What are Flush Diaphragms and where are they used?

There are certain media which cannot be used with conventional pressure sensors due to it being either too viscous or requiring a high level of cleanliness. 

In some test and research applications a sensor that changes the volume or shape of the vessel / pipe will lead to additional measurement uncertainties.

High Viscosity Liquids
Substances such as paint, oil, wax, pastes, tar, plastic melt or slurries can potentially block up the process connection of a pressure sensor. The media maybe too stiff to adequately fill the internal void of a pressure connector or the substance might wax once a process has cooled down. For these types of substances it is necessary to specify a flush fitting where the diaphragm is completely open. Where there are concerns about mechanical damage to the diaphragm there are options for recessed flush diaphragms on some pressure sensors.

Pharmaceutical, Food, Beverage and Biotech
For these applications the concern is not necessarily high viscosity but more to do with cleanliness. A flush surface which is fully welded stainless steel 316 and free from crevices is the preferred sensor construction to prevent the build up of process residue, facilitate cleaning and protect against chemical corrosion. For oil filled pressure sensors it is also a requirement to use a fill that is food compatible so that if the diaphragm was to fail without being detected it would not leak any fluid into the process that would be harmful to the consumer.

Waste Water
Exposure to a waste liquid containing solids, sludge and a mixture of chemicals will lead to a highly contaminated pressure sensor. To avoid problems with corrosion and blockage of the sensor, a robust open diaphragm which can withstand the knocks from solid waste and the corrosive effects of chemical interactions is essential. The sensor must also be easy to clean and able to withstand an intensive wash down at regular intervals

Threaded Style Process Connections
The most common form of flush face pressure sensor has a threaded connection with a completely open diaphragm on the end. Since the pressure connector must accommodate the surface area. of the diaphragm this leads to larger diameter threads such as 1/2 or 1 inch BSP being used in the mechanical design.

Clamped Style Process Connections
The main advantage of the clamped pressure seal over the threaded type is that there is no gap between the thread which would otherwise allow the build up of residues and bacteria. This makes it ideal for food and pharmaceutical production where very high levels of cleanliness are required.

Negligible Volume
In some test and research installations there is the requirement to keep the internal volume or the effect on flow to a minimum. A conventional pressure sensor has an internal volume which may contribute a significant change in the system / test volume. If flow characteristics of a chamber / pipe are critical to measurements, a conventional pressure sensor may interfere with the measurement by changing the flow performance of the apparatus. A flush face device mounted so that the diaphragm is level with the wall of the apparatus will not add any extra volume added to the system or interrupt the flow characteristics.

If you would like help in selecting a flush diaphragm pressure transducer or transmitter please complete this form.