on the flow range of the flowmeter.
If the pulses are counted it is easy to determine how many gallons in total passed through the system (the flowmeter) from start to finish, e.g. if 26250 pulses are counted it would mean that 3.5 gallons of liquid have passed. If the time is recorded between start and finish the average flow rate in gallons per minute can be determined. If the counting and timing is done on a continuous base the flow rate and total flow at any given moment is known.
The series 800, 900 and 1000 come with a calibration certificate over 6 or more points. It provides the values of the flow rate coming from the calibration rig, e.g. 0.154 gpm, and the frequency produced by the flowmeter for each calibration point. The devices to create the flow and measure the frequency are very accurate and are called calibration standards. They are checked and calibrated themselves by the designated authorities using higher standards, making all standards traceable to the main source; in the USA; for instance, the National Institute of Standards and Technology (NIST) and for the UK the National Physical Laboratory (NPL).
If the flow rate and the frequency are known for each calibration point, the K-factor for that point is determined as well. The K-factor is the frequency divided by the flow rate => [1/s] / [g/s] = 1/g. Knowing the K-factor for each calibration point, the factory determines the best fitting K-factor for that particular flowmeter . They do that for instance by averaging all K-factors, or use another appropriate mathematical method. The calibration certificate states “the best K-factor”, next to the information under what conditions the calibration has taken place, as temperature, pressure, liquid, etc.
Every paddle, Pelton wheel and turbine flowmeter produces a pulse output of some sort and each flowmeter has its own K-factor and they are all different. Installing a flowmeter means that the frequency device needs to be set up for the frequency range and the K-factor of that particular flowmeter.
Only then the frequency counter can indicate the flow rate and the total flow, and only then the system is accurate and according to specifications. If a flowmeter has to be replaced sometime in the future, the frequency counter has to be reprogrammed with the new K-factor. If that has not been done, the new flowmeter will run against the wrong K-factor and the indication of the flow rate and total flow will be several percentage points off, which can easily be 5 – 10 %.
If many flowmeters have to be installed on, for instance, a spraying machine, each set of flowmeter and frequency counter is set for a different K-factor. Coming from the same batch (shipment) of flowmeters, most likely the K-factors are close, but not the same. It is laborious and cumbersome to program each frequency counter with different settings, but if certain accuracies have to be achieved, it has to be done.
However, for these multiple flowmeter applications, there is now a very clever solution. We have seen that the frequency, flow rate and K-factor are proportional. Therefore, if an additional onboard electronic package can scale the frequency output proportional to the flow, the K-factor is determined. And if the K-factor can be set to be the same value for each flowmeter of a batch of a very large number of flowmeters, each installation is identical.
The so-called Pre-Scaler board fits inside the flowmeter housing of the series 800, 900 and 1000 and receives the pulses directly from the Hall Effect detector. As described above, as the flowmeter is calibrated at the factory the K-factor is determined. The software of the Pre-Scaler demands the input of the actual K-factor and the (fixed) required K-factor; after that the software does the rest.