What Are Level Transmitters Used For?
Level transmitters are used to tell you the level of something, usually within a tank, reactor, pit or a large enclosure used to store bulk solid materials like road salt. Often a level transmitter (LT) will ultimately be able to tell you the volume of the liquid or solid it is measuring by applying some formulas to the level measurement and converting the level to volume. This can be done onboard some level transmitters while others rely on an external device, like a programmalbe logic controller (PLC) to do the math.
Common Types of Level Transmitters
There are quite a few different types of level transmitters, but there are two overarching styles.
One style works by sending and receiving a signal from an antenna, where the signal bounces off of something and comes back to the antenna while the LT measures the time it took the signal to come back. Since the speed of the signal is known, the LT can perform a math function and figure out how far away the liquid level is by how long it tooks for the signal to be reflected off the liquid or solid and come back to the LT.
Think about it this way, if you can walk 1 mile in exactly 20 minutes, and you walk for 40 minutes, you’ve walked 2 miles. Since a beam from an LT must go from the transmitter to the surface and back again you must divide the distance travelled by 2. This would be like you walking 1 mile to the store (20 mins) and 1 mile back again (20 mins). You’ve walked 2 miles in 40 minutes, but the store is only 1 mile away.
We now understand how the LT knows how far away the surface being measured is from the antenna, but how does that tell us the level from the bottom of the tank or other equipment? The overall height from the bottom of the tank (or other equipmment or enclosure) to the antenna is can be determined by the LT’s literature and the dimensions of the tank or equipment can be measured by hand or taken from an equipment drawing. So you will be able to work out the overall height from the bottom of the equipment to the level transmitter’s antenna. Subtracting the distance from the antenna to the measured level of the solid or liquid from the overall height gives the level of the solid or liquid. This is a bit of a simplified example since there are other things like sloped sides and areas at the top (and sometimes bottom) of the probe that can’t accurately be used for measuring. Each LT is a little different in terms of these areas so I can’t discuss them all at this time, but the main principal is here.
See below for an illustration of how liquid measuring works using a through air type LT in a tank.
The other style of level transmitter uses differential pressure (check out my article on differential level transmitters here, where I explain how they can be used to measure level).
I’m going to be focusing on the style that sends and recieves a signal. There are many varieties of this type, I’ll discuss the most commone ones below.
Through Air Radar Type
This type of LT (also known as non-contact radar) uses radio frequency electromagneitic signals that are sent from a transmitter and then recieved again by that transmitter. It can be a good choice for measuring level where there aren’t many obstructions in the path of the signal. For instance, a large open tank.
Where this type of LT sometimes isn’t a great fit is when you have many obstructions, like a reactor with a mixer (especially if the mixer has multiple sets of blades up the shaft of the mixer), one or several baffle(s) and other items that can bounce the radar signal around at weird angles. Through air radar signals are typically wide compared to some other LT’s signals, so they have the tendency to come into contact with a higher number of surfaces. This can look strange to the LT as it can be picking up multiple signals and can cause errors in level measurement. Some of the more advanced through air radars can correct for these errors with some additional setup.
Other limitations of through air radar include not working well when the surface it’s measuring is not flat or is agitated by mixing or if foam is present.
These are a good choice when you have a flat calm surface and are reasonably priced.
High Frequency Through Air Radar (80 GHz)
Recently there have been some advances in the through air radar LT category, most notably those that operarate at a much higher frequency than the traditional ones. Since they operarate at such a high frequency they have a much narrower beam. This allows them to be installed on smaller fittings and work within smaller vessels and/or vessels with more obstructions.
Additionally they can accurately sense of a tank or other vessel through materials like Teflon (if desired) to allow the transmitter to be separated from the process and thus not be corroded by processing solvents and other materials. This can be achieved by mounting the transmitter on a Teflon blind flange and connecting the flange right onto the vessels nozzle or other fitting.
This combination of characteristics makes the higher frequency through air radar a good choice for pharmaceutical manufacturing given the smaller vessels and varied uses including many types of corrosive chemicals in a single reactor at different processing steps.
Guided Wave Radar Type
These instruments also use radio signals but come with a guide to send the signal down to the surface being measured. The guides are either single or double cables or rods. GWR’s radar signal is usually narrower than a through air radar signal. This allows for less chance of error from signal echoes or bounces. The rod or cable(s) can be secured to the bottom of a tank or reactor depending on the application.
The last few years have seen many advances in GWR technology, one of them is noise or false reading cancellation. Since the radar is applied over a wider area than that of the probe, it can detect when buildup has occured and ignore it. It does this by recognizing most of the signal that is returned isn’t due to the buildup since the signals bouncing off the buildup with generally be weaker than from the actual media’s surface that is being measured. This is due to the limited area of the buildup compared to the full area of the signal beam.
This type of instrument can measure multiple layers of liquid. Picture a bottle of salad dressing where all the oil has seperated and comes to the top of the bottle. This is a water/oil separation that has taken place. This same phenomenon can also happen within a tank or reactor. The top of each layer can be detected by the GWR since the propeties of the layers are different, most notably the dielectric constant. By programming the GWR transmitter with the dielectric constants of each layer, it will know how long it takes each signal to come back from the top of each distinct layer, and therefore how high the level is of each layer.
While these can be a little more expensive, they’re worth the extra cash for many applications.
These LT’s use sound waves that are at frequencies above that which humans can hear. Humans can hear up to about 20,000 Hz, where ultrasonic waves used for level detection have frequencies ranging from 40,000 Hz to 200,000 Hz. Since sound waves must propogate (or travel) through a medium such as air, nitrogen, solids, etc., ultrasonic LT’s can’t be used in a vacuum where there is the abscence of air (or anything really). If level measurement has to be performed under vacuum or partial vacuum, like in the case of a distillation of one solvent in another within a reactor, then another type of LT should be chosen.
This type of LT also does well even when urface it’s reflecting off of is not flat. So agitation (or mixing) within a tank or reactor, boiling a liquid (as long as the vapors are not of high density) are applications where these make sense a lot of the time. Ultrasonic LT’s also have success in measuring bulk solid materials, one application is controlling the level of material when feeding a powder or other materials through a hopper.
Similarly, the packing level on conveyors can be sensed to ensure they have the right density, for instance, if a row of packaged vegetables are being fed down a conveyor, you can tell if there are any that have landed on top of each other and measures can be taken before crating the packages to ensure the process flows optimally.
Take a look at this one as an example of an ultrasonic LT.
This type of LT doesn’t carry a huge price tag and often are very reliable, requiring little maintenance over their useful life, which can be quite lengthy. As with many other LT’s, these are often available in explosion proof and intrinsically safe versions, along with materials of construction that are resistant to wear and damage for many corrosive conditions.
Level transmitters have become all but essential devices in many industries including chemical, pharmacuetical and other manufacturing sectors. Selecting the right one based on cost, material compatibility, environmental conditions in the area the level measurements are taking place (foam, agitation, multiple liquid layers, etc.) all play an important role in selecting the right transmitter.
If you have any questions that I can help you with, please reach out in the comments below and I’ll be sure to get back to you.