Level Measuring Technologies: Ultrasonic, Capacitance, Hydrostatic, and Radar

With a variety of different level measuring devices available today, it is easy to be confused about which type of measuring device would be best suited for a particular application.  Provided below is a brief overview of four specific level measuring technologies: Ultrasonic, Capacitance, Hydrostatic, and Radar. For all of these above mentioned technologies, highlighted below is the basic operating principle of each, some application examples, as well as advantages and disadvantages of these technologies.

Hydrostatic Operating Principle
Hydrostatic operating level devices are based on the principle that fluids exert a pressure that is a function of height. Submersible level sensors work by measuring that hydrostatic pressure formed by the water column directly located above the unit and output a linear 4-20mA output 
     proportional to level. The piezoresistive pressure sensor is suspended below the level, and the output is then correlated to a level
     reading in feet or meters of water column. Hydrostatic pressure instruments, such as Dwyer’s’ submersibles, are provided at the
     lowest cost when compared to other liquid level sensing technologies. They feature easy installation with characteristic differences
     to suit a variety of applications. The Dwyer Series SBLT and MBLT feature small pressure openings that work great in clean water
     applications, with the MBLT featuring a small, 0.63”, housing diameter for borehole applications. For more robust units we offer
     our Series PBLT and FBLT with no pressure openings making them ideal for sludges and slurries.  Some advantages of this level
     technology is that it is great for applications that may contain vapor, foam, or any form of agitation.  Some disadvantages of
     submersibles are that you are limited by the use of only low viscosity liquids with a specific measuring range, compatibilty to the
     wetted material, and they can only be used in non-pressurized tank applications.  Standard units are calibrated for water and any
other fluid application with a SG variance, although water needs to be accounted for to maintain the accuracy of the unit. Another important note is that these units measure the pressure difference in reference to atmospheric pressure, so it is important that the breathing tube or vent tube of the unit be properly vented to the atmosphere and clear of any obstructions. The vent tube must also be free of any moisture that could condense, harming the electrical components. Our units are supplied with a hydrophobic, Teflon filter in order to avoid any moisture build up, with an option to purchase our A-297 reusable desiccant filter accessory, for higher moisture environments. 

 
Ultrasonic Operating Principle
Ultrasonic level sensors work by emitting high frequency acoustic signals that are reflected back and detected by the unit. The transit time of the signal, from the sensor to the target and back to the sensor is correlated to the level.  Some advantages in using an ultrasonic level sensor are that it provides a non-contact measurement, which virtually eliminates any compatibility concerns and is great for measuring the level of high viscous liquids, with no regards to the SG. The full measuring range of the ultrasonic is programmable and features a high F.S. accuracy. Ultrasonics can be used to measure level height in flumes and weirs in order to calculate open channel flow, as offered with our Series ULF. Some disadvantages of ultrasonic level sensing is that it is not to be used in high turbulence applications or applications that may have steam, foam, or high variances in the concentration of the process material. Turbulence and foam prevent the sound wave from being properly reflected back to the sensor, while steam and vapors absorb the acoustic signal. A stilling well may be used to prevent some of these issues but should be considered before purchasing an ultrasonic for these applications. High pressurized tanks or ones under vacuum have different sound coefficients and ultrasonics can be affected by the changing coefficient of sound due to moisture, temperature, or pressure.  Consequentially, it is important to understand that due to characteristics of sound waves when these changes exist in an application they may lead to inaccuracies, as well as a lost signal. Correction factors can be applied to the level measurement to improve the accuracy of measurement and is offered in the programing structure of our Series UTC and UTS. Direct overhead installation is required for the proper use of the unit making installation sometimes difficult. Since the unit is transmitting and receiving ultrasonic signals there is a small distance locatedat the end of the sensor where there are some mechanical vibrations that must be overlooked by the sensor in order for unit to accurately measure the level height. This small distance is known as the dead band and must be taking into account when looking level height of the process. Our Series ULT features software mapping to deal with tank intrusions such as ladders, pipes, or agitators, small dead band, and features an effective sensing area of only 7.6 cm or 3”.

Capacitive Operation
Capacitive units utilize low radio frequency to measure the conductivity of current in a closed circuit that varies proportionally to the level of the application. It’s important to remember that capacitance is a function of the dialectic constant of the fluid, the surface area of the capacitor, the probe, and the separation distance. All constants must maintain the same with only the level of the media allowed to change. Some advantages of capacitive units, such as the Series CRF2, is the vertical mounting, the FEP coating that increases chemical compatibility, and the fact that it works well in liquids with solids. Capacitance level units are not affected by dust, foam or varying specific gravity, and feature a programmable range. Some disadvantages to using capacitance level probes is that when it is used in applications with non-metallic tanks or tanks with irregularly shaped walls a probe ground reference must be used to properly operate and therefore must be purchased at time of order. It is also important that the dielectric constant of the measuring media is greater than 3mF and is used for measuring heights greater than 6in for water (other liquids as permitted by their dielectric constant).  As noted capacitance is a product of the surface area and therefore we discourage the purchase of a probe length less than 24” in length.  

Guided Wave Radar
Guided wave radar utilizes what is referred to as TDR or Time Domain Reflectometry technology. With this technology, impulses are transmitted along the probe at fast rates to the surface of the fluid and when these impulses hit the fluid a portion of the signal is reflected back up the probe. The time between the signal transmittal and receipt is used to determine the fluid level. Advantages of using radar are the top vertical mounting and the fact that it can be used with liquids with floating solids or coating liquids. Other added features of this technology is that it is unaffected by emulsification, dust, foam or vapors and it features a programmable output range, which is unaffected by dielectric or specific gravity changes of the liquid, with no real minimum probe length requirement. Some disadvantages of this technology are that the probe length must be the entire length of the sensing range and is typically offered at a higher cost. Attention must be taken when choosing the proper probe type in applications with bypass chambers or stilling wells.

Overall, there are vast amounts of ways to measure fluid level and every level measuring technology isn’t created equal. In order to avoid confusion and sometimes costly reinstallation fees, contact our Dwyer Technical Support Team at 1.800.872.9141 for more information on which level technology is best suited for your application.