Dynamic Density Level via Pressure

Calculate density

• First fill vessel to a known point:
  • Note the locations of sight gauges, float switches, etc. anything that can independently verify the current height
• Solve for density: $\rho =P/h$
  • Where $\rho$ is density, $P$ is pressure, and $h$ is height at a certain point. Gravity is already factored into the measurement of the pressure, so we can ignore it.
• Use density to find height to verify:
  • given $h=120in$ , $P=4.33PSIG$, and $\rho =0.0361lb/i{n}^3$: $h_1=4.33/0.0361=119.9446$

Implement in code

• If you have a float, then this is simple:

1. Find height in vessel of non-shutdown float (looking for internal height, not external. These tanks are often lifted above the ground by some amount)
2. Fill/drain tank to that level and move that pressure into a separate variable. You now have height and pressure, so solve for density.
3. Use density as a multiplier to the pressure to determine height based on a scale. In structured text it may look like:

(* float_trigger is the input from the float switch. If it is steady, a one-shot would be needed *)
 
IF float_trigger THEN
	pressure := PIT100.PV;                  // snapshot of pressure
	density := pressure / height_constant;  // calculate density
END_IF;
 
tank_ft := PIT100.PV / density;             // height is calculated