Eight reasons to choose a pressure instrument with a ceramic measuring cell
Pressure measurement is one of the oldest, most common means of process control. Users can choose a measuring cell material that best suits their processes depending on their requirements. The most popular cell material is metal, but ceramic is emerging as a cell material to measure corrosive liquids in challenging applications. Still, ceramic cells are relatively new to the market and users may be unaware of their benefits.
By nature, ceramic is abrasion resistant. The material’s tight, dense matrix makes a ceramic diaphragm ten times harder than stainless steel, rendering it better-suited to withstand harsh environments and any application where liquid is used to move solids. In these applications, process material often contacts pressure diaphragms, resulting in damage to the cells. This is not an issue with ceramic cells because surface scrapes do not leave marks, scratches or indentations. If product builds up on its surface over time, users can simply scrape it off with a hard metal object without harming the ceramic cell.
A longer lasting dry cell
Ceramic cells are known as dry cells, meaning they do not use filling oil to measure pressure. In typical pressure sensors, this oil acts as a transfer medium. To allow the oil to transfer, the metal used for diaphragms is thin and fragile, which leads to quick wear and tear. When metal diaphragms fail, oil contaminates process material and users may have to discard an entire batch. Conversely, ceramic cells use capacitive measurement at the point of pressure and do not rely on oil to move the pressure value. This eliminates the risk of wasting an entire batch and replacing damaged pressure sensors due to oil contamination.
Reduced hydrogen permeation
When hydro molecules penetrate through metal diaphragms, they get trapped and react with the filling oil. This reaction causes an expansion, which falsely indicates increasing pressure. To minimise hydrogen permeation, metal diaphragms are usually coated with gold or a similar material. This slows down the transfer of hydrogen molecules from the process to the oil filling behind the diaphragm, but hydrogen transfer still happens. The dense lattice of a ceramic cell also slows down the permeation of hydrogen molecules. However, since no oil filling is present behind the diaphragm, no effect occurs on the pressure reading.
Drift is the gradual offset from calibration after measurement cycles. Over time, pressure sensor drift distorts measurement accuracy and reliability. Drift for metal diaphragms can appear quickly as the thin metal fatigues and does not return to true zero. This drift is corrected by frequent field “rezeroing” – offsetting a sensor’s deficiencies by choosing a new value for zero. Ceramic cells are drift-free because they are limited in motion, creating less fatiguing and fewer required routine calibrations. This means that repetitive cycles and temperature extremes have a minimal effect on the life of the cell membrane. In turn, operators enjoy an extended cycle for routine maintenance and have no need for rezeroing.
Minimal product compatibility issues
In contrast with metallic cells, ceramic cells are compatible with most process materials because ceramic does not corrode. This is welcome news to applications that measure everything from saltwater to acidic solutions, because they do no damage to the ceramic cell. Additionally, expensive metallic-like tantalum is required for certain chemical applications, whereas ceramics are compatible with most chemicals.
High overload resistance
With minimal space between the diaphragm and the body of the ceramic cell, the diaphragm bottoms out on the base when pressure exists beyond the rated span. When that pressure is removed, the cell diaphragm returns to its original position and to full operation without recalibration. This means the cell can handle elevated overpressure beyond its span without permanent damage or offset. Even with ceramics, overload protection varies from cell to cell.
Some manufacturers offer ceramic cells with optional temperature measurement that can be output as a standard digital HART variable or assigned as the main or secondary 4 – 20 mA output. This additional measurement may allow users to avoid the expense of a remote temperature device in the process. It is important to note that this temperature value is usually used internally for thermal shock compensation and is not available for process compensation.
Detection of small pressure changes
To detect a pressure change with a metal measuring cell, oil must move through a diaphragm – even when the measurement range is small. A short distance requires a large diaphragm to register the pressure change. However, increasing the size of a metal diaphragm is risky because metal cells get weaker and wear out faster as they get larger. Since no oil moves in a ceramic cell, small changes can be detected without altering the size of the pressure transmitter.
This is not to say it is time for users to swap out all of their installed pressure instruments for new ones with ceramic cells. Metallic measuring cells have their place and by all means, if metal works, keep it. However, taking the benefits of ceramic in sum, users might want to consider switching to pressure instrument. Ceramic’s abrasion resistance and dense makeup make it a logical fit for harsh applications, and for measuring corrosive product. Ceramic’s high overload and temperature resistance make it a viable measurement cell option for tracking movement in pipelines, measuring pressure and level in pressurised batch vessels, and monitoring negative pressure in distillation columns. In difficult applications in which the measurement cell must be kept from the medium, ceramic is up to the task.