Combined with the U.S. 3-A sanitary standards, BCST analyzes the sanitary pressure transmitter contact surface and non-contact surface hygienic design principles to carry out, get with practical, hygienic pressure transmitter technical points for the production of high-performance pressure transmitters laid the foundation.
Hygienic pressure transmitters are an essential component in the process of monitoring food and pharmaceutical production equipment. With the increasing improvement of China’s production process and technology, the hygienic transmitter industry has also made significant progress, and its quality and performance are gradually approaching the international level. However, China’s pressure transmitter industry started late, pressure transmitter design standards are relatively backward and lacking, especially hygienic pressure transmitter hygienic design is still in a rather blank stage. Therefore, it is of great practical significance to research the sanitary design of pressure transmitters and propose specific indicators. 
1. Terminology conventions
1.1Â Hygiene standards
The main focus is on the American “3-A Hygiene Standard” and the German “EHEDG Standard” concerning transmitter hygiene standards. The American “3-A Hygiene Standard” of “74-06” is the hygiene standard for sensors, fittings, and connections. At present, many foreign pressure transmitter companies have obtained these certificates. With the increasingly stringent domestic requirements for transmitter hygiene, the major domestic transmitter manufacturers and users are also striving to obtain these certificates.
1.2 Hygienic design
The so-called hygienic design refers to the plan that enables food and pharmaceutical products to be produced, harvested, processed, transported, stored, and sold without being contaminated by harmful substances in all aspects. The purpose of the hygienic design of the pressure transmitter is to meet the cleaning requirements of the production site, i.e., to ensure that the cleaning meets the needs of cleanliness, to ensure that the product is not contaminated by secondary contamination, and ultimately to achieve hygienic requirements.
1.3 Contact and non-contact surfaces
The hygienic design of pressure transmitters is based on two concepts: contact surfaces and non-contact surfaces. Contact surfaces are all surfaces that come into contact with the product under test or where the product under test has the potential to drain, leak, diffuse or enter. Non-contact surfaces are all surfaces on which the product under test does not drain, leak, diffuse or enter.
2. Hygienic pressure transmitter structure and its material selection
2.1Â Transmitter construction
The hygienic pressure transmitter comprises a process connection and sensor, a variable transmission circuit module, and outlet protection. Figures 1 and 2 show the structure of a DIN connector type transmitter and a housing type transmitter.
2.2Â Hygienic pressure transmitter material selection
Process connections and sensors are pressure-sensitive components of the transmitter installation which require direct or indirect contact with the product to be measured. The transfer circuit module and outlet protection are components of the transmitter used to transmit signals, which do not require direct contact with the product under test, and the pressure transmitter’s non-contact surface. The poor choice of materials for both will, directly and indirectly, determine whether the pressure transmitter meets hygiene requirements, respectively.
2.3Â Contact surface materials
Hygienic pressure transmitters contact surface materials are usually made of high-quality stainless steel, commonly used 304 (06Cr19Ni10), 316 (06Cr17Ni12Mo2), 304L (022Cr19Ni10) and 316L (022Cr17Ni12Mo2). Following the 3-A Sanitary Standard requirements, pressure transmitters are required to conform to AISI (American Iron and Steel Technology Institute) 300 series stainless steel (except 301, 302, and 303) or ACI class. In contrast, any component of the stainless steel material must not be allowed to corrode with the product being tested and must be non-toxic and non-absorbent.
When welding is required, we require that the carbon content of the stainless steel must not exceed 0.08% and that alloys containing lead, leachable copper, or other toxic substances are not permitted. Non-metallic materials are also often used as transmitter contact surface materials and are generally used in various types of rubber and plastic. In addition, non-metallic materials are often used as auxiliary sealing materials and are selected for sanitary certification (e.g., US FDA approval).
2.4Â Non-contact surface materials
When we choose materials, the choice of non-contact surface materials is even broader. Firstly, it must be resistant to corrosion and have a non-absorbent surface; secondly, it must be durable and easy to clean. If it has a coating on the surface, its adhesion should be good. Parts that are disassembled for cleaning must not be painted.
3. Hygienic design of contact surfaces
3.1Â Surface roughness
The key point of the hygienic design is whether the cleanliness requirements of the production site are met. The product is affected by the ambient temperature and the product’s performance during production and processing. It will have some residues that will also inevitably adhere to the transmitter’s surface. If not cleaned in time, secondary contamination will occur.
3.2Â Welding
Hygienic pressure transmitters are usually connected and installed by welding the pressure-sensitive diaphragms, encapsulated joints, etc. The welding process is an essential part of clean design. Firstly, the welding must be continuous, generally not allowed to add material welding. If you need to add material welding, you must ensure that the material meets the hygienic requirements; secondly, the welding surface must be smooth and uniform, not allowed to exist in the dents, folds, cracks, cracks, and other parts.
3.3Â Sealing
Hygienic pressure transmitters are usually installed with gaskets, O-rings, or hard seals for sealing. The sealing material needs to be in contact with the product under test, and we need to prevent leakage to the outside. Gasketed seals must be designed to be removable or adhesive to facilitate cleaning and replacement for various applications. Gasket recesses should be prepared to be less in-depth than it is in width. For removable gaskets with cross-sectional dimensions greater than 1/4in, the breaks should not exceed 1/4in-depth or less than 1/4in width.
According to 3-A Hygiene Standard, the relevant requirements shall also be met for seal designs with a cross-sectional dimension ≤ 1/4in. In O-ring design and hard seal design, both the O-ring and hard seal contact line should be approximately flush with the installation contact surface, making the O-ring or hard seal contact line part of the surface exposed to the cleaning medium for easy cleaning and not harboring dirt (as shown in Figure 3 and Figure 4 respectively).
4. Conclusion
At present, we have the most difficulty in ensuring the development of hygienic design for transmitters because of the lack of the necessary knowledge of their clean design. In China, we have no relevant experience or standards to refer to, and we also lack a fundamental understanding of foreign standards. For example, the American 3-A standard is only from the contact surface and non-contact surface of two aspects of the sensor products for technical guidance overview. It does not involve specific industry health design details and methods. It is also designed to operate relatively vague.
With the development and growth of the domestic transmitter industry, more and more manufacturers have had to spend a lot of money on foreign standards and systems certification to enter the international market. Therefore, we have to establish our transmitter health standards and certification management system and strive to be internationally recognized to promote China’s sanitary pressure transmitter industry bigger and more robust to the world.