4 Advice to Choose a Liquid Level Transmitters

23, Jun. 2025

 

Level transmitter technology options: How to choose

The need for reliable level measurement is not uncommon. There are countless applications that require level measurement to function properly. However, even though the need is abundant, the knowledge surrounding it is not.

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There are several approaches one can take to achieve level measurement (through level transmitter technology), but they all have different pros and cons depending on the process they are earmarked for. More importantly, they all come with an associated cost. Some technology is on the expensive side, but if the level application in question does not require the features associated with said technology, a less expensive alternative can be just as effective. In fact, there are several key factors that need to be explored before settling on the appropriate level transmitter technology for a given application.

Application type, vessel design and process variation

There are three key questions to ask before beginning the decision-making process on which level transmitter technology to choose. It is important to collaborate with an expert when going through this process, but for that individual to provide you with the very best advice, you will need to gather the following information.

The first questions surround what the level application in question is. For example, what is the nature of the control of the process? Is it continuous or is there an on/off capability? The most important factor here is the repeatability of the process. Next, is the application a batch/recipe control application? If it is, then high accuracy is required. The same goes for a custody transfer application. In this case precision accuracy is paramount. 

Continuing the exercise of identifying what kind of level application it is, what kind of output is needed? This could be an analog 4-20mA, Modbus or Ethernet. Finally, do we need a visual indication or verification function? 

Identifying and understanding the requirements will ensure the right technology with the proper costs is selected.

The next step is to identify the vessel. The vessel type will play a part in dictating which technology is applicable for a certain requirement.

Tanks can come in different forms and have various features:

  • Above grade product tank
  • Below grade tank or sump
  • Floating-roof storage tank
  • Bullet or spherical pressurized tank
  • Material of construction
  • Instrument entry port size and location

Any combination of these elements will help guide your vendor in assisting you to apply the best instrument for your installation.

Finally, there are the process features to consider. The physical properties of the process will narrow the options available. A checklist of questions must be considered before making any final decisions.

Is the vessel pressurized? What is the temperature range? Is there a specific gravity or density? What is the viscosity? Is it Conductive/Dielectric Constant? Is there concern for corrosive or reactive behavior? Are there vapors or fumes in the dead space above the liquid? 

Contact us to discuss your requirements of Liquid Level Transmitters. Our experienced sales team can help you identify the options that best suit your needs.

Having answers to all these questions ensures that you’ll receive the very best guidance from your vendor, enabling you to make the most informed and effective selection.

Level transmitter technologies

The next question one may ask themselves is, “What are my options?” Once the decision-maker has all the relevant information applicable to their unique process, how do they make a selection? There are many different transmitter technologies available when attempting to achieve level measurement. They all have unique features and can sometimes be challenging to match up with the factors present in a certain process. What is most important is to consult with an expert before making any costly purchasing decision.

Pressure level transmitter technology comes in two distinct forms: Differential and submersible. Differential pressure can be used on pressurized above grade vessels. It is a technology that requires a stable density. Diaphragm seals associated with the differential pressure technology allows it to be ideal for dirty or viscous processes.

Submersible technology utilizes a wetted sensor and requires a small entry port at the top of the vessel or below grade sump, reservoir or lake, suspended by an electrical cable to the bottom.

Conductivity level transmitter technology is another option to explore, yet it requires the process to be a conductive media. Through this technology, a small top connection is required for the rigid or flexible probe to be easily installed. It is important to note that viscous material can be slow to respond to a falling level. However, it is good for corrosives when compatible probe materials are selected.

Guided-wave radar technology provides greater accuracy than conductivity transmitters, but the installations are very similar. Furthermore, the dielectric constant on the process must be stable. If not, either inaccurate measurement or a loss of signal is bound to occur.

Ultrasonic transmitters are the ideal choice for harsh processes, as the sensor never comes in contact with the product that is in the tank. They are also capable of long-range measurement, which is why a large number of ultrasonic transmitters are used in open tanks or sumps and reservoirs.

Radar transmitters are another option and considered to be one of the most accurate. This type of transmitter is commonly used for custody transfer applications due to the need for such precise measurements. It stands to reason that it is a very complicated installation and configuration.

The final transmitter technology type for the purposes of this discussion is the magnetostrictive variety. This utilizes a wetted sensor and float. Thus, it is not commonly used in viscous processes. What makes this technology unique is that it can give separate outputs for level, interface level and multi-temperature points in one probe and connection. It is also very accurate as we see it used frequently for custody transfer and batching. It is also a very convenient technology as through its digital “transit-time” signal processing, the operator can quite literally set it and forget it. Finally, it can be installed both internally and externally on magnetic level gauges.

It is safe to say there is much information to go through before making a decision on the level transmitter technology to choose. Each one of these technologies comes at a certain cost. Some are much more expensive than others, making the decision more impactful. It is important to avoid paying for unnecessary features that drive up the cost. Alternatively, cutting corners for cost saving purposes can be a critical mistake if the process requires more robust technology. The key is working with an expert that understands all the inputs for your specific challenge and can guide you through your decision.

If you want to learn more, please visit our website Ultrasonic Level Transmitter.

level sensors | PLCS.net - Interactive Q & A

For water I'd use hydrostatic (head pressure transmitter). It is VERY reliable. Engineering units are inherently water column units.

A restriction on hydrostatic measurement is that it is density/specific gravity sensitive, but clean water does not change its density, so that is not an issue.

I've had ultrasonic fail (temporary Loss of Echo) in heavy fog, or when condensation condenses on the face of the transducer, not uncommon in foggy conditions. In northern climates, an outdoor ultrasonic transducer will fail if it ices over. As Tom mentioned ultrasonic can not distinguish foam from water, it will give a reading for foam level.

The one consideration for submersible pressure sensors is to stick the cable's vent tube in a dessicant (water absorbing granules) to avoid an accumulation of condensation in the vent tube. Chemicals can involve changing densities/specific gravities, an issue with hydrostatic head pressure.

If it's not the same chemical of the same density in the same tank, you're asking for measurement interpretation errors with hydrostatic. That's why Tom mentioned 'clean water' as ideal for hydrostatic head pressure measurement. I agree.

Chemicals that have high vapor pressures so that the empty tank volume above the chemical level is chemical vapor that has displaced or mixed with air is an issue with ultrasonic, but not with radar. Ultrasonic is designed to shoot through air with the sonic velocity of air at different temperatures a well researched and documented phenomenon.

I suppose some ultrasonic vendor has a factor for compensating for the speed of sound through something other than air, but
a) the temperature compensation factor designed for air probably does not apply
b) you're likely to have to experiment to determine what the correction factor is

When people talk about measuring levels of liquid chemicals I lean towards radar, unless ordinary stainless steel wetted parts on a DP transmitter can be connected to a process port at a low elevation on the tank, which does not mean the drain/out-flow pipe because the level measurement will be affected by the hydraulic forces of a flow. Lots of personal care products and foods work well with SS and if the same product goes into the same tank, a flange mounted or impulse tube to a DP transmitter works great and is extremely reliable.

Corrosive chemicals that require a diaphragm seal have their own set of problems, which sometimes people tolerate because they don't have top mount access, enough clearance from the sidewall or the foot deep (350mm) blanking zone that radar requires. An isolating diaphragm always has a fluid fill on the transmitter side. The fluid fill is a 'closed system' which becomes a temperature measurement system where pressure changes due to temperature changes are added to or subtracted from the head pressure.

This recent (level) thread
http://www.plctalk.net/qanda/showthread.php?t=
shows two level trends, one on the first page, the second on the second page.

The first is a static (unchanging) level on an outdoor tank where the changing temperature of the transmitter body and its close coupled diaphragm seal (filled system) substantially affected the level readings on an hour to hour basis. There is no compensation for this error. It is what it is.

The second shows the apparent changes in level due to a poorly vented tank holding a chemical with a high vapor pressure. This level transmitter was a gauge pressure transmitter and the problem was fixed with differential pressure transmitter. Dear all,

What about measuring level in the inlet well sewage pumping station, (for sewage application) i am stuck into a debatable issue, either, i use non-contact level measurement like ultrasonic (radar is not very usefull since temperature is not a problem and it is more expensive, E&H has some types of ultraonic level sensor that are suitable for sewage application) or i use hydrostatic level sensor directly submerged into sewage (for overcoming the issue of foam) and i might have material issue, however, its cost is effective compared to radar which seems reliable for sewage application.

Your feedback is very valuable,

thank you!

charbel