But even the definition of head has some nuances that are not obviously apparent and must be taken into account in order to properly use the numerical values associated with it.
A key concept with head is that its numerical value is referenced to the density of the fluid under consideration. One foot of head of a given fluid is not the same energy content as one foot of head of a different fluid. Consider two site glasses containing two different fluids separated by a liquid/liquid interface at Points 1 and 2, as shown in Figure 2. In order for the two fluids to be in equilibrium at the interface, the total fluid energy at Point 1 (H1 due to the height of the column and density of Fluid 1) must be equal to the total fluid energy at Point 2 (H2 due to the height of the column and density of Fluid 2).
But it’s obvious that H1 does not equal H2, so how can the two values represent the same amount of fluid energy? To reconcile this apparent discrepancy, it’s important to understand that H1 quantifies the amount of fluid energy at the interface in reference to the density of Fluid 1, while H2 quantifies the same amount of fluid energy but uses the density of Fluid 2 as the reference.
If the two fluids are in equilibrium with no flow, the static pressure at Point 1 must equal the static pressure at Point 2.