A level sensing device locates the interface between a liquid and a vapor or between two liquids. Then it transmits a signal representing this value to process measurement and control instruments. As the level in the tank changes, the output reading changes proportionally. Hydrostatic head pressure is used to measure fluid level. To determine the height or level of a liquid the head pressure is measured and by knowing the specific gravity of the liquid the height can be calculated. Hydrostatic level gaging often use a differential pressure transmitter to compensate for the atmospheric pressure on the liquid. The high pressure port senses the atmospheric pressure on the fluid in the tank. The high side also senses hydrostatic head pressure. The difference between the pressures can be converted to level. The low pressure port senses only atmosphere.
In dip pipe applications, gas flows through a pipe that is submerged in the tank’s liquid. A differential pressure transmitter measures the back pressure on the tube caused by an increase in the tank level. The high pressure port senses the pressure increase caused by the back pressure in the dip pipe. The low pressure port is vented to atmosphere.
The same calibration procedure applies for any differential pressure level measuring system.
Input and Output Measurement Standards and Connections
A low pressure calibrator is the input measurement standard. It provides and measures low pressure values as required for calibrating hydrostatic level systems. A low pressure calibrator contains a pressure readout and pressure regulator.
A milliammeter measures the transmitter’s output. The milliammeter, power supply, and transmitter should be connected in series. For best calibration results, mount the transmitter in the same position as it is installed in the process. At the transmitter connect the source of pressure to the high pressure port and vent to atmosphere the low pressure port.
Five-Point Check
Determine the instrument’s range and test points for calibration.
For the lower range value measured in inches of water, divide the minimum height of the liquid in inches by the liquid’s specific gravity. The upper range value is the maximum height of the liquid in inches of water divided by its specific gravity. The span then, is the difference between these values. Perform the five-point upscale and downscale check.
Correct the zero at 10% of input span, adjusting zero until the output produced is 10% of the output span. Next, correct the span error, applying 90% input and adjusting the span until 90% output is produced.
Closed Tank Level Gauging
The procedure used in open tank applications is also used for closed tank applications. Closed tank applications must compensate for the static pressure in the vapor above the liquid. To accurately measure the head pressure of the liquid alone a reference leg is used. The reference leg is a pipe connecting the vapor space to the low side of the differential pressure transmitter. The reference leg must be either completely dry or completely filled with liquid.
Dry Reference Leg
The low pressure port receives the pressure of the vapor space. The high side receives vapor pressure in addition to the pressure from the liquid. The value measured by the transmitter represents only the pressure of the liquid because vapor pressure is applied to both the high and low sides of the transmitter. Calibrate with pressure to the transmitter’s high pressure port, and vent the low pressure port to atmosphere. Adjust the transmitter’s span for the specific gravity of the liquid in the tank. The low range is equal to the minimum level in inches, and the upper range value is equal to the maximum level in inches.
Wet Reference Leg
Often it is necessary to use a reference leg filled with liquid for gaging the level in closed tanks that contain volatile fluid. The column of fluid in the reference leg imposes additional hydrostatic pressure on the pressure side of the transmitter. This additional pressure must be compensated for to correctly gage level.
To determine the additional pressure that the reference leg will apply, take the height of the wet leg in inches and multiply it by the specific gravity of the fluid. The reference leg fill liquid may be different from the tank contents. Connect the low pressure calibrator to both ports of the transmitter. A regulator is used to add the hydrostatic pressure of the wet leg to the low side. Then, zero the output until 4 mA of output is produced. After zero is adjusted, perform a five-point check to the high side using a second regulator. In systems where the transmitter is mounted below the minimum measuring level, compensate for the additional static pressure by lowering the zero value. In systems where the transmitter is mounted above the minimum measuring level, compensate for the decreased static pressure by raising the zero value.
Calibrate the transmitter span first before compensating zero for transmitter height location.