Level Measurement

Level sensing can be

single point, continuous, direct, indirect

Continuous level monitoring

measures liquid level without interruption

What must be known for continuous level monitoring

the cross-sectional area of the container

Level measuring devices need to be

readily accessible

Why do level measuring devices need to be readily accessible

for inspection, maintenance, replacement

4 categories of level measurement

direct

indirect

single point

free-flowing solid measurement

Direct sensing means

the actual level is monitored

Indirect sensing means

a physical property of the liquid is sensed

Single point measurement

detects presence or absence of a liquid at a specific level

Which category of measurement measures the actual level

direct sensing

Which category senses a property of the liquid to determine the liquid level

indirect sensing

Which category of measurement only detects presence or absence at a specific point

single point measurement

Simplest method for direct visual reading

sight glass or gauge

How is a sight gas normally mounted

vertically, adjacent to the container and connected with shut off valves

Glass gauges should not be used with

hazardous liquid

Advantage of glass gauges

they are cheap

Disadvantage of glass gauges

can break easily

Two common types of float sensors

angular arm, pulley

Compare the density of the float to that of the liquid

the density of the float is less than the density of the liquid

Float with a pulley can be used with free-flowing solids, T/F

True it can be used w/free flowing solids

Advantages of a float sensor

almost independent of density of the liquid

accurate and robust

linear output with height of level

Which type of direct sensor has a linear output with height level

float sensor

What can affect the accuracy of float sensors

accumulation of stuff on the float

corrosion

chemical reactions

friction in the pulleys

Fuel level monitors are an example of what type of sensor

angular arm float

The angular arm float of a fuel gauge shows

output voltage from a potentiometer driven by the float

Disadvantage of angular arm float

nonlinear

Ultrasonic and sonic devices can be used for

single point or continuous level measurement

Ultrasonic/sonic devices: what is sent from a transmitter

pulse of sonic or ultrasonic waves

Ultrasonic/sonic devices: the waves are reflected from

the surface to the receiver

Ultrasonic/sonic devices: what is measured

the time it takes for the echo to reach the receiver

Ultrasonic/sonic devices: ___ gives the distance from the transmitter and receiver to the surface

the time delay

Ultrasonic/sonic devices can be used for

solids, corrosive and volatile liquids because there is no contact with the liquid

Which direct sensing device can be used to with solids and corrosive, volatile liquids

ultrasonic/sonic devices

Advantages of ultrasonic/sonic devices

reliable, accurate, cost-effective

can be used in high humidity

no moving parts

unaffected by density, conductivity

What does affect ultrasonic/sonic devices

vibration, high noise levels, dust

How can ultrasonic/sonic devices be used to measure the depth of the liquid

by placing the transmitter and receiver on the bottom of the container and measuring the time for an echo to be received from the surface back to the receiver

Common method of indirect level sensing

measure hydrostatic pressure at the bottom of a container

How is level determined using hydrostatic pressure

it is extrapolated from the pressure and the specific weight of the liquid

Pressure formula

p=yh

Instruments used to indirectly measure level using pressure

Capacitive probes, bubblers, resistive tapes, weight measures

Capacitive probe measures level in

nonconductive liquids

Capacitive probes consist of

inner rod with an outer shell

Bubbler devices require

supply of clean air or inert gas

Bubbler devices: gas from a __ is forced through a _

gas from a pressure regulator is forced through a tube

Bubbler devices: the open end of the tube is where

close to the bottom of the tank

Bubbler devices: how is the pressure required to force the liquid out of the tube related to the pressure at the end of the tube due to the liquid

they are equal

The pressure required to force the liquid out of the tube is the same as

the depth of the liquid multiplied by the specific weight of the liquid

Bubblers can or cannot be used with corrosive liquids

bubblers CAN be used w/corrosive liquids

Resistive tape: resistive element is placed where

close proximity to a conductive strip

The resistive element of resistive tape is inside of what

an easily compressed nonconductive sheath

When the liquid applies pressure to the resistive element what happens

a length of resistive element proportional to the depth of the liquid shorts out

Can resistive tape be used in corrosive chemicals and/or slurries?

yes they can

Device that is not recommended for use with explosive or flammable material

resistive tape

Advantage of resistive tapes

cheap

Disadvantage of resistive tapes

not rugged, not accurate

prone to humidity problems

Accuracy of resistive tapes depends on

material density

Load cells used to

measure the weight of a tank and its contents

Load cells are a direct or indirect measurement

indirect level measurement

What must be subtracted from the reading using a load cell

the weight of the container

Weight measurement well suited for continuous or spot measurement

continuous measurement

Types of probes used for single point sensing

conductive, thermal, beam breaking

Conductive probes used in what types of liquids

conductive and nonvolatile

Beam breaking probes are used for

pressurized containers

Beams from beam breaking probes can be

light, sonic/ultrasonic waves, radiation

Beam breaking probes can be affected by

deposits

Electric motor-driven paddle wheels used for solids in the form of

powder, grains, granuels

What feature of paddle wheels indicates the depth of the material

the torque