Siemens Sitrans, LC 500 Non-conductive or conductive contents, Operation & Application

The formula1 is: C =	-------------------------K⋅ ε ⋅ L
	Log(D ⁄d)

where C = capacitance K = constant

ε= dielectric constant

L = active measurement length D = diameter of tank

d = probe diameter.

(For detailed application examples, see page 91.)

The transmitter measures the capacitance of the measurement electrode relative to the tank wall (reference electrode) and transforms it to a 4-20 mA signal. Any material that covers the probe will cause an increase in capacitance relative to an uncovered probe surrounded by air. As the product level rises the capacitance will increase.

Non-conductive or conductive contents

In practice, the SITRANS LC 500 probe is usually insulated. If the vessel contents are non- conductive, the dielectric is composed of the vessel contents and the insulation, and the separation distance is from the probe to the tank wall. The tank wall is the reference electrode, and it must be connected to the ground point on the instrument.

dielectric = contents plus insulation (non-conductive contents) dielectric = insulation (conductive contents)

probe
dia. (d)	(insulation)
internal
	probe
tank wall
	sleeve

internal tank diameter (D)

Note: For simplicity, the probe is shown centrally mounted. If it is to be mounted off- centre, take care to ensure the electrode remains parallel to the tank wall.

If the vessel contents are conductive, the electrode must be insulated. In this case the dielectric is the insulation layer and the interface between the conductive contents and the insulating sleeve acts as the reference electrode. This reduces the separation distance for the filled portion of the tank to the thickness of the insulation. It also creates a linear reference electrode independent of the tank wall.

1.This formula applies to a centrally mounted probe: for a probe mounted off-centre, the formula must be adjusted.

7ML19985GE01 SITRANS LC 500 – INSTRUCTION MANUAL Page 9

Operation & Application