If products have a very low dielectric constant (
ε
r
<1.6), only a small part of the EM pulse is
reflected at the surface of the product. Most of the pulse is reflected at the probe end. TBF (tank
bottom following) mode is used to measure the distance to the product surface.
TBF mode (indirect measurement) compares:
•
The time for the pulse to go to the probe end and go back to the converter when the tank is
empty.
•
The time for the pulse to go to the probe end and go back to the converter when the tank is
full or partially filled.
The level of the product in the tank can be calculated from the time difference.
Level and interface measurement principle (direct measurement)
Figure 8-2: Level and interface measurement principle (2 liquids in the tank)
1
Time 0: The electromagnetic (EM) pulse is transmitted by the converter
2
Time 1: The pulse goes down the probe at the speed of light in air, V1
3
Time 2: Part of the pulse is reflected at the surface of the top liquid, the remaining pulse goes down the probe
4
Time 3: Part of the pulse goes up the probe at speed, V1. The remaining pulse goes down the probe at the speed of
light in the top product, V2
5
Time 4: The converter receives part of the pulse and records the signal. The remaining pulse is reflected at the inter-
face of the 2 liquids
6
Time 5: The remaining pulse is goes up the probe at speed, V2
7
Time 6: The remaining pulse goes up the probe at speed, V1
8
Time 7: The converter receives the remaining pulse and records the signal
9
The EM pulse moves at speed, V1
10
The EM pulse moves at speed, V2
11
Transmitted EM pulse
12
Received EM pulse (distance to the top liquid)
13
Received EM pulse (distance to the interface of 2 liquids)
Technology Solutions
Instruction Manual
Tek-Flex
4100A
www.tek-trol.com
1
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