analysis methodology that only one pressure and one temperature can exist at any one given point accordingly the pressures at the bottom of the tubing and the bottom of the piggy back line are equal to the hydrostatic pressure exerted by the fluid and gas column in the annular space static pressure sensing device 3 measures the pressure in the annular space since the hydrostatic pressure generated by the gas is negligible the difference between the pressure at the bottom of the piggy back line 17 and the pressure of any gas remaining in the well casing above the fluid column as measured by the static pressure sensing device 3 is due to the pressure generated by the fluid column in the annular space it is known that the pressure of a static fluid column at a given depth is equal to the given depth times the pressure gradient of the fluid therefore the fluid column depth in the annular space can be determined from dividing the pressure at the bottom of the fluid column less any pressure from residual gas above the filled column by the pressure gradient of the fluid the pressure at the bottom of the tubing or fluid column being the difference in pressure between the pressure at the bottom of the piggy back line as indicated by sensing device 7 and the pressure of any production gas in the annular space as indicated by.