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Measuring Liquid Level in a Vessel with Gaseous Head Pressure Introduction
Pressure measurement is commonly used to
determine liquid level for a variety of applications.
A column of liquid exerts a downward force due
to gravity. Pressure measurement at the base of
the column may be used to measure this force,
translate it into head pressure, which can then
be translated into liquid level depending on the
media specific gravity. Pressure measurement to
determine liquid level is particularly useful for
applications with foaming or bubbling liquid,
where other level measurement technologies
have difficulties.
Process liquids contained in vessels are often
kept under pressure with nitrogen gas. The
nitrogen gas maintains purity and can be used
to transport liquid without needing a pump.
In order to measure the liquid level, the gaseous
pressure in the head of the vessel must be mea-
sured with a second transducer. The head pressure
is subtracted from the overall pressure to obtain
the hydrostatic pressure due only to the column
of liquid.
Step 1. Compute pressure difference, P,
between the gaseous head pressure (P2) and
overall pressure (P1). If there is no gaseous
head pressure P2=0.
Step 2. Multiply by conversion factor.
Step 3. Divide by specific gravity of liquid, s.g.
The second transducer is not needed if the vessel
is vented and maintained at atmospheric pressure.
Example1. No Head Pressure
30% hydrogen peroxide is in a 70 tall vessel at
room temperature. The head of the vessel is at
atmospheric pressure. The specific gravity of
room temperature 30% hydrogen peroxide is 1.11.
An NT
®
Flow-through Pressure Transducer, model
4210, that measures from 0 to 3 PSIG (0 to 21 kPa)
is installed on the outlet at the base of the vessel.
Using the formula above, each PSI of pressure
represents 25.0 of liquid height. A full vessel
with liquid level at a height of 70 will measure
2.8 PSIG (19 kPa). An empty vessel will measure
0.0 PSIG. A monitoring device measuring the
pressure transducer output can be scaled to
display the liquid level in inches, such as the
NT
®
Dual Channel Display.
MEASURING LIQUID LEVEL IN A VESSEL
WITH GASEOUS HEAD PRESSURE
Liquid Level
(inches)
=
27.7/PSI
×
(P1[PSIG] - P2[PSIG])
Liquid specific gravity P
=
P1 - P2
=
27.7
×
P
s.g.
Liquid Level (inches)
=
10.07cm
×
P
s.g.
Liquid Level (centimeters)
or
27.7
×
P (PSI)
PSI
10.07 cm
×
P (kPa)
kPa
or
Figure 1: NT® Pressure Transducers Entegris
®
is a registered trademark of Entegris, Inc.
NT
®
is a registered trademark of NT International, an Entegris company
ENTEGRIS, INC.
Corporate Headquarters / 3500 Lyman Boulevard / Chaska, Minnesota 55318 USA
Customer Service Tel. 763-502-0200 or Toll Free 877-503-0200 / Customer Service Fax 763-502-0300
www.entegris.com / www.entegrisfluidhandling.com
The materials integrity management company
©
2004 Entegris, Inc. Printed in USA 3960-1676MAX-0204
Example 2. Head Pressure Present
98% sulfuric acid is in a 42 tall vessel at room
temperature, specific gravity 1.83. Two NT
®
Flow-
through Pressure Transducers that measure 0 to 6
PSIG (0 to 41 kPa) are mounted one each on the
outlet at the base of the vessel and on the inlet at
the top of the vessel. The vessel is under a 3 PSIG
(21 kPa) gaseous nitrogen blanket, as measured
by the second transducer.
Using the formula, each PSI of pressure represents
15.1 of liquid height. A full vessel with liquid level
at 42 will measure 5.8 PSIG (40 kPa) from the
lower transducer. An empty vessel will measure
3.0 PSIG (21 kPa). An NT
®
D80 Dual Channel
Display can be used to simultaneously measure
both pressure transducers, display the vessel
liquid height in inches, and alarm at high or low
liquid levels.
Entegris designs and manufactures measurement
instruments for the high purity and corrosive
chemical environments of the semiconductor
industry. Our products measure flow, pressure,
and level for the various acids, caustics, solvents
and slurries used in the industry.
For More Information
For more information on Entegris liquid level
measurement instruments or our complete line
of fluoropolymer fluid handling solutions, contact
your local Entegris distributor or Entegris, Inc.
To review our complete line of sensing and
control product solutions visit Entegris Web
site at www.entegrisfluidhandling.com or
contact Entegris Customer Service.
Figure 2: Measuring liquid level in a vessel with gaseous head pressure using two
pressure transducers and a dual channel display.
Figure 3: Sulfuric acid liquid level vs. pressure measurement for
Example 2.
Sulfuric acid liquid level (inches)
Example #2
0
42
PSIG
3.0
5.8
0
21
4.4
Channel B
Display liquid level in inches
NT® Pressure Transducer
Measures head pressure
NT® Dual Channel Display
Channel A
NT® Pressure Transducer
Measures total pressure
Level being measured
42
(1,067 mm)