In production environments such as food, beverages, dairy products, fermentation, and biopharmaceuticals, differential pressure measurement is often directly related to filter pressure differential, tank liquid level, flow conditions, or equipment blockage warnings. For quality control and safety management personnel, accurate readings are certainly important, but more challenging issues often arise after cleaning: Is there residual liquid at instrument connections? Has material accumulated around diaphragm edges? Have new hygiene risks been introduced after disassembly and reassembly?

The value of a sanitary differential pressure transmitter lies not only in transmitting differential pressure signals to the control system, but also in enabling the measurement point to be incorporated into the complete CIP cleaning-in-place, SIP sterilization, or manual cleaning process. When sensor structure, installation position, and cleaning procedures are not properly coordinated, even highly accurate instruments may become potential sources of microbial residue, cross-contamination between batches, or abnormal readings.

Cleaning dead legs are usually not where they can be seen

So-called cleaning dead legs are not limited to pipe bends or the bottoms of vessels. For sanitary differential pressure transmitters, risks are often concentrated around process connections and isolation diaphragms: excessively deep internal connection cavities, grooves formed after gasket compression, overly long pressure tapping branches, and connection orientations that hinder drainage can all prevent cleaning solutions from fully contacting surfaces or completely removing residues.

Traditional differential pressure measurement sometimes uses impulse lines to connect the high- and low-pressure sides to the transmitter. This approach is relatively common in general industrial operating conditions, but when used with media subject to high hygiene requirements, slender impulse lines, valve manifold cavities, and low-point liquid accumulation areas can easily become residue zones. In particular, sugary beverages, emulsions, protein liquids, fermentation broths, or high-viscosity slurries do not naturally disappear simply because “cleaning has been performed” after material adheres to surfaces. If flow velocity, temperature, concentration, and contact time are insufficient, residues may still remain inside.

Therefore, in hygienic applications, the design focus should shift from “whether the instrument can be installed” to “whether media-contact surfaces can be completely cleaned, drained, and verified.” This is also why the selection of sanitary differential pressure transmitters should not be based solely on measuring range, output signal, and accuracy class.

Design the measurement point as part of the pipeline, rather than as an additional accessory

The most effective way to prevent dead-leg contamination is to reduce unnecessary media-contact spaces. For measurement points in direct contact with process media, sanitary diaphragm designs with flush diaphragms and no retention cavities should be prioritized, so that the diaphragm is as flush as possible with the vessel wall, pipeline inner wall, or equipment connection. In this way, cleaning solution can cover the diaphragm surface during flow, reducing the likelihood of material buildup at edges and localized deposits.

Connection standards also need to be consistent with the hygiene level of the production line. Clamp-type sanitary connections are commonly used in locations requiring regular maintenance because they are relatively convenient for disassembly, assembly, and inspection; specific specifications should be confirmed based on existing equipment interfaces, sealing materials, operating pressure, and cleaning temperature. If interface sizes do not match and reducers or extension pieces are used on site as “temporary transitions,” new recesses and blind spots can easily be created in the connection section.

For applications such as differential pressure monitoring before and after filters and auxiliary assessment of sterile filtration integrity, the pressure tapping points on the high- and low-pressure sides should be as close as possible to the equipment being measured, while maintaining relatively consistent structures and response characteristics on both sides. If one side is directly connected by a short straight connection while the other is routed through a long line, hygiene risks will increase, and temperature effects, gas-liquid column effects, or inconsistent dynamic responses may also occur, ultimately causing the differential pressure value to lose its intended reference significance.

Installation orientation determines whether residues can drain naturally

Many contamination issues are not caused by the product itself, but by improper installation orientation. The isolation diaphragm of a sanitary differential pressure transmitter should not face upward and form a “cup-shaped” liquid accumulation area; pressure ports, short connecting pipes, and valves should also avoid low points that cannot drain by themselves. If process equipment must retain branches, their length, angle, and internal continuity should be confirmed during the design stage, rather than remedied by increasing flushing cycles at acceptance.

In applications involving gaseous media or possible bubble formation, attention should also be paid to gas accumulation at high points. Trapped gas can cause differential pressure readings to fluctuate and can also prevent cleaning media from fully filling the measurement chamber. For liquid processes, low-point liquid accumulation and solids sedimentation must be prevented. A simple yet practical assessment method is: after shutdown and draining, determine whether media could still remain near the measurement point without being discharged by gravity; if the answer is “possibly,” that location warrants reassessment.

During installation, transmitters should also be kept away from locations subject to strong vibration, frequent mechanical impact, or direct radiation from heat sources. Hygienic design does not mean focusing only on surface finish. Long-term vibration may loosen clamps, gaskets, or wiring seals, thereby affecting sealing performance and measurement stability.

CIP procedures should cover instruments rather than bypass them

When developing CIP procedures, some production lines focus on vessels, main pipelines, and filling equipment, while assuming that instrument connections will be cleaned incidentally. In reality, locations where sanitary differential pressure transmitters are installed often have lower flow velocities. In particular, filter bypasses, tank-top connections, branch ends, and small-diameter connection points require confirmation that cleaning solution truly provides complete coverage.

It is recommended that quality control and equipment teams jointly verify the following items:

• Whether the cleaning solution flow path can pass through all media-contact surfaces on both the high- and low-pressure sides;
• Whether cleaning temperature, concentration, flow velocity, and circulation time match the characteristics of the process medium;
• Whether acid cleaning, alkaline cleaning, hot water, or steam sterilization conditions are within the allowable range of the diaphragm, seals, and fill system;
• Whether sufficient drainage is available after cleaning to prevent cleaning solution from remaining mixed with residual process liquid;
• Whether additional targeted flushing and verification are required after batch changeovers, extended downtime, or abnormal blockage.

A common misconception should be avoided here: higher cleaning parameters do not necessarily mean greater safety. Temperatures, pressures, or chemical concentrations exceeding the applicable limits of instrument materials and seals may cause diaphragm deformation, seal aging, or zero drift. The correct approach is to match the parameters to the actual structure, wetted materials, and process cleaning system of the sanitary differential pressure transmitter, and incorporate the relevant parameters into equipment documentation management.

When verifying that cleaning is complete, do not rely only on appearance and trend curves

On-site confirmation of cleaning effectiveness should not be based solely on a clean exterior or the restoration of normal transmitter signals. For high-risk production lines, instrument connection points should be included in the hygiene inspection checklist: inspect clamps, gaskets, diaphragm edges, and adjacent short sections for discoloration, scaling, adhered material, or seal damage; during planned maintenance, conduct targeted inspection of wetted parts after disassembly.

At the same time, attention should be given to the “abnormal signals” in the measurement data itself. For example, if the differential pressure baseline of the same filter remains consistently high under similar flow rates and temperatures, it may indicate a change in filter element load or localized scaling at the pressure tapping port. Repeated zero drift or slow recovery after cleaning requires investigation into factors such as diaphragm contamination, trapped bubbles, installation stress, or liquid accumulation in the connection structure. Analyzing process trends, cleaning records, and instrument calibration results together usually makes it easier to identify problems than reviewing any single item alone.

Key questions to clarify during the selection stage

When purchasing sanitary differential pressure transmitters, it is recommended not to provide only basic information such as “differential pressure measurement” and “4-20mA output.” It is more valuable to clearly provide process information in one step, including medium viscosity, whether particles are present, operating temperature and pressure, CIP/SIP conditions, existing connection standards, installation space, cleaning flow direction, and whether remote diaphragm seals are required.

Xi'an Shenghongchuang Instrumentation Co., Ltd. and its production base, Shaanxi Qinkong Sensor Technology Co., Ltd., provide selection, manufacturing, and non-standard customization services for differential pressure transmitters and supporting instruments for industrial measurement and control applications. For special installation requirements in hygienic processes, communication can be conducted regarding wetted structures, process connections, range matching, output methods, and on-site installation conditions, avoiding forced adaptation through complex adapters after products arrive on site.

Ultimately, avoiding cleaning dead legs with sanitary differential pressure transmitters cannot be achieved by relying on a single component. It is the result of the combined effect of structure, connections, installation, cleaning, and maintenance. Only by making measurement points cleanable, drainable, and inspectable can contamination risks be minimized before every production run while ensuring reliable data.