The Anderson SL Pressure Type Level Transmitter

As we’ve discussed in previous posts, process control and information- such as level, pressure, and temperature- are valuable pieces of information, essential to optimal process performance. Previous posts have focused on some of the flow and temperature products we offer. Today, we’ll take a look at the first of three different level technologies we deal with all the time at Holland and how we can apply it,  measuring pressure to determine level.

To begin, why are tank or process pipe level measurements important? By knowing what the level of a tank is, we are able to plan for production more efficiently, identify and reduce plant losses, track product, and it also help with financial accounting. Some bulk products, such as milk, are bought and sold by volume. If we know the amount of product in a tank, we can figure what that product is worth and how much we received.

A level measurement also helps us to optimize process. Balance tanks, for example, are a constant level tank that provides a constant supply of milk (by creating uniform head pressure) to a pasteurization system.

As alluded to above, there are several ways to measure the level in a sanitary system. The first technology we’ll look at is pressure level measurement. Pressure based level transmission has been around for a while. It’s straightforward and easy to install. Sanitary pressure based level transmitters are used to make continuous level measurements. If a measurement is being made on an open vessel, a general gauge pressure transmitter will work just fine. In an open vessel, the head pressure of the liquid is measured to help us infer a liquid level. A column of liquid exerts a force on a point, based on its own weight. We call that force hydrostatic pressure. As discussed in previous posts, hydrostatic head pressure is very important in sizing a pump. We’ve gone over how to calculate your hydrostatic pressure in previous posts, but here is the equation again in case you forgot:

Hydrostatic pressure (ft.) = Height (ft.) x Specific Gravity

Equation 1.1

If the liquid level of our tank changes, our hydrostatic pressure changes proportionally. Consequently, we can simply install a sanitary pressure gauge at the lowest point of our vessel and determine the level. Taking it a step further, we can use a pressure based level transmitter to convert this pressure to a 4-20 mA output.
Things get a little trickier with a pressurized tank. In a pressurized tank application, a sanitary differential pressure transmitter must be used to compensate for the vessel pressure. Gauge pressure transmitters won’t work in these applications because they can’t distinguish between pressure changes due to liquid level or change in vessel pressure. To solve this issue, we take two pressure measurements that are compared to one another. Changes in vessel pressure affect the high and low pressure taps equally, so effects of the pressure are cancelled out.

Now that we know how a sanitary pressure level transmitter works, let’s look at the advantages and disadvantages of this technologies. Pressure transmitters are economical and easy to use. They work in almost any style of tank and can handle a wide range of liquids including slurries and suspensions. Temperature swings aren’t really an issue and they stand up well against turbulence from a mixer and even foam.

The large contributor to inaccuracy with a sanitary pressure level sensor are changes in fluid density. So fluids that are very thick and change in composition as their concentration increases will create challenges with pressure sensors. These level sensors will generally need to be located at the low point of a vessel, so ports and tank design needs to be well thought out ahead of time. Anderson Instruments does offer a top mount style pressure level transmitter, the LD (the LA is the pharmaceutical version), there are some vessel limitations for this type of transmitter, however. Future posts will focus more on specific product offerings.
In today’s high purity, high precision applications, effective level measurement is essential. Selecting the right technology is critical. For more help with your next level application, contact a Holland Sales Engineer today.