If a vapor with a significant density exists above the liquid, the hydrostatic
pressure added needs to be considered if accurate transmitter output is required.
EXPLAIN the process of density compensation in level
detection systems to include:
Before examining an example which shows the effects of density, the unit "specific volume" must
be defined. Specific volume is defined as volume per unit mass as shown in Equation 3-1.
Specific Volume = Volume/Mass
Specific volume is the reciprocal of density as shown in Equation 3-2.
Specific volume is the standard unit used when working with vapors and steam that have low
values of density.
For the applications that involve water and steam, specific volume can be found using "Saturated
Steam Tables," which list the specific volumes for water and saturated steam at different
pressures and temperatures.
The density of steam (or vapor) above the liquid level will have an effect on the weight of the
steam or vapor bubble and the hydrostatic head pressure. As the density of the steam or vapor
increases, the weight increases and causes an increase in hydrostatic head even though the actual
level of the tank has not changed. The larger the steam bubble, the greater the change in
hydrostatic head pressure.