KROHNE optiplex 1300 c, Руководство пользователя

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Handbook OPTIFLEX 1300 C
85 OPTIFLEX 1300 C
8. Measuring principle
8.1 General principle
This Guided-Radar (TDR) level meter has been developed from a tried and tested technology called Time
Domain Reflectrometry (TDR). Other modern applications include checking for and locating damage along
telecommunication cable lines.
The device sends low-power electromagnetic pulses of one nanosecond width along a rod or cable
conductor. This pulse travels at a known speed: the speed of light. Upon reaching the surface of the product
to be measured, the pulses are reflected back with a signal strength that is dependent on the dielectric
constant,
ε
r
, of that product (e.g. up to 80% of the pulse strength is reflected from the surface of water).
A product’s dielectric constant,
ε
r
, is an electrical property. The strength of pulse reflection from the surface
of the product being measured is registered by the instrument and converted into a signal amplitude in
volts.
The instrument measures the time between the emission and the reception of the signal: half of this time
corresponds to the distance from the instrument reference point (the flange facing) to the product surface.
This value of time is converted into a 4 ... 20 mA output current or a digital signal.
Readings taken using this technology have the advantage of being uninfluenced by dust, foam, vapor,
agitated and boiling surfaces.
Pressure, temperature and density variations also have no effect.
Refer to the Measurement modes section for variations and further details on the principle.
The diagram below shows how TDR measurement of level works. At time, t, represented by a vertical line,
the following occurs:
•
t0: EM pulse emitted.
•
t1: Pulse travels down probe at the speed of light in air, V1.
•
t2: Pulse reflected back from the top product.
•
t3: Pulse travels back to the receiver at speed V1.
•
t4: The receiver receives and records the level return signal.
The waveform below the diagram shows a snapshot of what the user would see on an oscilloscope.