There are five common automatic tank gauge solutions widely used for both inventory and custody applications.
Manual gauge
This solution is the reference for all other solutions, and it utilizes a dip tape. Manual gauge uses either the innage (from the top flange to the bottom of the tank) or ullage (from the top flange to the top liquid layer) method to measure the total liquid level inside the tank. API MPMS chapter 3.1A covers the procedure and requirement for manual gauge, which is normally three consecutive readings with a difference not exceeding ±3 mm.
Float and tape gauge
This solution uses a float attached to a spring via a perforated tape. The spring provides constant tension, which balances the float on the liquid level. The perforated tape is connected to a mechanical counter assembly.
Servo gauge
The servo gauge uses the displacement measurement principle. A small displacer (weight) on a measuring wire from a drum is accurately positioned and balanced in the liquid medium using a servomotor. These devices can meet the inventory and custody accuracy requirements.
Radar tank gauge
Radar tank gauge (RTG) is the most common solution for tank gauge. It used to be the Saudi Aramco standard tank gauge for LPG LNG solution for inventory tanks. This technology is microwave-based, which measures the distance from the top connection to the liquid surface. The two available techniques are frequency modulated continuous wave (FMCW) and time-of-flight time domain reflectometry (TDR). These devices can meet the inventory and custody accuracy requirements.
Hydrostatic tank gauges
A hydrostatic tank gauge for LPG LNG system has up to three pressure transmitters and one temperature transmitter. Two pressure transmitters are installed close to the bottom of the tank and are used to calculate the density. A third transmitter measures the vapor pressure at the top of the tank to increase the accuracy.