Why Cleanroom Differential Pressure Monitoring Requires Careful Measurement Point Placement

Cleanroom differential pressure is an important control parameter for maintaining airflow from areas of higher cleanliness to areas of lower cleanliness. It is also a direct indicator for assessing enclosure sealing performance, supply and exhaust air balance, and door management status. When Fuba differential pressure transmitters are used for cleanroom differential pressure monitoring, sensor placement should not be based solely on installation convenience; it should be determined comprehensively in conjunction with room cleanliness classification, process risks, relationships with adjacent areas, and HVAC automatic control strategies.

The core of proper measurement point placement is to enable the differential pressure transmitter to collect representative static pressure values. If pressure taps are located near supply air outlets, return air outlets, exhaust air outlets, or door gaps, local airflow velocity can cause pressure fluctuations, resulting in frequent fluctuations in displayed values and consequently affecting alarm assessment and damper interlocking. For areas requiring stable production, monitoring points should primarily reflect the overall room static pressure rather than locally disturbed pressure.

Differential pressure control in clean areas commonly targets the pressure difference between adjacent rooms or between a cleanroom and a corridor. Positive-pressure cleanrooms generally maintain positive pressure relative to the outside, allowing air to flow from cleaner areas to areas with lower cleanliness; rooms involving dust, odors, pathogen risks, or hazardous media may use negative-pressure control to prevent contaminants from spreading outward.

During the engineering design phase, a differential pressure gradient diagram should first be prepared to define the design differential pressure of each room relative to adjacent rooms, corridors, or outdoors, and then determine the connection directions of the high-pressure and low-pressure ports of the Fuba differential pressure transmitter. In general, the number of measurement points should not be determined solely by “room size”; differential pressure control boundaries and risk levels should be the primary basis.

Establish Differential Pressure Gradients and Monitoring Boundaries Before Point Placement

Use Differential Pressure Between Adjacent Areas as the Basic Monitoring Unit

A Fuba differential pressure transmitter typically corresponds to one clearly defined differential pressure relationship, such as “core cleanroom to buffer room,” “buffer room to clean corridor,” or “clean corridor to general area.” The high-pressure port is connected to the higher-pressure side, and the low-pressure port is connected to the lower-pressure side. Reversing the connections will result in opposite display polarity. Although this may not necessarily affect the absolute value, it will distort alarm logic and on-site assessment.

For areas with continuous differential pressure gradients, monitoring relationships should be established step by step according to airflow direction. For example, where four levels are formed from a core operating room, buffer room, clean corridor, and general corridor, key boundaries should be monitored rather than installing only one instrument in the innermost room. This allows timely detection of differential pressure imbalance caused by insufficient supply air, excessive exhaust air, or door leakage at any level.

When a cleanroom has multiple entrances and exits leading to areas with different pressure levels, separate monitoring is recommended according to the actual control boundaries. In particular, pressure relationships around personnel doors, logistics doors, and pass-through boxes may differ. If only one direction is monitored, abnormal differential pressure at another location may not be identified in time.

For large clean workshops with many partitions or clearly defined supply and return air zones, verification points may be added in addition to the design differential pressure boundaries. Verification points do not necessarily all need to be connected to the automatic control system, but they can be used for commissioning, inspection, and periodic review to help determine whether the main monitoring points remain representative over time.

Specific Installation Locations for Fuba Differential Pressure Transmitters

Pressure Taps Should Avoid Areas of Local Airflow Disturbance

Pressure taps should be located where they can represent indoor static pressure, typically in wall areas at a certain distance from door openings, supply air outlets, return air outlets, and exhaust air outlets. In engineering practice, pressure taps may be arranged approximately 1.5 m or more away from supply or return air outlets and adjusted based on on-site air velocity conditions. For rooms with laminar supply airflow or high air change rates, areas directly impacted by airflow should be avoided.

The pressure tap height should suit personnel activities and process operation areas. A commonly used installation height is 1.2 m to 1.5 m above the floor. This height facilitates maintenance and viewing while reducing the direct impact of overhead supply airflow and floor-cleaning airflow on sampled pressure. Where project standards or process specifications contain explicit requirements, project documentation shall prevail.

The differential pressure transmitter itself may be installed on a wall, in an instrument enclosure, or in a low-voltage control cabinet. However, the impulse tubing between the pressure taps and transmitter should be as short, straight, and sealed as possible. The lengths and routes of impulse tubing on both sides should be as consistent as possible to reduce the effect of temperature differences, resistance differences, and condensate accumulation on low-range measurement. For low differential pressure applications, the airtightness of the pressure transmission system is particularly important.

After installation, check wall penetrations, fittings, tees, and impulse tubing connection points to prevent air leakage from causing zero drift or low readings. Impulse tubing should not have obvious bends, flattening, or sagging sections where water can accumulate. Where ambient humidity is high, anti-condensation measures should be taken, and tubing flow-path inspections should be included in the maintenance plan.

Principles for Measurement Point Placement Near Door Areas

Door areas are where differential pressure changes are most pronounced, but this does not mean pressure taps must be located beside doors. Door opening, closing, and personnel passage can all cause transient pressure fluctuations. If a pressure tap is too close to a door gap, transmitter readings will be directly disturbed. It is recommended to use the sensor to assess differential pressure under stable conditions and use door contact signals to identify door-open status, with the two types of signals used together.

For frequently opened logistics doors or interlocking doors, appropriate delay and alarm suppression logic should be set in the control program. For example, instantaneous differential pressure alarms may be temporarily suspended while a door is open, followed by an observation period of 30 seconds to 120 seconds after the door closes before confirming whether the differential pressure has returned to the set range. The specific time should be determined through on-site commissioning and should not be applied directly as a fixed value.

Where the two sides of a pass-through box belong to areas with different differential pressure levels, the need for separate differential pressure monitoring may be assessed according to its sealing structure and frequency of use. For pass-through boxes used only as local equipment, overall room differential pressure is generally the primary consideration; for transfer areas handling critical material transfer with frequent opening and closing, boundary differential pressure verification should be strengthened.

During on-site commissioning, tests may be conducted for door opening, door closing, HVAC system startup and shutdown, and exhaust equipment switching, while recording the response trend of the Fuba differential pressure transmitter. Under normal conditions, readings should stabilize within the system-designed recovery time after the door is closed. If fluctuations remain large over a prolonged period, investigate air volume balance, door seals, ceiling sealing, and impulse tubing installation.

Coordinated Selection of Range, Accuracy, Control, and Alarms

Cleanroom differential pressure is typically a low differential pressure measurement. When selecting a Fuba differential pressure transmitter, the range should cover normal operating values, startup fluctuation values, and fault abnormal values, while retaining an appropriate margin. An excessively large range will reduce the resolution of small differential pressure signals, while an excessively small range may create an overrange risk during fan startup and shutdown or frequent door opening.

For example, in areas with design differential pressures of 10 Pa, 15 Pa, or 20 Pa, engineering selection may use suitable low-range products according to the actual control range and system fluctuations. For demanding applications, attention should be paid to accuracy, long-term stability, temperature drift, response time, and zero adjustment capability, rather than comparing only full-scale values. A display resolution of 0.1 Pa does not necessarily mean that the on-site measurement system has equivalent accuracy.

Alarm values should be set around the design differential pressure and process-allowed deviation, with a distinction recommended between early warnings and interlock alarms. Early warnings alert maintenance personnel to check air volume or door status, while interlock alarms serve critical process protection. Excessively tight alarm thresholds can lead to frequent false alarms, whereas excessively wide thresholds will undermine the practical purpose of the differential pressure barrier; therefore, they need to be determined based on commissioning data.

The output signal of a Fuba differential pressure transmitter can be connected to a PLC, DDC, building automation system, or independent digital display control instrument to enable on-site display, remote trend recording, and audible and visual alarms. For critical clean areas, it is recommended to retain historical trend curves and focus on differential pressure changes during shift changes, cleaning, equipment startup and shutdown, and peak traffic periods, providing a basis for subsequent maintenance and verification.

Key Points for Commissioning, Verification, and Long-Term Maintenance

Use Measured Data to Confirm Whether Point Placement Is Effective

After installation is completed, zero checks and differential pressure comparisons should be conducted with doors and windows closed and the HVAC system operating stably. A calibrated micro-differential pressure gauge may be used for spot checks at the same differential pressure boundary to confirm whether the deviation between the Fuba differential pressure transmitter display value and the actual on-site value meets project requirements. Commissioning records should include test time, fan status, door status, and environmental conditions.

When readings remain unstable, troubleshooting should begin with airflow organization and installation conditions: first confirm whether supply air, return air, and exhaust air volumes are balanced; then check the sealing of door gaskets, wall panels, and ceilings; and subsequently inspect impulse tubing fittings, pressure tap blockage, and transmitter zero point. Avoid repeatedly modifying alarm thresholds before the cause is identified, as this may conceal the actual fault.

During routine maintenance, regularly check whether trend curves show gradual decline, periodic large fluctuations, or sudden changes. Gradual changes may be related to increasing filter resistance or changes in fan performance; sudden changes may result from damaged door seals, abnormal damper operation, exhaust equipment switching, or detached impulse tubing. Trend data are more useful than a single reading for identifying potential issues.

Xi'an Shenghongchuang Instrumentation Co., Ltd. can provide Fuba differential pressure transmitter selection, measurement point planning, and technical service support according to cleanroom floor layouts, differential pressure gradients, control interfaces, and on-site installation conditions. Before project implementation, room functional zoning, supply, return, and exhaust air arrangements, door locations, and target differential pressure requirements should be provided to develop a monitoring configuration plan better suited to on-site operating conditions.