In this first video of a two part series
we’re going to introduce you to
“Open Vessel level measurement using a Differential Pressure Transmitter”.
The second part of our two part series
will take you through the process of predicting Differential Pressure Transmitter outputs
of open and closed tank level control loops
before we get started, you might want to review 3 of our other videos
"What is a Level Sensor?"
“What is a Pressure Sensor”,
and “Differential Pressure Transmitter Explained”.
You can find the links to these videos in the description.
Even with the development of new technologies
and highly sophisticated level measurement instruments,
Differential Pressure Transmitters are still in service today
measuring vessel fluid levels.
In this video, we're going to:
Discuss the two categories of level measurement
Talk about the difference between Direct
and Indirect methods of continuous vessel level measurement
Explore how a Differential Pressure Transmitter is used to measure vessel level
and discuss variables that influence the measurements
Ok, …off we go!
Level measurement can be divided into two categories:
“point” and “continuous”.
Point level transmitters are used to mark a specific single liquid height.
For example, the point level sensor would send a signal to a PLC
at a specific level to start a pump.
A continuous level sensor
measures the fluid level within a measurement range
rather than at a specific single point.
Said a different way,
continuous level sensors provide level measurement
over the full 0% to 100% range of a vessel.
Because we are interested in the height of a liquid in a vessel,
we take vertical measurements taken from the surface
to a fixed reference point.
The reference point is usually near the bottom of the vessel.
You might have noticed that the vessel Reference or Zero percent point
is not at the very bottom of the vessel.
This is to make sure that the vessel is never completely drained.
We can take advantage of the fact that the pressure measured
at the base of a liquid storage vessel
will vary with the height, or level, of the liquid in the vessel.
Armed with that knowledge,
we can determine the level of the vessel
from the indirect “inferred” measurement of the pressure.
We don’t have to worry about the size, shape, or volume of the tank
because none of those parameters have any effect on the pressure at the reference point.
But, we do have to consider other parameters
such as “specific gravity” and “temperature”…
More on that later.
Alright, we’re ready to look at how a Differential Pressure Transmitter
can be used to measure the vessel level.
We said earlier that the pressure at the bottom of a vessel
is directly related to the level of the liquid in the vessel.
In that case, if we connect a Differential Pressure Transmitter to the reference,
or zero percent point of that vessel,
we can use the pressure measurement to determine the level.
Another variable that has a major effect
on the pressure at the reference point of the vessel
is the relative density of the liquid in that vessel.
Relative density is also called specific gravity.
There is a relationship between pressure, relative density, and liquid height.
That relationship can be expressed as:
"Pressure in inches of water
is equal to the relative density of the liquid
multiplied by the height of the surface of the liquid in inches”
Now… let’s see how we can measure the level of an open vessel
using a differential pressure transmitter.
The High Pressure Port is connected at the 0 meters point
and the Low Pressure Port is vented to atmosphere.
How can we predict the pressure at the bottom of the vessel?
Consider our discussion earlier about the relationship between liquid height,
relative density, and pressure in inches of water
to determine the pressure at the bottom of the vessel for any liquid level.
You can convert the resulting inches of water pressure value
into any pressure scale you need such as psi, kPa, bar, etc.
Let’s review what we’ve discussed in this video…
Level measurement can be divided into two categories: “point” and “continuous”
Point level sensors are used to mark a specific single liquid height
A continuous level sensor measures the fluid level
within a measurement range rather than at a specific single point.
Vessel Level can be determined indirectly
using a Differential Pressure Transmitter
by measuring the pressure at the 0% point of the vessel and “inferring” the level.
For vessel measurements, Pressure in inches of water
is equal to the relative density of the liquid
multiplied by the height of the surface of the liquid in inches.
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