Well control
Fluid is any substance that flows; e.g. oil, water, gas, and ice are all examples of fluids. Under extreme pressure and temperature almost anything will become fluid.
Fluid exerts pressure and this pressure is as a result of the density and the height of the fluid column. Most oil companies usually represent density measurement in pounds per gallon (ppg) or kilograms per cubic meter (kg/m) and pressure measurement in pounds per square inch (psi) or bar or pascal (Pa). Pressure increases as the density of the fluid increases.
To find out the amount of pressure a fluid of a known density exerts for each unit of length, the pressure gradient is used.
A pressure gradient is defined as the pressure increase per unit of the depth due to its density and it is usually measured in pounds per square inch per foot or bars per meter.It is expressed mathematically as;
"pressure gradient = fluid density × conversion factor".
The conversion factor used to convert density to pressure is 0.052 in English system and 0.0981 in Metric system.
Relative density
"Buoyant Force Transducer": the buoyancy force produced by a float in a homogeneous liquid is equal to the weight of the liquid that is displaced by the float. Since buoyancy force is linear with respect to the density of the liquid within which the float is submerged, the measure of the buoyancy force yields a measure of the density of the liquid. One commercially available unit claims the instrument is capable of measuring relative density with an accuracy of ± 0.005 RD units. The submersible probe head contains a mathematically characterized spring-float system. When the head is immersed vertically in the liquid, the float moves vertically and the position of the float controls the position of a permanent magnet whose displacement is sensed by a concentric array of Hall-effect linear displacement sensors. The output signals of the sensors are mixed in a dedicated electronics module that provides a single output voltage whose magnitude is a direct linear measure of the quantity to be measured.