Manual

ManualsBrandsDwyer ManualsSensorsCRF2

OPERATING PRINCIPLE

Capacitance and Dielectrics

Capacitance is the property of two or more conductors to store a charge

when there is a voltage difference between the conductors. In other

words capacitance relates the voltage between two conductors and the

amount of charge that can be held on the conductors (i.e., the number

of electrons). Capacitance is measured in Farads. Since a Farad of

capacitance represents a very large charge storage capacity, most

capacitance encountered is generally measured in microFarads (µF, 10-

6) or picoFarads (pF, 10-12). Capacitances encountered in level sensing

applications are generally in the 10’s or 100’s of pico Farads range.

The material between the conductors affects the capacitance also.

Insulating materials do not allow free movement of electrons, however

in an electric field the molecules of these materials will tend to align with

the field thus storing energy. This is called the dielectric effect and these

materials are often referred to as dielectrics. When placed between two

conductors the energy storage capability of these dielectrics will allow

more charge to be stored on the conductors for a given voltage

difference thus increasing the capacitance between the conductors. The

ratio of capacitance change caused by these dielectrics is referred to as

the dielectric constant. Different materials have differing dielectric

constants and will consequently change the capacitance between two

conductors more or less depending on the value of this constant. This

value ranges from 1.0 for a vacuum to over 100 for certain materials.

The dielectric constant for air is very close to 1.0 and usually assumed

to be exactly 1.0.

Capacitive level sensors determine the level of material by changes in

probe capacitance resulting from the movement of dielectric materials

between the probe and the reference ground electrode such as a tank

wall. Since measuring very small capacitance changes (less than 1 pF)

can be problematic in industrial environments, capacitance level sensing

tends to be most effective for materials with a dielectric constant greater

than 1.2. Since the difference in capacitance is being measured, it is

also possible to detect the level of two immiscible liquids that have

different dielectric constants such as oil and water.

Measurement

The CRF2 uses an impulse RF admittance measurement technique to

measure the probe capacitance. The impulse admittance measurement

offers advantages over other techniques in that it produces minimal

emissions to interfere with other communication or instrumentation

systems. The CRF2 continuously measures the probe capacitance.

Using this capacitance measurement, it computes a linear value with 0%

at the zero calibration value and 100% at the span calibration value.

From this the output current is computed and generated. Since no

assumptions are made regarding the relative value of the zero and span

calibration capacitances, the output can be set to measure from low to

high capacitance or high to low capacitance.

INSTALLATION

Unpacking

Remove the CRF2 from the shipping carton and inspect for damage. If

damage is found, notify the carrier immediately.

Materials

The CRF2 may be used to detect level of a variety of materials.

Conductive materials such as water require an insulated probe for

proper operation. When used with a conductive material, the material

itself must be grounded to the reference ground of the CRF2. This may

be done through a conductive tank wall or using an optional reference

ground electrode. Dry non-conductive materials may use either an

insulated or uninsulated probe. Capacitance level measurement is best

applied when the material dielectric constant is greater than 1.2. With

non-conductive materials, particularly low dielectric materials, the probe

should be spaced more closely to the reference ground to increase the

base capacitance and ensure reasonable sensitivity. The limiting factor

for spacing will be to ensure that material buildup around the probe is

avoided. For conductive materials this will be less of a concern since the

dielectric insulator around the probe is the predominant factor in the

capacitance changes with level.

Example

Series

Enclosure

Probe Type

Ground

Process

Connection

Probe Length

Options

CRF2

048

XXX

M20

CRF2-WR01T-048-M20

Capacitive Level Transmitter

Weatherproof

Remote Mount Weatherproof Housing

Rod

Cable

None Included

Attached ground rod (3˝ or 4˝ flange process connection types only)

Unattached ground rod

3/4˝ NPT male

1˝ NPT male

1-1/2˝ NPT male

3/4˝ BSPT

1˝ BSPT

1-1/2˝ BSPT

1˝ sanitary clamp

1-1/2˝ sanitary clamp

2˝ sanitary clamp

2˝ 150# flange, 316 SS

2˝ 150# flange, PVC

3˝ 150# flange, 316 SS

3˝ 150# flange, PVC

4˝ 150# flange, 316 SS

4˝ 150# flange, PVC

Insertion length in inches. Example 048 is 48˝ length.

Rod Type: minimum: 24˝, maximum: 144˝

Cable Type: minimum: 24˝, maximum: 360˝

M20 conduit connection with cable gland