LEVEL MEASUREMENT Laboratory Procedure Background InformationDirect Level MeasurementA dev...

LEVEL MEASUREMENT Laboratory Procedure Background Information

Direct Level Measurement

A device called a gauge glass, or sight glass, can be used to read the level of a lank directly. Transparent glass allows the technician lo actually see the level and read it in a direct manner. The principle of operation is simple; gravity causes the level in two interconnected containers to be the same. Therefore, the liquid level seen in the sight glass should be the same as the level in the tank.

Indirect Level Measurement

A differential pressure (d/p) transmitter can be used to infer level by measuring the hydrostatic head pressure exerted by the liquid above the transmitter’s datum (zero) line. When using a differential pressure transmitter to measure the level in a tank, a correlation between the head pressure produced by the liquid and the output of the transmitter must be established. The act of establishing this correlation is called calibration. A d/p transmitter is an indirect method as opposed to the direct reading provided by the sight-glass. When a transmitter is properly calibrated, its output signal will have a linear relationship lo the level in the tank. For example, the output of a typical 4-20 mA analog transmitter at 0% level should be 4 mA and at 50% level should be 12 mA.

Materials Needed

• tall cylindrical tank (2-4 feet tall)

• pressure or differential pressure transmitter with an appropriate calibrated range matching the height of the cylinder

• gauge glass instrument attached to a tank containing a liquid

• 24 VDC power supply to supply power to the transmitter

• digital multimeter with a milliampere test scale

Safety Requirements

Safety glasses are required in the lab.

Procedure

Perform the following lab activities:

FIRST ACTIVITY: Identify and describe level instruments

Procedure

a. Identify and describe the function of each of the level measuring instruments displayed in the lab.

b. Record your answer and observations.

SECOND ACTIVITY: Perform a Direct Level Measurement

Procedure

a. Read the liquid level as accurately as possible and record your results.

Level reading = ______ inches

b. Convert this reading to hydrostatic head press for use in Activity #3.

Head pressure = ______ inches w.c.

c. Explain how you lined up the liquid in the gauge glass with the associated scale.

THIRD ACTIVITY: Perform an Indirect Level Measurement Procedure

a. Attach or make sure that the high-pressure side of the d/p transmitter is connected lo the lowest point in the lest cylinder.

b. With no liquid in the cylinder and power applied to the transmitter, confirm that its output is reading 4 mA (3.0 psi for a pneumatic transmitter). Output reading: ______ mA (psi)

c. Fill the cylinder to a point above the datum line and open the bleed valve on the high side of the d/p transmitter to bleed air from the tubing and transmitter chamber. Once all the air is bled and water is flowing steadily, close the bleed valve.

d. Fill the cylinder with water until the output of the transmitter is equal lo 8 mA, then slop and record the results. Then move to 12. mA, 16 mA, and 20 mA stopping and recording each lime in the same manner as with 8 mA. (Pneumatic signal: 6 psi, 9 psi. 12 psi, and 15 psi)

Findings

For Activity 1, write a description of each type of level transmitter studied in this lab. Explain the basic concept behind each instrument as well as any additional observations you may have made while examining it. Look in your book and/or handout for additional information about these transmitters.

For Activities 2 and 3, use all readings taken in this lab to further explain the basic concept behind the specific measurement techniques used. For example, explain how the liquid in the gauge glass directly represents the liquid level of the tank or how the head pressure equates to liquid level. Finally, explain the difference between direct and indirect level measurement using the information gathered in this lab.

Attach the following information to the back of the written lab report:

• All measurements taken during the lab experiment.

• All pages associated with this lab including the lab handout itself.

• Answers to the following questions. Use classroom notes to help answer these questions.

• What can cause the level of the gauge glass (sight glass) to be different from the level in the tank?

• If the liquid in the gauge glass were concave in appearance, would you lake your reading with the liquid next to the tube wall or with the bottom of the meniscus?