Level measurements

1. LEVEL MEASUREMENT
Dr Sourav Poddar
Department of Chemical Engineering
National Institute of Technology
Warangal, TS, India

2. Measurement of material levels contained in a tank or silo is an integral activity in any controlled process
Level measurement sensors fall into two main types.
• Point level measurement sensors which are used to detect whether material in the container has reached a
preset level condition.
• Continuous level sensors which measure fluid/material level within a range, rather than at one point
Float: In these point level sensors, a magnetic float moves with the liquid surface, actuating a hermetically sealed
"reed switch" in the stem. The simple, low-maintenance design installs readily; minimizes shock, vibration, and
pressure; and works with a variety of media. The reed switch can be single pole, single throw (SPST) or single pole,
double throw (SPDT). The float of material that is lighter than the fluid follows the movement of the liquid level.
The position of the float, perhaps attached to a rod, can be determined to measure the level.
Reed-Relay Float Switch

3. Displacement: By Archimedes principle, a body immersed in a liquid is buoyed by a force equal to the weight of the liquid
displaced by the body. Thus, a body that is denser than the liquid can be placed in the vessel, and the amount of liquid
displaced by the body, measured by the weight of the body when in the liquid, can be used to determine the level.
Displacement level transmitter
Displacement level transmitter

4. Differential pressure:
The difference in pressures between to points in a vessel depends on the fluids between these two points. If the difference
in densities between the fluids is significant, which is certainly true for a vapor and liquid and can be true for two different
liquids, the difference in pressure can be used to determine the interface level between the fluids. Usually, a seal liquid is
used in the two connecting pipes (legs) to prevent plugging at the sensing points.

5. Capacitance: A capacitance probe can be immersed in the liquid of the tank, and the capacitance between the probe and
the vessel wall depends on the level. By measuring the capacitance of the liquid, the level of the tank can be determined.
Capacitance sensors can handle point or continuous level measurement. They use a probe to monitor liquid level changes in
the tank, electronically conditioning the output to capacitive and resistive values, which are converted to analog signals.
The probe and the vessel wall equate to two plates of a capacitor, the liquid to the dielectric medium. Because the signal
emanates from level changes alone, material build-up on the probe has no effect. Non-conductive fluid vessels may dictate
dual probes or an external conducting strip. The probe, which can be rigid or flexible, commonly employs conducting wire
insulated with PTFE. Rigid probes offer higher stability, especially in turbulent systems, where swaying of the probe can
cause signal fluctuations.
Capacitance Continuous Level Measurement Probes

6. Ultrasonic Sensors: A low-energy ultrasonic device within these sensors measures liquid level at a certain point.
Consisting of a field-mounted sensor and integral solid state amplifier, contact ultrasonic sensors have no moving parts
and require no calibration. Typically, they are equipped with terminal blocks for connection of a power source and
external control devices. The ultrasonic signal crosses a one-half inch gap in the sensor, controlling relay switches when the
gap contains liquid. The sensing level is midway along the gap for horizontally mounted sensors, at the top of the gap for
vertically mounted sensors. As liquid falls below this level, the ultrasonic signal attenuates and ultimately switches the
relay to its prior state. These sensors are used in vessels or pipes to automatically operate pumps, solenoid valves, and
high/low alarms. Two would be required to fill and empty tanks, and to meter liquid volumes. Compatible with most
liquids, they are unaffected by coatings, clinging droplets, foam, and vapor. However, highly aerated liquids and liquids
viscous enough to clog the sensor gap may cause problems.
Radar Level Sensor: In applications requiring non-contact
measurement of liquid levels, an ultrasonic transmitter is the
logical extension of ultrasonic sensors. They are ideal where
vapor, dust, or a foaming surface prevents ultrasonic
measurement. Their radar sensor uses microwave pulse
technology to track target liquids from the antenna tip to the
bottom of a tank.
Non-contact
Ultrasonic Sensor

7. Level
measurement
devices

8. Table 1. Level sensors

9. LEVEL GAUGES
Description Gauges are commonly used for indicating fill levels in
liquid holding tanks. Liquid level gauges are frequently employed to
monitor the level in a liquefied petroleum gas (LPG) tank or heating
oil tanks. The Babbitt LG-Series magnetic level gauge consists of a
float chamber, a float, and an external indication device. The float
chamber is basically a column of 2 1/2 inch pipe with process
connections to match those of the storage tank, reactor, drum,
column or other equipment where level is to be measured. These
connections may be side couplings or flanges, or top and bottom
flanges.
Symbol
Figure: Magnetic Liquid Level gauge

10. SPECIFICATIONS
Material : All wetted parts and external metal parts are stainless steel.
: Indicator tube is glass or polycarbonate.
Float chamber : 2 ½ inch pipe with RF slip-on/blind drain flange
(standard)
Pressure range : Up to 4000 psig
Min.specific gravity : 0.25
Temperature rating : -200oF to +1100oF continuous
Scale : Feet and inches standard with ½ inch scale divisions;
Resolution : Approximately ¼ inch
Indicator : 7/8 inch diameter by 11/4 inch long, brightly coloured red
Testing : Hydrostatic tested to the pressure rating specified
Welding : Boiler/piping codes ASME B31.1/B31.3

26. THANK YOU