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Calculate the capacitance for a time constant of 65 μSec, using the following formula:
t
=
R C
is the sum of the load resistor and cable resistance.
R
is the sum of the cable capacitance and the capacitances of the connected
C
device/devices.
The time constant of the complete setup is given by:
t
=
Rsense
Where
Rsense
Rcable
= the length of the cable
L
Cdevice
Ccable
Example
A twisted pair cable with a conductor cross-section of 1 mm
a copper resistance of 73.6 Ohm/km and a capacitance of 100 pF/m (or 1 nF/10m).
For a standard 250 Ohm sensing resistance, with a 100 meter cable:
Rsense
Rcable
= 100 m
L
Cdevice
Ccable
t
=
t
=
t
=
20s
Since 20
s
Multi-drop applications: maximum cable length
In a multi-drop application, the total capacitance of all the devices must be calculated.
With three devices, at 3 x 68 nF, the allowable cable length will be considerably limited.
Notes:
•
If the device is part of a multi-drop setup, the SITRANS LC500 sets the current to 4
mA, which inhibits analog signalling, including fault signalling.
•
Multi-drop is a HART mode where devices are set to a fixed current, and the
device is interrogated periodically. The maximum number of devices on one loop
is 15, one of which can be an analog mode device.
Page 26
SITRANS LC500 – OPERATING INSTRUCTIONS
(time constant = Resistance * Capacitance)
+
Rcable L
= The resistance of the HART sensing resistor
= The resistance per meter of cable
= the capacitance of the LC500 = 68 nF
= the capacitance per meter of cable
= 250 Ohm
= 0.0736 Ohm/meter
= 68 nF
= 0.1 nF/m
Rsense
+
Rcable L
250
+
0.0736 100
is less than 65
, 100 meters of this cable is suitable.
s
Cdevice
+
Ccable L
Cdevice
+
Ccable L
9 –
10
10
68
+
0.1
2
(AWG 18 equivalent) has
9 –
100
7ML19985GE04
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