MODEL CHART
Example
CRF2 -W R 0 1T -048 -M20 CRF2-WR01T-048-M20
Series
CRF2
Capacitive level transmitter
Enclosure
W
R
Weatherproof
Remote mount weatherproof housing
Probe
Type
R
C
Rod
Cable
Ground
0
A
U
None included
Attached ground rod (3˝ or 4˝ flange process connection types only)
Unattached ground rod
Process
Connection
1T
2T
3T
1B
2B
3B
1S
2S
3S
1F
2F
3F
4F
5F
6F
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
Probe Length
XXX
Insertion length in inches. Example 048 is 48˝ length. Rod type min: 24˝, max: 144˝; Cable type min: 24˝, max: 360˝
Options
M20 M20 conduit connection with cable gland
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.
