Operating Principle

A Diaphragm Seal System consists of a measuring instrument, typically a pressure transmitter or pressure gauge, one or two Diaphragm Seals and either a direct mount construction or with capillary lines, filled with a fill fluid. A correctly prepared and filled Diaphragm Seal System will accurately transfer process pressure on the diaphragm to the sensing element of the measuring instrument. This is based on Pascal’s principle which states that a pressure exerted on a fluid is transmitted undiminished through that fluid in every direction. The process pressure exerts a force on the outside face of the seal, the flexible diaphragm. As the diaphragm flexes under this force it pushes inwards and attempts to compress the transmission or fill fluid behind the diaphragm. The transmission fluid is designed to withstand compression so the force is channeled proportionally and directly in to the measuring instrument to produce a resultant reading on the connected instrument.  

When to use diaphragm seals

Diaphragm Seals are typically used to protect the measuring instrument. There are many different situations in which a Diaphragm Seal should be typically considered: 

• Corrosive medium: When the process medium is corrosive it would chemically attack the wetted parts of a standard pressure measuring instrument.  
• Viscous medium: When the process medium is highly viscous or contains solid particles, either of which could result in the instrument’s pressure inlet getting blocked. 
• Solidification: When the process medium is prone to solidification, crystallization and/or polymerization over time. The medium may freeze when the temperature drops, it may set as it dries, or it may be subject to polymerisation. In these situations the pressure inlet can get blocked. 
• High/low temperatures: When the process medium temperature is very high or very low and exceeds the temperature limits of the pressure measuring instrument resulting in damaged measurement instruments. 
• Sanitary requirements: When the process is easily affected by the formation of bacteria on or in the process connection.  The presence of bacteria in the process medium can lead to rejection of production batches. These applications where hygiene is of paramount importance are often found in the pharmaceutical, food and beverage industry.
• Specific process connections: When the location of the pressure measurement is not suitable for a direct mounting of a pressure measuring instrument. A diaphragm with remote mounting, by means of capillary, ensures easy visual check of the instrument. 
• Replacing ‘wet legs’: As a substitution for so-called ‘wet legs’ for liquid level measurements in pressure retaining tanks. Hydrogen permeation: Also Diaphragm Seals are used when in the process there is a chance of presence of hydrogen ions (H+) that can permeate the diaphragm. In those cases, a diaphragm seal with gold plating offers the required protection. 

Performance considerations

Mounting a Diaphragm Seal to a pressure instrument changes the performance of the instrument. The Diaphragm Seal System will have additional temperature effects and response time depending on the system configuration. The performance of the entire Diaphragm Seal System needs to be evaluated when specifying a new application to ensure satisfactory performance when mounted in process.  

• System volume: The fill volume in the Diaphragm Seal System needs to be minimized as much as possible. The more volume in the system, the higher the (potential) effect of temperature and mounting. For better performance always keep the system volume as small as possible, for instance by keeping capillaries as short as possible. Also it is advised not to use different capillary lengths at HP and LP of a DP instrument. 
• Mounting effect: The mounting effect on a Diaphragm Seal System is the variation of the pressure represented by the vertical fill fluid column between the Diaphragm Seal and the instrument, due to the variation of the gravity of the fill fluid as a result of ambient temperature deviation.  
• Zero shift: The most common application of Diaphragm Seals is for level measurement. Proper ranging of a transmitter for level service requires considering the specific gravity of both the fill fluid and the process fluid, and the transmitter range. 
• Temperature effect: Changes in volume of the Diaphragm Seal System are referred to as temperature effects. They are caused by changes in volume and density of the fluid in the system and occur when the fill fluid expands or contracts caused by fluctuations of the process and/or ambient temperatures. This change in fill volume drives a change in the internal pressure of the Diaphragm Seal System.  
• Diaphragm characteristics: The characteristics of the diaphragm itself are important for the performance. In general, a larger diaphragm diameter allows for more flexibility and is more sensible to changes in the volume due to temperature influences. Other diaphragm characteristics as the material, the thickness of the diaphragm as well as the convolution pattern, are an important factor as they all have an effect on the performance of the Diaphragm Seal System.  
• Fill fluid characteristics: Each fill fluid has its own characteristics, such as density, viscosity, thermal expansion, and vapour pressure. These characteristics are influenced by the systems pressure and temperature and determine the performance of the Diaphragm Seal System. The selection of the fill fluid depends on factors such as temperature, pressure, volume to be displaced (response time) and process safety. Most used fill fluids are silicone oil, glycerin, or vegetable oils. Also special inert fill fluids, such as halocarbon® for chloride and oxygen applications and other special filling fluids for high temperatures (up to 420 °C) are used.