Chapter 2
Input Devices and Sensors
Liquid Level Measurements
Liquid level measurements are typically used in DDC control systems
for HVAC applications to monitor and control levels in thermal storage tanks,
cooling tower sumps, water system tanks, pressurized tanks, etc.
Types of Liquid Level Sensors
Numerous sensing technologies are available. Common technologies
applicable to HVAC system requirements are based on hydrostatic pressure,
ultrasonic, capacitance and magnetostrictive-based measurement systems.
Hydrostatic
Level measurement by hydrostatic pressure is based on the principle that the
hydrostatic pressure difference between the top and bottom of a column of
liquid is related to the density of the liquid and the height of the column.
For open tanks and sumps, it is only necessary to measure the gauge pressure at
the lowest monitored level. For pressurized tanks it is necessary to take the
reference pressure above the highest monitored liquid level. Pressure
transmitters are available that are configured for level monitoring
applications. Pressure instruments may also be remotely located, however this
makes it necessary to field calibrate the transmitter to compensate for
elevation difference between the sensor and the level being measured.
Bubbler type hydrostatic level instruments have been developed for
use with atmospheric pressure underground tanks, sewage sumps and tanks, and
other applications that cannot have a transmitter mounted below the level being
sensed or are prone to plugging. Bubbler systems bleed a small amount of
compressed air (or other gas) through a tube that is immersed in the liquid,
with an outlet at or below the lowest monitored liquid level. The flow rate of
the air is regulated so that the pressure loss of the air in the tube is
negligible and the resulting pressure at any point in the tube is approximately
equal to the hydrostatic head of the liquid in the tank.
The accuracy of hydrostatic level instruments is related to the
accuracy of the pressure sensor used.
Ultrasonic
Ultrasonic level sensors emit sound waves and operate on the principle that
liquid surfaces reflect the sound waves back to the source and that the transit
time is proportional to the distance between the liquid surface and the
transmitter. One advantage of the ultrasonic technology is that it is
non-contact and does not require immersion of any element into the sensed
liquid. Sensors are available that can detect levels up to 200 feet from the
sensor. Accuracy from 1% to 0.25% of distance and resolution of 1/8" is
commonly available.
Capacitance
Capacitance level transmitters operate on the principle that a capacitive
circuit can be formed between a probe and a vessel wall. The capacitance of the
circuit will change with a change in fluid level because all common liquids
have dielectric constant higher than that of air. This change is then related
proportionally to an analog signal suitable for DDC analog inputs. Resolution
of 1/8" and accuracy of 1% to 0.25% of span are available.
Magnetostrictive
Magnetostrictive level transmitters (Figure 2.23) operate on the principle that
an external magnetic field can be used to cause the reflection of an
electromagnetic wave in a waveguide constructed of magnetostrictive material.
The probe is composed of three concentric members. The outermost member is a
protective, product-compatible outer pipe. Inside the outer pipe is a
waveguide, which is a formed element constructed of a proprietary
magnetostrictive material. A low-current interrogation pulse is generated in
the transmitter electronics and transmitted down the waveguide creating an
electromagnetic field along the length of the waveguide. When this magnetic
field interacts with the permanent magnetic field of a magnet mounted inside
the float, a torsional strain pulse, or waveguide twist, results. This
waveguide twist is detected as a return pulse. The time between the initiation
of the interrogation pulse and the detection of the return pulse is used to
determine the level measurement with a high degree of accuracy and reliability.
Accuracy and resolution of 1/16" or better are available from some
manufacturers.
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