Displacer type level Transmitter
This is also one of the indirect ways of level measurement commonly used in the industry. As a differential pressure transmitter is used to measure the tank level using the hydrostatic pressure, the displacer type level transmitter work by measuring the difference in weight of the rod as the level increases. It works on the base of Archimedes’ principle, as the liquid level keeps on increasing the rod attached to the transmitter experience a loss of weight due to the buoyancy force. The difference in weight is calculated proportional to the level and configured in the transmitter to get a real-time level measurement.
In order to find the URV and LRV we need some basic information about the rod and fluid to be measured.
- Radius of Rod (r)
- Length of Rod (L)
- Weight of Rod (W)
- Specific Gravity of Fluid (SG)
Now let’s see the formula to find the loss of weight experienced by the rod due to the rising and falling of process fluid inside the displacer vessel.
Loss of weight = π * r² * L * SG
Now by looking over with current example, you can get a clear picture of how this principle is used in level measurement. let’s consider a closed tank filled with fluid in ambient conditions and a displacer level transmitter is installed. The detail necessary for range calculation is given bellow
- Radius of Rod (r) = 4
- Length of Rod (L) = 40
- Weight of Rod (W) = 2600
- Specific Gravity of Fluid (SG) = 0.83
At Zero level (0% Level)
At an empty level, fluid will not impact the rod. So the rod will not lose its weight as in this example the fluid not gonna change its state, so the LP side will not disturb the rod’s weight.
LRV = TOTAL WEIGHT – LOSS OF WEIGHT
LRV = 2600 -0
LRV = 2600
At Full level (100% Level)
When the tank is fully filled with process liquid, the rod will lose related to the specific gravity of the liquid. Let’s calculate the loss in weight..
Loss of weight = π * r² * L * SG
Loss of weight = 3.14 * 4² * 40 * 0.83
Loss of weight = 3.14 * 16 * 40 * 0.83
Loss of weight = 1667.968
URV = TOTAL WEIGHT – LOSS OF WEIGHT
URV = 2600 – 1667.968
URV = 932.032
Interface level measurement
Displacer type level transmitter is also used to measure the interface level, as the lighter fluid remains above the level of the higher fluid. We need to take both specific gravities of the liquid into consideration. As we saw above at the empty level the rod didn’t face a loss of weight. But here during 0% level the lighter vapor impact the rod and result in the loss of some weight. The calculation helps you to understand.
- Radius of Rod (r) = 4
- Length of Rod (L) = 40
- Weight of Rod (W) = 2600
- Specific Gravity of Fluid (SG1) = 0.9
- Specific Gravity of Fluid (SG1) = 0.75
At Zero level (0% Level)
At an empty level, lighter fluid vapor will impact the rod. So the rod will lose its weight as in this example the fluid changes its state, so the LP side will disturb the rod weight.
Loss (W) = (π * r² * L * SG1) + (π * r² * L * SG2)
= ( 0 ) + (3.14 *4²*40*0.75)
=1507.2
LRV = Total Weight – loss of Weight
=2600 – 1507.2
LRV = 1092.8
At Full level (100% Level)
When the tank is fully filled with process liquid, the rod will lose related to the specific gravity of the liquid. Let’s calculate the loss in weight..
Loss of weight = (3.14 * 4² * 40 * 0.9) + (0)
Loss of weight = 3.14 * 16 * 40 * 0.9
Loss of weight = 1808.64
URV = TOTAL WEIGHT – LOSS OF WEIGHT
URV = 2600 – 1808.64
URV = 791.36
