You have a tank. That tank is filled with a liquid (we’re sticking to liquids here). The level or volume of liquid in that tank is important to you. How can you tell how much you have? There are so many options for getting this info, but not every method is appropriate for each tank or liquid type. Below is a list of level indication/measurement technologies and some info on where to use them.
Types of Technologies:
Eyeball – Your eyes work well when you can see the product, but there is no way to transmit or record that data automatically to a control or inventory system. You can look down into a tank, or see through a clear tank. You can augment your eyeball with a sight glass on the side of the tank or a mag gage with a float to see indirectly. Use this for low cost/no cost low priority inaccurate applications.
Mechanical Automatic Tape Measure – This technology connects a measuring tape to the roof of your floating cover or to a float in your tank and has a readout at eye level. This can be read using your eyes or can be transmitted back to the control system. Use these when you are cost conscious and need a general idea of your level and know that mechanical systems will need maintenance over time.
Displacer/Float – These can be simple floats that give an idea of the top of the liquid level and send a 4-20mA or relay back to the control system. These can also be servo-controlled displacers that are the most accurate level sensors in existence. They are mechanical systems and will fail and require maintenance over time, so use sparingly.
Hydrostatic/Differential Pressure – This works by measuring the pressure created by the weight of the fluid above the sensor pushing down on it. This is usually a reliable measurement and is the go-to when it works. Your liquid needs to have a consistent density to be consistent. If your material temperature changes, the densities will change, and the level will be off. If it is an enclosed tank, you will need to use a differential sensor to compensate for the varying or non-atmospheric pressure in the top of the tank. If that pressure varied and wasn’t measured, it would appear that the level in the tank changed. Don’t use this when there will be build up, pressure shocks, or damaging grit in your vessel.
Bubbler – This technology uses compressed air that is regulated at a specific flow rate and to bubble up from the bottom of a tank. The pressure it takes to overcome the weight of the fluid is measured by a pressure transmitter and gives you the level of the fluid. Use this in applications with buildup, hazardous environments, places where you don’t want your instruments to touch, etc.
Ultrasonic – This was the first open air, top down, contactless level device. It emits ultrasonic waves which contact the liquid and reflect to the transmitter. The time it took for the sound to travel out and back is measured and since the speed of sound is known, the distance can be calculated. That gives you level. Ultrasonic is fast. It is also fairly accurate and inexpensive. The only downside is that it doesn’t work in soft/loose foam, in a vacuum, or when the temperature at the sensor differs from the air above the liquid. This can be compensated for by a separate RTD but adds to the complication and cost. Use this technology when the level changes rapidly and the tank is vented.
Radar – This is a top down, non-contact level measuring method. It is like ultrasonic but uses electromagnetic waves of various frequencies to reflect off the liquid surface rather than sound waves. This allows for measurement in pressurized and vacuum tanks, but there are other considerations. Radar reflects off liquids more or less strongly depending on the dielectric (dk) constant of the liquid. If the dk is too low (~<2.0) the radar will pass through the liquid and just measure the bottom of the tank. Radar is slow compared to ultrasonic or hydrostatic pressure, so don’t use in rapid changing processes. Steps also need to be taken to deal with condensation in the radar horn. Radars are a good all-around choice for level. My personal favorite after hydrostatic pressure.
Guided-wave Radar – These are a modification of an open-air radar. Instead of sending radar through the air like a flashlight, you send it down a rod or cable. This has the benefit of concentrating the radar in a small area. This allows for measuring of lower dk fluids (~>1.2). The drawback to guided wave is that it is in contact with the fluid and therefore is susceptible to buildup and interferes with cleaning systems and agitators.
Capacitance – This was one of the first electronic level technologies. It uses a change in capacitance to infer the level of liquid in a tank. This is a contact measurement where the transmitter is one plate of a capacitor, the tank is the second plate, and the liquid or air between them is the dielectric. By varying the dielectric, the capacitance changes. The ratio of liquid and air contacting the sensor rod is proportional to the measured capacitance. These are fast responding, work on many fluid types, and can even detect interface level. They do have the downside of being in contact with the process and possibly getting buildup. They also must extend the entire distance to be measured, so are a poor choice for tall tanks.
Load Cells – These are strain gages mounted to a frame that “weigh” the tank. They need to be arranged so that the tank is the only weight they see and that there is no side loading. This means the tank must be isolated from the process with expansion joints and loops and that vibration needs to be minimized. Since they don’t touch the process, they can be used on just about anything. These are just more expensive and not always doable because of the need for isolation or the tank is just too big.
Nuclear Absorbance – This technology works on everything, but you don’t use it unless you must. It consists of a gamma radiation emitting source mounted to one side of the tank and a long sensor (scintillating tube) mounted to the other. The amount of radiation that passes through the tank and liquid varies with the amount of liquid in the way. This gives you the ability to “see” the level. The downside to this is that you will now have radiation on your site and must report to the NRC and have a nuclear safety officer. You will also have to pay for wipe tests and other safety related paperwork which raises the cost and complication. Only use these where you must.
There are many, many choices when it comes to determining your tank level and using that information to control your process. Take the initiative and call me or the fine folks at Jasper Engineering if you have level applications and questions. We can analyze your process and determine the correct technology for your specific case. We can help you out and we love the adventure of solving the problem.