Siemens Sitrans, LC 500 instruction manual Operation & Application, Measurement, f ≈

			Conventional Capacitance
					Measurement
		R	= (C1 + C2 + C3) + Ca
				(C1 + C2 + C3) + Ca + Cm
R	=	Ratio between initial
		capacitance and total			Measuring-
		capacitance				Circuit
Ca	=	Initial capacitance (air)
Cm	= Capacitance Increase
				C1
		(product)
C1	= Capacitance connection			C2
		point		C3
C2	= Capacitance connection
		cable					Ca
C3	=	Capacitance Process					Cm
		connection (includes active
		part)

SITRANS LC 500 with Active Shield

R = Ca

Ca + Cm

Measuring-

Circuit

Ca

Cm

The measurement is further protected from interference by a buffer, which applies the frequency signal from the measurement section to the active shield section. This effectively eliminates any electrical potential difference between the shield and the measurement section and prevents additional changes in capacitance occurring.

SITRANS

LC 500

active shield	empty	full
	tank	tank
active		100%
measurement
section
probe seal		buffer
		frequency	(f) ≈	K
(inactive)
		0%		capacitance (C)

The relative lengths of the measurement section and active shield section can be specified to suit a particular application. If the measured range will be short relative to the total length of the electrode, specify a short measurement section. This increases the achievable resolution of the measurement, since any change in level will be greater relative to the length of the measurement section.

Operation & Application

7ML19985GE01 SITRANS LC 500 – INSTRUCTION MANUAL Page 11