Revision 8 SLG 700 SmartLine Level Transmitter User’s Manual Page 17
TDR for Interface and Flooded Measurements
The Time-Domain Reflectometry (TDR) principle can also be used to measure an Interface
Level as well as the upper level. The position of the level interface has to be calculated with
knowledge of the dielectric constant (DC
U
) of the upper layer.
The SLG 700 can measure levels of different materials in the same tank and can detect the
echo from the boundary between Vapor and the Upper Product (UP), and between the Upper
Product (UP) and the Lower Product (LP). This allows calculating the level for each material
and the interface thickness as in Figure 2-3.
If an interface level is being measured, the pulses pass through the upper medium before
reaching the interface.
Distance to Product in the Interface equation:
𝑑
𝐼
= 𝑑
𝑠
+
∆𝑡 × 𝑉
𝑤𝑔
2 ×
√
𝐷𝐶
𝑈
Where:
d
S
= Distance to surface
∆t = change in time for a pulse to travel the distance through the Upper Product
v
wg
= speed of light in a vacuum on the probe
DC
U
= Dielectric Constant of Upper Product
Surface and interface measurements can be made if:
DC
U
= where the DC Upper Product is less than 9 and the DC difference between the
upper and lower product is greater than 8.
The minimum thickness of the interface layer is 7cm.
Figure 2-2 shoes the distances to surface and interface can be calculated as shown in this
sample echo curve.
Figure 2-2: Sample Echo Curve
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