Dynamic Vapor Compensation - Emerson 5301HxxxxxxxxxxxxxxZZ Series Reference Manual

Reference Manual
00809-0100-4530
D.6

Dynamic Vapor Compensation

Rosemount 5300 Level Transmitter is based on the Time Domain Reflectometry (TDR)
technology where low power nanosecond microwave pulses are guided down a probe
submerged in the process media. When a radar pulse reaches a media with a different
dielectric constant, part of the energy is reflected to the transmitter. The time difference
between the transmitted pulse and the reflected pulse is converted into a distance from
which the total level or interface level is calculated.
For radar level gauging, the actual measured quantity is the propagation time through the
empty space between the radar level transmitter and the liquid surface. For typical radar
level transmitter accuracy, the propagation speed of the radar signal should be close to
the velocity of light in vacuum. However, in some important cases, the deviation is not
negligible and must be taken into account for accuracy. High tank pressure in combination
with certain gases is an example.
High pressure water steam can influence radar level transmitter measurements. This is
due to the high pressure as well as the polar structure of water molecules. In such cases,
the Rosemount 5300 Level Transmitter can be configured for compensation for this effect.
Water has high critical temperature and pressure (705 °F / 374 °C and 3205 psi / 221 bar,
respectively). However, Dynamic Vapor Compensation has a maximum temperature and
pressure of 676 °F / 358 °C and 2610 psi / 180 bar. Above these limits, level measurement
may not be possible since the separation between gas and liquid becomes indistinct.
In a closed vessel containing water liquid and water vapor, existing databases (referred to
as a Mollier diagram) have been used to calculate pressure and vapor density, and deduce
the dielectric constant of the vapor from this. The dielectric constant changes as given in
Figure
Figure D-6: Dielectric Constant versus Temperature for Saturated Water Vapor
2.25
2.0
1.75
A
1.5
1.25
1.0
A. Dielectric constant
B. Temperature (°F/°C)
C. Pressure (bar)
Reference Manual
D-6.
32 (0) 212 (100)
A
C
572 (300)
392 (200)
B
Advanced configuration
April 2019
200
C
100
752 (400)
405