In the system line, moving liquid reaches on both sides of the bluff body and shed alternately vortices. The speed of flow 5 defines the rate of shedding of the vortices. The shedding of the vortices causes the sensor to detect a vibration in the bluff core. Pulses are sent from the sensor to the transmitter if the appropriate loop output is developed.
Input and Output Measurement Standards:
Calibration of the transmitter of a vortex shedding flowmeter requires an input level capable of simulating the transmitter’s electrical pulses. This input is supplied by a frequency generator. See the manufacturer’s literature for a more detailed explanation of the features of a particular frequency generator.
Settings and Connections:
Determine the vortex shedding frequency 16 prior to making the connections. Normally the manufacturer provides the vortex shedding frequency, but if it is not specified in the manufacturer’s literature, measure the frequencies using this formula:
Vortex Shedding Frequency = RF x CF x ( URV/TIME)
The vortex shedding frequency is represented in pulses per second or PPS.
PPS = represents the alternate shedding of vortices on either side of the bluff body
RF = stands for reference factor which can be found on the transmitter’s data plate and is usually represented in pulses per US gallon
CF = is the conversion factor, and is a number found in the manufacturer’s conversion table, the CF converts the RF to actual volume or mass flow rate units
URV = is the upper range value in US gallons per minute
TIME = is related to the increment in which the flow is measured
For the coarse span jumpers, see the manufacturer’s instructions for the appropriate setting.
The frequency generator can be connected to the transmitter’s input terminals once the PPS has been determined and the span jumpers set in their proper positions. The calibration loop’s output side is connected in series. Despite all the connections are made, set the fine span.
Adjustments and Accuracy:
The frequency generator is set to the transmitter’s upper range value for setting the fine span. Adjust the fine span screw until the milliameter shows 100 percent output. Upon completion of the fine span adjustment, as indicated on the output standard by a 20 mA reading, adjust the zero.
Disconnect the generator frequency and connect the signal to the coaxial cable shield.
Zero is calibrated to the low range value so that after the fine period change the generator has to be set to the appropriate setting.
The zero is set at the lowest input value until the output signal matches the input signal. It is important to adjust and adjust the span and zero until both are right.