Emerson Process Management 5300 manual Dynamic Vapor Compensation, Dielectric constant

Reference Manual

00809-0100-4530, Rev BA July 2009

Rosemount 5300 Series

DYNAMIC VAPOR COMPENSATION

Rosemount 5300 Series 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 Series Level transmitter can be configured for compensation for this effect.

Water has high critical temperature and pressure (705 ºF / 374 ºC and

140 bar, respectively). Level measurement may not be possible above these limits. However, some applications, such as power plants, may use water near the limits. 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 C-12.

Figure C-12. Dielectric constant versus temperature for saturated water vapor.

Dielectric constant

32 (0)	212 (100)		392(200)
32 (0)	2	2 (100)

Temperature (oF/oC)

Dielectric constant

Pressure

Pressure (Bar)

572(300) 752752(400)(400)

The standard version of a Rosemount 5300 Series level transmitter can be configured for static compensation of vapor by manually entering the dielectric constant of vapor, see “Dielectric Constant Settings” on page C-14.For applications with a varying pressure and/or temperature, certain models of the Rosemount 5300 Series have a built-in function (Dynamic Vapor Compensation) that automatically compensates for varying vapor dielectric constants.

C-15