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LEVELtrol-II Installation & Operating Instructions
7. Principle Of Operation
General
Operation
Determining the
Liquid Quantity
for Various Tank
Geometries
Determining Liq-
uid Level
7.1 General Operation
The determination of the level and the amount of material in a tank is a
commonly needed industrial measurement. In other applications it is desired
to dispense liquid from one container to another. This instrument is intended to
satisfy these needs.
A variety of displays, analog outputs, pulse outputs, alarms, and
communications ports are provided to suit the various application needs which
might be encountered.
7.2 Determining the liquid quantity for various Tank Geometries
Various tank geometries are used in industry as containers for liquids. These
tank geometries may be classified as vertical cylindrical, horizontal cylindrical,
or spherical. Many “calibrated” tanks come with a “strapping chart” which
equates the volume in the tank for various liquid level measurements.
This instrument permits the user to either enter his tank shape and dimensions
or strapping table. The unit then computes the volume in the tank for the
measured liquid level based on mathematical equations solved by the
instrument.
Tanks often have a discharge location which is somewhat above the base of
the tank so that the tank can never be fully drained. The quantity of fluid below
the discharge point is sometimes called a “reserve” amount. The instrument
computes both the total amount and the available amount from the discharge
location information entered by the user.
7.3 Determining Liquid Level
A liquid level measurement is required in order to compute the quantity in a
tank.
There are a wide variety of liquid level measurement techniques to choose
from. Among the most common measurement techniques are:
a. hydrostatic pressure measurement
b. ultrasonic level measurement
The equations necessary to compute the liquid level are different for the two
techniques. This instrument uses the correct equation for the level sensor
type you have chosen. The level calculations also include adjustments for the
location of the sensor above the base of the tank.
7.4 Corrections for density in hydrostatic pressure based level
measurements
The hydrostatic pressure measured at the base of a tank is a function of the
liquid level and the density of the fluid.
The LEVELtrol-II provides for the connection of a compensated input, a
temperature or density transmitter, which will enable accurate determination of
the liquid level in the tank. Stored fluid properties are used to infer the density
from a temperature measurement.
