Calculate the level using pressure differences
Surface calculation using pressure difference: Compression difference level ( DP ) is used to measure the interface of two fluids that have a specific gravity (S1 & S2). To measure the interface, the overall surface must be at low pressure or higher at all times. It is important that this surface be large enough to create a reasonable DP between two specific gravity limits. This measurement can be performed with or without seal diaphragm and capillary. However, from a maintenance point of view, it is easier to use sealing media and capillary assembly. Keeping your feet wet at a constant height can be difficult in some applications. To determine the calibrated range for a transmitter, four assumptions must be made:
- At a lower calibration value, the 4 mA point, the tank fills with a lighter liquid.
- At the high calibrated value, the 20 mA point, the tank is filled with heavier fluid.
- The shut-off pipe that is transmitted to the transmitter is flooded at all times. The overall level should be equal to or higher than the high (low pressure) faucet.
- There is always a reference surface seen by the low pressure side. This can be done with a remote sealing system or with a wet foot. The reference surface must have a constant height and density. This calculation changes slightly for leg systems when the density of wet feet and low pressure are different.
The measured surface consists of a combination of two liquids:
L = L1S1 + L2S2
When the tank is filled with a lighter liquid, the transmitter is at 4 mA (or 0% orifice) and L2 = L:
HP = L2S2 + dSf
LP = dSf + hSf
At 4 mA, DP = HP – LP = L2S2 – hSf
When the tank is filled with heavier liquids, the transmitter is at 20 mA (or 100% orifice) and L1 = L:
HP = L1S1 + dSf
LP = dSf + hSf
At 20 mA, DP = HP – LP = LS1 – hSf
Compression difference level gauge interface level measurement
This tool is used to calculate the DP compression difference level gauge range for interface measurement applications
Formulas used to calculate the calibration amplitudes of a DP compression difference level gauge for interface measurement application
The Transmitter 4ma or LRV formula
∆Pmin or 4ma = LS2 – HSf
The Transmitter 20ma or URV formula
∆Pmax or 20ma = LS1 – HSf
Using the above tools, we calculate the range of the transmitter. Now calibrate the transmitter and consider the line. Now write down the DP value of the current transmitter to calculate the interface level. Now enter the Span, L and LRV parameters of the transmitter on the DP scale (4ma transmitter value on the DP scale) and use the transmitter calibration tool to measure the above interface. DP is measured as we line the transmitter. Now replace all the parameters in the following interface calculation tool.
Interface level measurement formula
H = Distance between taps
L = Total Measured Level
S1 = Specific gravity of lighter fluid
S2 = Specific gravity of heavier fluid
Sf = Specific gravity of reference leg
Measured DP= After transmitter calibration, present DP value
Span = Difference between absolute of maximum and minimum range
LRV = Lower range value of transmitter in DP scale
I = Interface level
Interface measurement using DP transmitters
- Low costs
- Simple installation
- No additional components required
- The opening should be large enough to be measured. Or the distance between the taps or the special charm difference of the m7ust is large
- The top valve should always be covered.
- Small openings are prone to temperature errors.
A tank needs to measure the interface at a level of 150 inches. This tank has two liquids with a specific gravity of 1.0 and 1.08.
At 4mA, DP = L2S2 – hSf
DP = (150 x 1.0) – (150 x 0.934)
So at 4 mA = 9.9 inH2O DP
At 20mA, DP = LS1 – hSf
DP = (150 x 1.08) – (150 x 0.934)
So at 20 mA = 21.9 inH2O DP
The calibrated aperture is 9.9 to 21.9 inH20. When the transmitter reads 9.9 inH20, the tank fills with a lighter liquid. When the transmitter reads 21.9 inH20, the tank fills with heavier liquids. Use the DP reading as the percentage of span to determine the interface of the fluid mixture. For example, if the output of the transmitter is 18.4 mAh or DP 20.7 inH2O, the interface is calculated using the formula:
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