1. Mechanical Measurement and Instrumentation MECN 4600
Professor: Dr. Omar E. Meza Castillo
Department of Mechanical Engineering
Inter American University of Puerto Rico
Bayamon Campus

2. Tentative Lecture Schedule
Topic
Lecture
Basic Principles of Measurements
Response of Measuring Systems, System Dynamics
Error & Uncertainty Analysis
1, 2 and 3
Sensor & Transducers
Basic Electronics, Signal Processing 4
Measurement of Pressure 5
Measurement of Temperature
6, 7, 8
Measurement of Stress-Strain 9
Measurement of Time Constant 10
Measurement of Level 11

3. Differential Pressure Cell (DP Cell)
Topic 11: Level Measurement
Differential Pressure Cell (DP Cell)

4. Course Objectives
To measure the level of a tank using a differential pressure cell (DP Cell).

5. Introduction:
The sensing and control of product levels in containers involves a wide range of materials-liquids, powders, slurries, and granular bulk. All level measurement involves the interaction of a sensing device, element, or system with material inside a container. You can use a wide variety of physical principles to measure level-sight, pressure, radiation, and electric and sonic principles.
Three sight-type level sensors are glass gauges, displacers, and tape floats.

6. Glass gauges: are the most widely used instruments for measuring process tank level. Two types of level glass gauges measure liquid level in process tanks: tubular and flat gauges. The tubular type works on the same principle as a manometer. As the liquid level in an open tank rises or falls, the liquid in the glass tube will rise or fall. The gauges consist of glass, plastic, or a combination of the two materials.

7. Tape float: One of the most simple, direct methods of float level measurement is the tape float gauge. A tape connects to a float on one end and to a counterweight on the other to keep the tape under constant tension. The float motion results in the counterweight riding up and down a direct-reading gauge board, thereby indicating the level in the tank.
Standard floats are normally cylindrical for top-mounted designs and spherical or oblong for side-mounted designs. Small-diameter floats see use in higher density materials.

8. Bubblers: The air bubbler is another pressure-type level sensor where you install a dip tube in a tank with its open end a few inches from the bottom. A fluid forces itself through the tube; when the fluid bubbles escape from the open end, the pressure in the tube equals the hydrostatic head of the liquid. As liquid level (head) varies, the pressure in the dip tube changes correspondingly..

10. Ultrasonic Level Measurement
Ultrasonic level sensors measure the time required for sound waves to travel through material. Ultrasonic sound waves generally have frequencies above 20 kiloHertz. Ultrasonic instruments operate at frequencies inaudible to the human ear and at extremely low power levels, normally a few thousandths of a watt. The velocity of a sound wave is a function of the type of wave transmitted and the density of the medium in which it travels.

11. Ultrasonic Level Measurement

12. Pressure-type instruments: DP CELL
Another common example is closed-tank level measurement. If the pressure in the closed tank changes, an equal force applies to both sides of the differential pressure (dP) transmitter. Because the dP cell responds only to changes in differential pressure, a change in static pressure on the liquid surface will not change the output of the transmitter. Thus, the dP cell responds only to changes in liquid level when the specific gravity of the liquid is constant.

13. Pressure-type instruments: DP CELL

14. Pressure-type instruments: DP CELL

15. The Principle of the Differential Pressure
Small and moderate pressure differences are often measured by using a device know as a MANOMETER, which mainly consists of a glass or plastic U-tube containing a fluid such as mercury, water, alcohol, or oil.

16. The Principle of the Differential Pressure
Analysis
P2 = P1
AP1 = APatm + W
W = mg = ρVg = ρAgh
P1 = Patm + ρgh (kPa)
La pressure difference can be expressed as:
ΔP = P1 – Patm = ρgh (kPa)
h = ΔP/(ρg) = [(kgm/s2)/m2]/[(kg/m3)(m/s2)]= m

17. DP Cell System
In the DP Cell System, the pressure is transformed by a system of data acquisition (LABVIEW) and converted in Volts
The expression for this is a lineal expression:
Where a and b are calculated by the Least Squared Method

18. Laboratory: Measurement of Level in a tank
Table of Data
Setup the system
Take the data discharging the tank #
Height (m)
Voltage (V) Group 1
Voltage (V) Group N
Average
Voltage (V) 1 30 2 28 . 11