The basic construction of the turbine flowmeter incorporates a bladed turbine rotor installed in a flow tube. The rotor is suspended axially in the direction of flow through the tube. The turbine flowmeter is a transducer, which senses the momentum of the flowing stream. The bladed rotor rotates on its axis in proportion to the rate of the liquid flow through the tube.

TURBINE ROTATION
As the liquid product strikes the front edge of the rotor blades, a low-pressure area is produced between the upstream cone and the rotor hub.
The blades of the turbine rotor will tend to travel toward this low-pressure area as a result of this pressure differential across the blades. The pressure differential (or pressure drop) constitutes the energy expended to produce movement of the rotor. The initial tendency of the rotor is to travel downstream in the form of axial thrust. But since the rotor is restrained from excessive downstream movement, the only resulting movement is rotation.
Fluid flowing through the meter impacts an angular velocity to the turbine rotor blades, which is directly proportional to the linear velocity of the liquid. The degree of the angular velocity or number of revolutions per minute of the turbine rotor is determined by the angle of the rotor blades to the flowing stream of the approach velocity.

ROTOR BALANCE
With axial thrust forcing the turbine rotor downstream, the friction resulting from contact between the turbine rotor and the downstream cone would cause excessive wear if there were not some means of balancing the turbine rotor on its axis between the upstream and the downstream cone.
Bernoulli's Principle states that when flow velocity decreases, the static pressure increases. Therefore, a high-pressure area exists at the downstream side of the turbine rotor exerting an upstream force on the rotor. As a result, the turbine rotor is hydraulically balanced on its axis.

SIGNAL OUTPUT
Electrical output is generated using the principle of reluctance. A pickup coil, wrapped around a permanent magnet, is installed on the exterior of the flow tube or the meter body immediately adjacent to the perimeter of the rotor (Figure 1). The magnet is the source of the magnetic flux field that cuts through the coil. Each blade of the turbine rotor passing in close proximity to the pickup coil causes a deflection in the existing magnetic field. This change in the reluctance of the magnetic circuit generates a voltage pulse within the pickup coil.
Each pulse generated represents a discrete amount of volumetric throughput. Dividing the total number of pulses generated by the specific amount of liquid product that passed through the turbine flowmeter determines the K-Factor. The K-Factor, expressed in pulses per unit volume, may be used with a factoring totalizer to provide an indication of volumetric throughput directly in engineering units. The totalizer continuously divides the incoming pulses by the K-Factor (or multiplies them with the inverse of the K-Factor) to provide factored totalization. The frequency of the pulse output, or number of pulses per unit time, is directly proportional to the rotational rate of the turbine rotor. Therefore, this frequency of the pulse output is proportional to the rate of the flow.

By dividing the pulse rate by the K-Factor, the volumetric throughput per unit time of the rate of flow can be determined. Frequency counters or converters are commonly used to provide instantaneous flow rate indication. Plotting the electrical signal output versus flow rate provides the characteristics profile or calibration curves for the turbine flowmeter.
Electrical output is also generated using the principle of inductance. A pickup coil is installed on the exterior of the flow tube immediately adjacent to the perimeter of the turbine rotor. The magnetic source of the flux field in this type of output is either the rotor itself or small magnets installed in the rotor. In the case of the rotor, the material of construction would be nickel or some other easily magnetized flux field. The results are identical to that of the reluctance principal.

ACCURACY
The accuracy of a turbine flowmeter is derived from its output (electrical or mechanical) and is the measure of the deviation of an indicated measurement from the referenced standard. Turbine meter accuracy is dependent upon several items.
The accuracy must include the error associated with the calibration standard. In the USA, the National Institute of Standards and Technology represents the flow standard.
Linearity is the variation of the flowmeter K-factor from a nominal value of a point on a curve. Normally during calibration, a value is chosen which makes linearity fall in line with accuracy. Linearity may remain constant during meter life although the absolute accuracy level has changed.
Repeatability is the ability of a turbine flowmeter to reproduce its output indefinitely under constant operating conditions at any point over its specified operating range.

SPECIFIC GRAVITY
The specific gravity of a liquid is the ratio of its density to that of water at 4BC (39.2BF) and is dimensionless. While changes in specific gravity do not affect the average turbine meter K-factor value, the overall linear range of the flowmeter is changed (Figure 2). The linear range represents the minimum to the maximum flow rate within which the linearity of the flowmeter is specified.
As stated previously, the rotor rotates due to pressure differential across the rotor blades. Specific gravity is one of the factors affecting this pressure differential. As the specific gravity decreases, the pressure differential decreases. On a fluid with a low specific gravity and a low flow rate, the pressure differential across the blades is very low. This leaves almost no energy for turning the rotor. Consequently, the rotor cannot turn in proportion to the liquid throughput and the K-factor drops off.

Therefore, the angle of the rotor blades is changed to help compensate for the change to a lower specific gravity. This allows products with lower specific gravity's to be measured accurately by the turbine flowmeter.