Chapter 2: Input Devices and Sensors
Part 2: Analog Devices

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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