Today we are going to learn the process of calibrating some of the more common instruments used for level measurement.
Open Tank
Let’s begin with the formula P = pgh. P is the pressure exerted by our tank in Pascals. p is the density of our fluid. This is also known as Specific Gravity. Specific Gravity is the ratio of the density of a given liquid to water. g is gravity acting on the fluid. h is the height of the fluid in the tank.
Notice how volume plays no role in the amount of pressure exerted by our tank. It is a common mistake to assume that the bigger the tank the more the pressure.
We do not want our values in Pascals however, we would like PSI. 1 PSI is equivalent to 27.7” of water.
For example, if we have a full tank with no lid that is 240″ tall and full of water, what should our Pressure Transmitter be reading if it is mounted at the bottom of the tank?
First, you have to do is divide our 240″ by 27.7. This will give us to give 8.66 PSI.
What is our reading if we have a 240″ tank full of a liquid with a Specific Gravity of 0.5?
First, we will multiply our 240″ by our Specific Gravity of 0.5 which gives us 120.
Next, we will do the same as before and divide 120 by 27.7 giving us 4.33 PSI
Closed Tank
Dry Leg
Things get a little more complicated when measuring the level of a closed vessel. Closed vessels are naturally pressurized and this pressure must be taken into account for an accurate level reading.
In this case, we will use a Differential Pressure transmitter. A Differential Pressure Transmitter has both a High side and a Low side. The transmitter will output the difference between the high side and the low side.
Our high side of the transmitter will go to the bottom of the tank as before but our low side will go to the top of the tank. The internal pressure is now acting on both sides of the transmitter effectively cancelling it out.
Wet Leg
Let’s say that we are measuring liquids that may be harmful to the sensor whether it be too hot or corrosive. In this case we will use what is known as a wet leg. We will fill up the low side with water this time so only the water will come in contact with the sensor. The low side will still be attached to the top of the tank.
We will have to account for the extra pressure now that we have a full column of water acting on the low side. For example, let’s say that we have a 240″ tank and no fluid in the tank. Our Differential Pressure transmitter will have 0 psi on the high side and 8.66 PSI on the low side which gives us a reading of -8.66 PSI as it subtracts the reading from the high side by the low side. This would be considered our zero.
What would our pressure transmitter read if the tank was at 192″
First, we divide 192″ by 27.7”. This gives us 6.93 PSI on the high side and don’t forget about our 8.66 PSI on the low side.
Our transmitter will read the difference which is -1.73 PSI.
Common Mistake
Occasionally air can get stuck in your wet leg tubing which will cause erratic readings on your transmitter as air is compressible and water is not.
Every time you remove your tubing from your transmitter you must bleed the lines.
The key is to slowly open one of the bleed ports of the transmitter until you see the process fluid drip out. The line is now free from air.
Ultrasonic
Ultrasonic is another common level measurement device. An advantage of ultrasonic is that it doesn’t physically contact the process.
It is best used for corrosive liquids or chemicals and bulk solids such as grain or ore.
We measure the distance from our sensor to the bottom of the tank. This will be what is known as our empty reading.
Next, we will measure our full reading. It is a common mistake to input the distance from the top of the tank to the bottom of the tank as the full input. The full input is actually the distance from the sensor to the max level you would like to measure.
Conclusion
I hope to have described the basic fundamentals of level measurement and calibration in that you can confidentally look at a level reading and determine its accuracy. If you liked this article please check out our other posts on both temperature and pressure calibration!
