Breaking the Fog Bank: Mitigating Atmosphere’s Effect on Pressure Sensors

Hydrostatic pressure sensors are a common method of liquid level measurement in many industries. Hydrostatic pressure refers to the pressure created by the mass of a stationary liquid at a given point of measurement.  As level increases inside a tank, the hydrostatic pressure increases proportionally. Higher hydrostatic pressure equals higher level; lower hydrostatic pressure equals lower level. 

A pressure sensor mounted at the bottom of a vented tank measures more than the mass of the liquid inside, as the force of the atmospheric pressure inside a plant also exerts itself on the liquid. Think of it this way: A large open tank might have a true level of four feet, but if one were to cap that tank and force air on the fluid, a pressure sensor would read the spike in pressure inside the tank as a level increase even though the amount of liquid inside has not changed. Thus, in order to achieve an accurate level measurement using a pressure sensor, a plant must account for atmospheric pressure.

Compensating for atmosphere on a pressure device is simple. All relative pressure sensors include a filtered vent open to atmosphere that compensates the level measurement for the varying atmospheric pressure on top the fluid in a vented or open vessel. In a sense, the pressure instrument works like a balance scale with atmospheric pressure in one weighing pan and the total hydrostatic pressure in the other. But when the potential of condensation exists as it did in the dairy plant, the accuracy and repeatability of the measurement is vulnerable. The vent can become clogged, reference the incorrect atmospheric pressure, and deliver an incorrect output. More on that later.

Users in the dairy plant opted for a pressure sensor with a ceramic measuring cell. They chose the ceramic cells for their high overload protection, a necessity considering the plant’s wash down method, and their unsurpassed durability. Further, ceramic cells measure without fill fluid so there was no risk of contaminating their product if the cell were somehow breached. Given all of the variables involved, users decided a pressure transmitter with a ceramic cell was the most reliable option available.

Plant personnel had an instrument they liked for a critical application but they forgot to consider the complete environment and the atmosphere in the facility did not cooperate. Here is what they did to compensate. 

Inside the Costa Rican dairy, plant personnel use steam or hot water during the CIP/SIP process when cleaning the vessels and surrounding areas. This sudden injection of heat into and onto cold tanks creates a fog bank and condensation on colder surfaces inside a plant. The moisture in the air changes the atmospheric pressure and alters the total hydrostatic pressure measurement. 

The expectation of the plant personnel was exactly what it should have been:  The level measurements would be accurate because pressure instruments compensate for changing atmospheric pressure conditions. However, what they didn’t fully understand at the time was that the vent handling the compensation was directly in the line of fire of their cleaning process. Even though it was protected, a constant direct stream of hot water jammed the filter on the vent altering the compensation for the changed atmospheric pressure. They made the assumption that the diaphragm was damaged or that the electronics in the instrument were bad, causing an inaccurate level output. Naturally, they sent the units back to the U.S. for repair. However, by the time the pressure sensors reached the States the condensation in the vent line had dried and the instrument functioned perfectly. In effect, the sensors they sent us and the ones we received were different instruments. 

While the sensor was in transport to us for review, they installed their spare and decided to move the sensor’s electronics housing (and vent) to a remote location, far away from the direct spray of water. This is common practice in many plants. By keeping the electronics covered, safe, and vented, pressure sensors inside a plant can still compensate for changing atmospheric pressure. By leaving the measuring cell attached to the vessel and the electronics elsewhere, the risk of water ingress is eliminated.  The downside is that the compensation of the pressure measurement is done by reading the atmospheric pressure in an area away from where the cell is actually measuring.

To safeguard completely against condensation spoiling measurement accuracy, plant personnel considered installing an absolute pressure cell. An absolute pressure cell is referenced against complete vacuum instead of the atmosphere around the cell. Since this cell is uninfluenced by atmospheric changes, there is no need for ventilation and thus, no vents to jam. As this was an option, the benefits of an absolute pressure cell were cause for celebration in the Costa Rican dairy. 

However, this option has a downside: Absolute pressure cells do not automatically compensate for changing atmosphere.  This requires a different mindset and does not provide an accurate level indication in an open or vented vessel. Since the sensor is reporting the total hydrostatic pressure (fluid level plus atmosphere) operators need to think about what the sensor is telling them. When the vessel is empty the output of the sensor relates to the existing atmospheric pressure. At the time of installation the output could be scaled to subtract this value and provide an accurate level output. But when the atmospheric pressure changes from this set value, the level again will be incorrect.

Plant operators weighed the benefit of having no vent to plug during the cleaning process against the loss of level accuracy. In their final evaluation, they decided changing to an absolute pressure transmitter outputting a level not compensated for atmosphere was not worth it. They went back to the drawing board searching for a solution to their challenge.

The first two options have sacrificed one benefit in exchange for another; users solve one problem and potentially create a new one. There is a win-win choice available however, a third option that combines all the benefits of both remote electronics and an absolute pressure cell while maintaining an accurate level output. Users can install an absolute pressure cell and pair it with climate-compensated electronics. With interior technology similar to the weather app in a smart watch, these electronics can measure and compensate for changing atmospheric pressure without a vent directly connected to the ceramic measurement cell. 

Climate-compensated electronics are potentially transformational for users struggling with constantly changing humid atmosphere, like those in the Costa Rican dairy. This solution truly has it all. The absolute pressure cell measures against vacuum, eliminating potential vent plugging, and a remote electronics, mounted close by (but out of the line of spray) to compensate for atmospheric pressure changes close to the point of measurement. 

For industrial plants dealing with changing humid atmospheric conditions disturbing their level measurements, installing an absolute pressure cell and combining it with climate-compensated electronics is one of the best options available. Users get a consistent, reliable measurement no matter what happens to atmospheric pressure in the plant without making their instrument vulnerable to condensation. Additionally, when users choose a pressure device with a ceramic measuring cell, they have a solution that can stand up to thermal shock and tackle harsh conditions without experiencing potential measurement drift.

I showed personnel in the Costa Rican dairy such a setup already in use at a Wisconsin dairy and they immediately jumped at the chance to adopt the solution in their own facility. I personally helped them setup the system and they’ve enjoyed accurate, consistent measurements ever since. It took them some time to find the right solution, but now those users in the tropical dairy can break the fog bank.