Technical Documentation
CT-1000
The potentiometric level sensor
Edition: 03/2011
Version: 5
Article no.: 233945
Gems Sensors & Controls One Cowles Rd Plainville CT Telephone 800-378-1600
Page 2/15 CT-1000
Table of contents
Field of application........................................................................ 4
1
2 Safety instructions......................................................................... 5
3 Design and function description ..................................................6
4 Installation ..................................................................................... 8
4.1 CT-1000 Mono (single rod)
4.2 CT-1000 DU (two rods)
.................................................................................9
......................................................................................9
5 Electrical connection.................................................................... 10
6 Configuration...............................................................................12
6.1 Measuring span at the level sensor ...................................................................12
6.2 Current draw in failure mode ...........................................................................13
7 Technical data .............................................................................. 15
© Copyright:
Reproduction and translation only with the written consent of Gems Sensors & Controls.
Gems Sensors & Controls reserves the right to carry out product alterations without prior notice.
CT-1000 Page 3/15
1 Field of application
The purpose of the CT-1000 level sensor is to provide continuous level gauging of
electrically conductive liquids ( > 1 μS/cm). During the measurement no insulating layer
must be formed on the probe tube.
The level sensor is available in lengths of 8” to 19.7’ .
With the “single-rod probe” model (CT-1000 Mono), the inside of the container must
have a conductive surface (metal containers/downpipes). The “two-rod probe” model
(CT-1000 DU) is compatible with non-conductive containers.
The standard sensors work by outputting the measured fill level as an analog value in the
range 4 to 20 mA.
With the HART option, the configuration and retrieval of measured values by the digital
HART protocol are supported, see
Technical Documentation for CT-1000 with HART protocol– art. no. 350044
In conjunction with the standard level sensors, wetted parts are in stainless steel 1.4571,
insulators are in PTFE. On request, the probe tubes are also available in Hastelloy, tantalum, titanium or stainless steel 1.4404.
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2 Safety instructions
The purpose of the CT-1000 level sensor is to provide continuous level gauging of liquids.
The level sensor must be used exclusively for this purpose. The manufacturer accepts no
liability for any loss or damage arising from improper use.
The level sensor has been developed, manufactured and tested in accordance with the
latest good engineering practices and generally accepted safety standards. Nevertheless,
hazards may arise from its use. For this reason, the following safety information must be
observed:
Do not change or modify the level sensor or add any equipment without the prior
consent of the manufacturer. The installation, operation and maintenance of the level
sensor must be carried out only by expert personnel. Equipment operators, set-up
technicians and service technicians must comply with all applicable safety regulations.
This also applies to any local safety regulations and accident prevention regulations which
are not stated in these operating instructions.
The safety instructions in this manual are labelled as follows:
Failure to observe these safety instructions could result in a risk of
accident or damage to the CT-1000 level sensor.
Useful information designed to ensure continued and correct operation of
the CT-1000 level sensor and to make your work easier.
CT-1000 Page 5/15
3 Design and function description
The design of the CT-1000 level sensor is illustrated in Figure 1 with the screw-in unit
version as an example. Inside the sensor head (1) and concealed by the cap (2) are the
protected terminals and configuration buttons. The electrical connection is established by
means of an M16x1.5 screwed cable gland (3) and screw terminals or by an M12 plug-in
connection. Ground connection (4) is required only in conjunction with the CT-1000
Mono version (see section 5 Electrical connection). On the probe neck, there is either a
screw-in unit (5) or a flange (not shown) for installing the probe in the container.
Figure 1: The CT-1000 level sensor
A current pulse generates a linear voltage drop (< 100 mV) on the probe tube (6) electrically insulated from tank potential. The voltages at the bottom end and top end of the
tube are measured.
As long as the probe tube is not immersed, it has no path to the tank potential. This state
is detected by the signal processor, which outputs an “empty” signal. As soon as the
probe tube is immersed, the voltage measured at the immersed tube end corresponds to
tank potential and the voltage measured at the top end is added to the voltage drop
along the non-immersed part of the probe tube.
With the probe tube fully immersed, the voltages measured at the top and bottom ends
of the tube are equal in respect of the potential reference point. The region in between is
mostly linear, depending on the tank geometry and the distance between the tube end
and tank bottom, which makes it possible to calculate the fill level from the ratio of the
measured voltages.
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The fill level is calculated from the ratio of the measured voltages by a microcontroller.
The ratio of the two voltages also enables the orientation of the CT-1000 sensor to be
detected, i.e. whether the sensor has been installed from top down or bottom up.
The level sensor is fed from a 2-wire 4 to 20 mA interface and requires a voltage between
8 and 30 V. The positions of the 4 mA and 20 mA points on the probe tube are freely
adjustable (see chapter 6 Configuration). To prevent galvanic processes from occurring in
the liquid, both the discharging and charging currents flow through a galvanically
isolating transformer. The alternating voltage produced across the very low-resistance
probe tube is in the 100 mV range.
The table below shows the signal characteristics of the sensor depending on sensor
orientation and measured value output setting (normal/inverted).
Measured value output
normal
Fill level “Empty”
signal
Installation
top down
Installation
bottom up
20 mA
4 mA
20 mA
4 mA
3.8 mA
3.8 mA
Measured value output
inverted
Fill level “Empty”
4 mA
20 mA
4 mA
20 mA
signal
20.5 mA
20.5 mA
Figure 2: Measured value output
Bottom up orientation: The 0 point is at the probe base
Top down orientation: The 0 point is at the sensor head
Normal measured value output: The 20 mA point is higher than the 4 mA point
Inverted measured value output: The 20 mA point is lower than the 4 mA point
CT-1000 Page 7/15
g
g
4 Installation
All local safety and accident prevention regulations not expressly
referred to herein must be observed.
CT-1000 Mono
50
160
Installation length
Measuring range
6
Cap, WAF 19
removable
Screwed
cable gland
M16x1.5
Sensor head
Earth
connection
Screw-in
unit R ¾ min.
or flange
Flange or
screw-in unit R 2
Probe tube
6
Counter electrode
24
CT-1000 DU
50
160
e
ran
Installation length
Measurin
29
Figure 3: CT-1000 variants
Level gauging with CT-1000 is possible only in electrically conductive
liquids.
The level sensor can also be fitted into the tank from underneath. The
orientation is detected automatically.
CT-1000 Mono: If the probe tube is immersed in the liquid only a short
way, the linearity of the measurement is affected by the short distance between the probe tube and reference potential.
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4.1 CT-1000 Mono (single rod)
With the single-rod probe, the tank acts as the counter electrode. For this reason, the
material on the inside of the tank must be conductive and there must be good electrical
contact between the tank and sensor head. For this purpose, the ground connection
must be connected to the tank.
4.2 CT-1000 DU (two rods)
With the two-rod probe, the second rod acts as the counter electrode. As the distance
from reference potential is constant, the measurement result is unaffected by the
geometry of the tank. While the second rod also adds to the stability of the mechanical
design, this does require a larger process connection for installation.
CT-1000 Page 9/15
5 Electrical connection
For the wiring of the level sensor, you need a two-core unshielded cable, which is
terminated inside the sensor head of the level sensor.
It is essential that the correct minimum wire gauge be selected: the supply voltage at the
level sensor must not fall below 8 V in the event of maximum current draw (21.5 mA).
To connect the level sensor (see Figure 4):
Unscrew cap (1) using an open-ended spanner (WAF 19)
Loosen union nut (2) of screwed cable gland (3)
5
Figure 4: Connecting the CT-1000 level sensor
Feed two-core cable (4) through union nut (2) and fit screwed cable gland (3)
and then retighten the union nut.
Connect two-core cable (4) to the screw terminals on the sensor head marked (+)
and (–). Observe proper polarity.
Screw on cap (1)
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The earth connection on the underside of the sensor head can be used for earthing or
equipotential bonding.
With the CT-1000 Mono, ground connection (5) must be connected
to the electrically conductive tank.
Protect the sensor head against the ingress of water. An external
cable diameter of 5 to 10 mm ensures reliable sealing of the cable
entry. Make sure that the cable gland is screwed tight, and close the
cap firmly.
During all installation operations, it must be ensured that the negative
terminal of CT-1000 is internally connected to the housing.
CT-1000 Page 11/15
6 Configuration
6.1 Measuring span at the level sensor
To enable configuration of the 4 mA and 20 mA points at the CT-1000 level sensor, two
buttons and an LED (light emitting diode) are provided near the terminals inside the
sensor head.
By default, the level sensor is set to maximum span with 4 mA at the probe base and
20 mA at the sensor head. The measuring span is configurable for adaptation to the tank
concerned. However, the minimum distance between both points must be at least
10 mm. If this minimum distance is not observed, the display direction of the level sensor
will be reversed automatically. The measured value would then be inverted.
Measuring span configuration (see Figure 5):
Figure 5: Measuring span configuration
Unscrew cap (1) using an open-ended spanner (WAF 19).
Press and hold 4 mA button (2) or 20 mA button (3) for at least 3 seconds.
The level sensor is now in configuration mode. Green LED (4) “Cal/Err” flashes. The
current draw of the level sensor is 12 mA. If no button is pressed again, the level sensor
remains in configuration mode for 20 seconds before reverting to measuring mode and
discarding any changes.
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In configuration mode, the 4 mA or 20 mA point, or both reference points, can be
modified in any order.
To define a reference point while the sensor is in configuration mode
briefly press the 4 mA button (2) (1-2 seconds) to define a current draw of 4 mA
for the present immersed position of the sensor.
briefly press the 20 mA button (3) (1-2 seconds) to define a current draw of 20
mA for the present immersed position of the sensor.
When the 4 mA button is pressed, the LED goes out for 5 seconds. When the 20 mA
button is pressed, the LED lights up permanently for 5 seconds. The sensor then remains
in configuration mode for a further 15 seconds before storing the change and reverting
to measuring mode.
The new configuration will not be adopted by the level sensor until it has
switched from configuration mode to measuring mode automatically (LED
goes out). The level sensor must, therefore, not be disconnected from the
power supply until this time.
6.2 Current draw in failure mode
If a malfunction is preventing the level sensor from detecting levels correctly, the sensor
will enter failure mode after a short time. Failure mode signalling conforms to the
NAMUR NE43 recommendation. The failure current is set by default to 21.5 mA but this
value can also be set to 3.6 mA.
Configuration of current draw in failure mode (see Figure 5):
Unscrew cap (1) using an open-ended spanner.
Press and hold both the 4 mA button (2) and the 20 mA button (3) s
ously for at least 3 seconds.
Green LED (4) “Cal/Err” flashes rapidly. The current draw of the level sensor is 16 mA.
After 5 seconds, the LED stops flashing and then indicates the selected failure current
draw for 2.5 seconds. If the LED lights up permanently, the failure current is 21.5 mA; if
the LED goes out, the failure current is 3.6 mA. If no button is pressed again, the level
sensor remains in failure mode for a further 2.5 seconds before reverting to measuring
mode and discarding the change.
To set a current draw of 3.6 mA during the 10-second dwell period in failure mode
imultane-
briefly press the 4 mA button (2) (1–2 seconds).
CT-1000 Page 13/15
To set a current draw of 21.5 mA during the 10-second dwell period in failure mode
briefly press the 20 mA button (3) (1–2 seconds).
The new configuration will not be adopted by the level sensor until it has
switched from configuration mode to measuring mode automatically (LED
goes out). The level sensor must, therefore, not be disconnected from the
power supply until this time.
Screw on cap (1).
If, during operation, the level sensor detects that the level cannot be output
correctly due to an insufficient supply voltage, it enters failure mode and
sets current draw to 3.6 mA (regardless of any failure current settings).
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7 Technical data
Electrical connection
Power supply
Output signal
“Empty” signal
Failure signal
Process connection Screw-in unit (standard 2” NPT)
Sensor head Height above screw-in unit/flange 6.3”
Probe tube Length 8” to 19.7’
Two-line terminal
8 to 30 V DC
4 to 20 mA
3.8 mA or 20.5 mA
21.5 mA or 3.6 mA
Flange on request
For material, see probe tube
Diameter 1.97”
Index of protection IP 68
Material: stainless steel
Cable diameter 0.2” to 0.4”
Temperature –40 to +185 °F
Diameter of single rod 0.236”
Standard material: 1.4571
Optional material: tantalum, titanium, Hastelloy,
or 1.4404
Measuring range freely configurable (> 10 mm)
Seal PTFE
Accuracy
Digital component
Analog component Temperature drift better than ±0.01 %/K
Process temperature NT: -40 to +302 °F
Process pressure 150 bar at 68°F, 25 bar at 302 °F
Process conductivity > 1 μS/cm
Linearity better than ±0.02” or ±1 %
Resolution better than 0.1 %
Resolution better than 0.5 μA
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