Pressure and level measurement
Level measurement in open, vented vessels
In hydrostatic level measurement in open or vented basins or vessels,
a continuous pressure compensation of the ambient air with the gas
phase above the liquid takes place. Thus the ambient pressure that
acts on the medium as an additional ‘force’ always resembles the
ambient pressure acting on the whole system, including the level
sensor. If one therefore uses a pressure sensor with a relative pressure
measuring cell, a pressure sensor that is compensated or vented (just
like the tank) to the ambient pressure, it ‘automatically’ compensates
for the effect of this ambient pressure on the level measurement. This
means that a relative pressure sensor in vented vessels and tanks
completely ‘cancels out’ the atmospheric pressure overlying on the
liquid from the level measurement. Thus, the hydrostatic pressure
corresponds only to the filling height of the liquid (Figure 2). There-
fore, the filling height of an open tank or vessel is calculated using
the following equation:
h = p / (
ρ
* g)
p = hydrostatic pressure [bar (relative)]
ρ
= density of the liquid [kg/m³]
g = gravitational force or gravitational acceleration [m/s²]
h = height of the liquid column [m]
As a simple rule of thumb for water as a medium, one can assume that
a pressure of 1 bar (relative) corresponds to a filling height of 10 m.
Rule of thumb (media of density ~ 1 000 kg/m³ ): h = 1 bar (relative) /
(1 000kg/m³ * 10 m/s²) = 10 m
The rule of thumb demonstrates that the hydrostatic pressure in a
vessel is proportional to the filling height of the medium, as long as
it maintains a constant density, completely independent from shape
or fill quantity.
Figure 2: Level measurement in open, vented tanks and vessels.
Level measurement in sealed, gas-tight vessels
The level measurement in sealed, gas-tight vessels, which is fre-
quently found in the chemical industry, requires a compensation of
the pressure of the enclosed gas phase above the liquid. The enclosed
T
ake note
pressure of the gas phase acts as an additional force on the liquid
and distorts any hydrostatic pressure measurement at the base of the
vessel. Thus, this distorting influence must be compensated through
an additional pressure measurement of the gas phase. Frequently,
a second pressure sensor is used for the measurement of the gas
pressure. This application effectively represents a differential pres-
sure measurement, where the two separate pressure measurements
are offset against each other (Figure 3). This offset calculation can
be made either by two individual sensors or via an integrated dif-
ferential pressure sensor. In this application, the sensors used can
either be relative (sensor with ambient pressure compensation) or
absolute pressure variants (sensor with sealed vacuum reference).
Thus, the filling height of a sealed tank or vessel is calculated using
the following equation:
h = (p
2
- p
1
) / ((
ρ
* g)
p
2
= hydrostatic pressure [bar]
p
1
= pressure of the enclosed gas in the vessel [bar]
ρ
= density of the fluid [kg/m³]
g = gravitational force or gravitational acceleration [m/s²]
h = height of the liquid column [m]
Figure 3: Level measurement in sealed, gas-tight tanks and vessels.
Types of hydrostatic level sensors
In hydrostatic level measurement, one can differentiate three types
or designs of level sensors: conventional pressure transmitters, proc-
ess pressure transmitters and submersible pressure transmitters,
available in relative, absolute and differential pressure variants. For
application in tanks and free-standing vessels, conventional pressure
transmitters (Figures 4 and 5) or process pressure transmitters (Fig-
• Level measurement has moved from being purely mechanical to involving
complex electronics.
• Hydrostatic level measurement however, remains the most important
measuring principle.
• Hydrostatic sensors are expected to continue to show increased market
share.
51
June ‘13
Electricity+Control
