Robertshaw 5318B, Instruction Manual, Page: 13 / 30

MODEL 5318B INSTRUCTION MANUAL
Level Tek II , Capacitance Level Switch 909GF295
Page 7
4.4.1
DETERMINING THE CONTROL
RANGE
The first item that must be selected is the control
range. The Model 5318B offers three (3) control
ranges, but the default value should be sufficient
for most typical, horizontally mounted probe,
installations.
Applications involving vertically mounted probes
may require use of one of the other ranges. The
signal range is a function of the probe and the
media being sensed. The setpoint(s) must fall
within the control range selected.
The control range is determined by selecting the
appropriate update rate. The relationship
between the control range and update range is
as follows:
20* – 250 pF ...................... 4/second
20* – 500 pF ...................... 2/second
125* – 1,000 pF ................. 1/second
* These values are 30 pF for the low
frequency version.
EXAMPLE #1:
Consider a vertically mounted Model 740A-
B048-N0 probe, 48 inches long used in a
conductive liquid. This probe has a gland
capacitance of 24 pF and the pF/Ft in air is
4.5.
Assuming that the material being sensed
has a dielectric constant of 60 the pF/Ft of
the length submerged in the material is 53.
With the product level below the end of the
probe the capacitance input to the Model
5318B will be:
A) Gland ...........................24 pF
B) Probe (4 x 4.5).............18 pF
C) Total ............................42 pF
With all but 6 inches of the probe
submerged in the material the capacitance
input to the Model 5318B will be:
A) Gland ..................... 24.00 pF
B) Probe; In Air
(0.5 x 4.5).................2.25 pF
C) Probe; Immersed
(3.5 x 53)............. 185.50 pF
D) Total .................... 211.75 pF
These calculations show that the input
capacitance for this application will be 42 to
212 pF and the default sampling rate of
4/second (0-250 pF) will be acceptable.
EXAMPLE #2:
Consider a vertically mounted Model 740A-
B120-N0 probe, 12 feet long used in water.
This probe has a gland capacitance of 24 pF
and the pF/Ft in air is 4.5.
Using a dielectric constant of 80 the pF/Ft of
the length submerged in the material is 56.
With the product level below the end of the
probe the capacitance input to the Model
5318B will be:
A) Gland...........................24 pF
B) Probe (12 x 4.5) ..........54 pF
C) Total ............................78 pF
With all but 1 foot of the probe submerged in
the water the capacitance input to the Model
5318B will be:
A) Gland......................24.00 pF
B) Probe; Air
(1 x 4.5) ...................4.50 pF
C) Probe; Immersed
(11.5 x 56) ...........644.00 pF
D) Total .....................672.50 pF
These calculations show that the
capacitance input for this application will be
78 to 673 pF and the sampling rate of
1/second would be required.
4.4.2
CHOOSING THE DISPLAY
FORMAT
During normal operation the results of the
previous input conversion, measurement, are
displayed on the right hand side of the bottom
line of the LCD display. The user has the option
of displaying this value either as the actual,
RAW , measurement or as a converted,
CNVRTD , value.
The actual measurement (RAW) is a non-linear
function and the converted (CNVRTD) value is
linear. Regardless of the display format
selected the Model 5318 operates internally
using the actual measurement value.