Treadmill User Manual

Page 24 SITRANS LC 500 – INSTRUCTION MANUAL 7ML19985GE01

mmmmm

Interconnection

Cable

Selecting the correct instrumentation cable

• you need to know the cable length, the barrier type (if applicable), and the

measurement resistance

• select a cable that will give you a capacitance time constant of less than 65 µSec

1. Calculate the capacitance for a time constant of 65 µSec, using the following

formula:

(time constant = Resistance * Capacitance)

is the sum of the load resistor and cable resistance.

is the sum of the cable capacitance and the capacitances of the connected

device/devices.

2. Determine the cable length allowed, by subtracting the capacitance value of the

device (or devices) on the loop from the total capacitance, and using the maximum

allowable limit of 100 pF per meter (or 1 nF per 10 meters).

Example

1. Calculate the cable capacitance which will give a time constant of 65 µSec:

A twisted pair cable with a conductor cross-section of 1 mm

(AWG 18 equivalent) has

a copper resistance of 73.6 Ohm/km and a capacitance of 100 pF/m (or 1 nF/10m).

For a standard 28 V 280 Ohm barrier and a 250 Ohm measuring resistance, with a

100 meter cable:

Resistance = 280 (barrier) + 250 (sensing device) + 7.36 (cable)= 537.36

s = nF

Notes:

• To maintain reliable transfer of the HART modem signals, the RC

time constant of

the connections should be less than 65 µSec.

• Cable capacitance must also be considered when selecting cable for intrinsically

safe installations.

• For output signals (from the SITRANS LC 500), only the cable and barrier

resistance are relevant. For input signals the measurement resistance is also

relevant.

• Use twisted pair cable, screened as a pair.

RC = Resistance * Capacitance

Or, if you use a common screen over a cable containing multiple twisted pairs,

do not use other pairs for signals that could interfere with HART signals.

tRC×=

CtR⁄=

65 10

6–

× 537.36 C×

C 65 10

6–

× 537.36⁄()121==