Dwyer CRF2 - Manual (Page 2)

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
CRF2 W
W
R
R
R
C
0
0
A
U
1T
1T
2T
3T
1B
2B
3B
1S
2S
3S
1F
2F
3F
4F
5F
6F
048
XXX
M20
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