What Requirements Should a Differential Pressure Gauge Meet Under ISO 14644?

Differential pressure gauges are widely used to monitor pressure relationships between cleanrooms, airlocks, corridors, production areas and the surrounding environment.

The measured value helps operators assess airflow direction, detect deterioration in the HVAC system and maintain the environmental conditions established during design and qualification.

ISO 14644 does not specify one gauge model, one measurement range, one accuracy class or one universal room-pressure value for every cleanroom. Instrument requirements should be established according to the cleanroom class, manufacturing application, contamination risk, HVAC configuration and monitoring plan.

A highly accurate gauge can still produce unreliable information if it has an unsuitable range, is installed at the wrong location or is not calibrated correctly. Instrument selection should therefore be addressed throughout design, procurement, installation, qualification, operation and maintenance.

What does ISO 14644 require for cleanroom differential pressure?

ISO 14644-1:2015 classifies air cleanliness according to the concentration of airborne particles. It uses airborne-particle-counter data to determine the ISO cleanliness class rather than classifying a room by its differential pressure.

A differential pressure gauge does not directly establish whether a room meets ISO Class 5, ISO Class 6, ISO Class 7 or ISO Class 8. It supports the HVAC and airflow conditions needed to maintain control of airborne particles.

ISO 14644-2:2015 specifies minimum requirements for a cleanroom monitoring plan based on parameters that measure or affect airborne-particle concentration. Differential pressure may form part of that plan where the pressure relationship affects cleanroom performance.

ISO 14644-3:2019 provides test methods supporting the assessment of cleanrooms and clean zones. It covers performance testing in as-built, at-rest and operational occupancy states, depending on the purpose of the assessment.

ISO 14644-4:2022 addresses the process of creating a cleanroom from requirements through design, construction, start-up and verification. It does not prescribe one technological solution, so project-specific performance requirements should be defined by the user and design team.

ISO 14644-5:2025 requires an operations control programme covering personnel, materials, cleaning, maintenance and monitoring. A differential pressure instrument should therefore remain under operational and maintenance control throughout the cleanroom lifecycle.

Does ISO 14644 require a 10 Pa pressure difference?

ISO 14644 does not specify a universal 10 Pa differential for every cleanroom.

Ten pascals is frequently referenced in certain industry-specific standards and guidance documents, particularly for pharmaceutical applications, but it should not automatically be applied to every ISO 14644 cleanroom.

Electronics, semiconductor, optical, food, medical-device and pharmaceutical facilities may use different pressure arrangements.

The required pressure difference should be based on:

• Intended airflow direction.

• Cleanliness of adjacent areas.

• Room leakage.

• Supply, return and exhaust airflow.

• Door-opening frequency.

• Personnel and equipment loads.

• Product-protection requirements.

• Containment requirements.

• Cross-contamination risk.

• Qualification and risk-assessment results.

Two rooms of the same ISO class may still require a pressure difference to control personnel flow, material flow or cross-contamination.

Some applications may require negative pressure to contain hazardous materials. The selected gauge must therefore match the project’s intended pressure condition rather than a default industry value.

Define the purpose of the measurement

Before selecting an instrument, the measurement duty should be clearly identified.

Typical applications include:

• Pressure between two cleanrooms.

• Pressure between a cleanroom and corridor.

• Pressure across an airlock.

• Pressure between a cleanroom and an unclassified area.

• Isolator, RABS or Clean Booth pressure.

• HEPA-filter differential pressure.

• Pre-filter or intermediate-filter differential pressure.

• Pressure monitoring on a Dispensing Booth, Pass Box or cleanroom device.

A room-pressure gauge and a HEPA-filter pressure gauge should not automatically use the same range. Room-to-room pressure is generally much lower than filter pressure loss.

The intended use also determines whether the instrument only needs local indication or requires signal transmission, alarms and continuous data recording.

Select a range that matches the design pressure

Measurement range is one of the most important specifications.

An excessively wide range makes small variations around the normal operating value difficult to see. A range that is too narrow may overrange during door opening, HVAC start-up or temporary system imbalance.

For low-pressure room-to-room applications, a range such as 0–25 Pa or 0–50 Pa may be considered where it matches the design pressure. These values are engineering examples and are not mandatory ISO 14644 ranges.

Where both positive and negative pressure must be detected, a bidirectional range such as ±25 Pa or ±50 Pa may be suitable.

The final range should consider:

• Target operating value.

• Alert level.

• Action limit.

• Door-opening transients.

• Maximum credible pressure.

• Sensor overpressure capability.

The normal operating value should fall within a clearly readable portion of the span while leaving sufficient capacity for abnormal conditions.

Accuracy should support the operating limits

ISO 14644 does not prescribe one accuracy class for all differential pressure gauges.

Required accuracy should reflect the intended use and the separation between the normal value, alert level and action limit.

For example, where a room operates at 15 Pa and the action limit is 10 Pa, the combined error of the sensor, signal transmission and display system must be sufficiently small to distinguish reliably between the two conditions.

Instrument assessment should consider more than percentage-of-full-scale accuracy. Important characteristics include:

• Accuracy within the actual operating range.

• Repeatability.

• Zero drift.

• Long-term stability.

• Temperature effects.

• Display resolution.

• Overpressure resistance.

• Calibration uncertainty.

Zero stability is especially important for low-range sensors. A shift of only a few pascals may represent a significant proportion of the actual room pressure.

A resolution of 0.1 Pa or 1 Pa may be appropriate depending on the range. Additional display digits do not necessarily indicate better measurement accuracy.

Mechanical gauge or electronic transmitter?

A mechanical differential pressure gauge may be suitable for local observation or periodic manual recording.

It has a simple construction, requires no electrical supply and provides an immediate visual indication.

A basic mechanical gauge normally cannot:

• Record data automatically.

• Generate a remote alarm.

• Store event time stamps.

• Display pressure trends.

• Communicate with a BMS or EMS.

• Detect signal loss.

An electronic differential pressure transmitter or digital gauge is more appropriate where continuous monitoring, alarm generation or central-system integration is required.

ISO 14644-2 requires a monitoring plan based on parameters that affect cleanroom performance. Whether pressure should be monitored continuously or periodically should therefore be established through the monitoring plan and risk assessment rather than determined only by the ISO class name.

Signal outputs and system integration

An electronic gauge should provide an output compatible with the project control system.

Common options include:

• 4–20 mA.

• 0–10 V.

• Modbus RTU.

• Modbus TCP.

• BACnet.

• Alarm relays.

• Ethernet-based communication where available.

A 4–20 mA output is widely used because it is suitable for practical transmission distances and can be integrated with a PLC, BMS, EMS or SCADA system.

Where the central-system value is used to determine cleanroom status, the complete measurement loop should be verified from the applied pressure through to the displayed and stored value.

The system should be capable of identifying signal loss, out-of-range signals or sensor faults. A failed transmitter should not result in a false normal indication.

Alarm settings should be based on operating data

Pressure alarms should not be copied from one cleanroom to every other room.

A practical alarm strategy normally includes:

• Target operating value.

• Normal operating range.

• Alert level.

• Action limit.

• Alarm delay.

• Reset hysteresis.

• Defined operator response.

An alarm delay can reduce nuisance events caused by short door openings. An excessive delay, however, may conceal an actual loss of segregation.

Alarm values should be established after HVAC balancing, commissioning and observation of pressure variation during normal operation.

Alarm functions should be challenged during qualification rather than verified only by changing a software setting.

The instrument should be suitable for the cleanroom environment

A cleanroom-mounted instrument should minimize particle accumulation and allow effective cleaning.

The front face should preferably be flush or nearly flush with the wall panel. Gaps around the device should be sealed using materials compatible with the cleanroom surface.

The housing, display and seals should withstand the cleaning agents, disinfectants or process chemicals used in the area.

For electronics and semiconductor facilities, the project may also need to consider chemical emissions, particle-shedding materials and electrostatic-discharge requirements.

The instrument should be suitable for the expected temperature, humidity and pressure conditions at the installation location.

Pressure taps should measure representative static pressure

The high- and low-pressure ports must be connected to the correct spaces. Reversing the connections will produce an incorrect sign or pressure direction.

Pressure taps should not be positioned:

• Directly in a supply-air jet.

• In a high-velocity airflow region.

• Too close to a fan or return-air grille.

• Directly at a door gap.

• Where they may be obstructed or damaged.

• Where liquid can accumulate.

The objective is to measure representative room static pressure rather than a local dynamic-pressure effect.

Pressure tubing should be leak-tight and protected against kinking, crushing and liquid accumulation. High and low lines should be clearly identified to prevent incorrect reconnection during maintenance.

Calibration requirements

The gauge should be calibrated before use and at intervals defined by the equipment-management, maintenance or monitoring programme.

Calibration should not be limited to a zero check. Test points should cover:

• The normal operating region.

• Alert level.

• Action limit.

• The upper part of the working range where relevant.

The reference instrument should have suitable accuracy and metrological traceability. Where required by the contract or quality system, calibration may be performed by a laboratory operating under ISO/IEC 17025.

The calibration interval should reflect instrument criticality, manufacturer recommendations, operating conditions and historical drift. The same interval should not automatically be assigned to every pressure gauge.

Where an instrument is found out of tolerance, the facility should assess the possible effect on previously recorded data and the cleanroom condition during the affected period.

Verify the complete 4–20 mA loop

Where the official value is read from a BMS or EMS, calibrating only the sensing element is not sufficient.

Acceptance testing should verify the relationship between:

• Pressure applied by the reference instrument.

• Local transmitter display.

• Output current.

• PLC analogue input.

• HMI, BMS or EMS value.

• Alarm-relay status.

• Historical data record.

Error can arise in the sensor, signal converter, analogue input module, software scaling or configuration.

A complete loop check can identify discrepancies that are not shown on the transmitter’s individual calibration certificate.

Testing in the required occupancy state

ISO 14644-3:2019 covers cleanroom performance testing in as-built, at-rest and operational states. The applicable state should be specified in the project requirements and test plan.

Differential pressure acceptance testing should consider:

• Stable HVAC operation.

• Door opening and closing.

• Pressure recovery time.

• Personnel and equipment effects.

• Fan failure or reduced supply airflow.

• Sensor-signal loss.

• Power failure and restoration.

• Alarm operation.

• Agreement between local and central indications.

ISO 14644-4:2022 covers requirements, design, construction, start-up and verification. Differential pressure acceptance criteria should therefore be defined in the URS or design specification before installation rather than decided only after the equipment has been commissioned.

Required technical and qualification documents

The instrument documentation package should include:

• User Requirement Specification.

• Technical datasheet.

• Model, serial number and measuring range.

• Installation-location drawing.

• High- and low-pressure connection diagram.

• Electrical wiring diagram.

• Calibration certificate.

• On-site test results.

• Signal-loop test record.

• Alarm-challenge record.

• Operating and maintenance instructions.

• Recommended spare-parts list where applicable.

• Inspection, calibration and deviation procedures.

• Maintenance and configuration-change history.

ISO 14644-5:2025 requires an operations control programme that includes maintenance and monitoring. Instrument records should therefore remain controlled and updated throughout operation rather than ending with the original handover package.

Common mistakes when selecting a gauge

Common errors include:

• Selecting one excessively wide range for every room.

• Confusing room differential pressure with HEPA-filter pressure loss.

• Assuming ISO 14644 requires every room to maintain 10 Pa.

• Reviewing only full-scale accuracy and ignoring zero drift.

• Locating pressure taps directly beside supply-air diffusers.

• Reversing high- and low-pressure ports.

• Calibrating the transmitter without checking the signal loop.

• Failing to challenge alarms under operating conditions.

• Failing to determine pressure recovery after door opening.

• Failing to assess historical data following an out-of-tolerance calibration.

Another important mistake is assuming that differential pressure monitoring replaces cleanroom classification.

ISO 14644-1 classifies air cleanliness according to airborne-particle concentration. Differential pressure is a supporting parameter used to help maintain the required environmental condition.

FAQ: ISO 14644 differential pressure gauges

Does ISO 14644 require every cleanroom to maintain 10 Pa?

No. ISO 14644 does not establish one universal 10 Pa value for every cleanroom. The design pressure should be based on the application, airflow direction, HVAC configuration, contamination risk and industry-specific requirements.

Does an ISO Class 7 or ISO Class 8 room require continuous pressure monitoring?

The ISO class alone does not determine the monitoring frequency. The monitoring plan should define the parameters and frequency according to their effect on cleanroom performance. ISO 14644-2 specifies minimum requirements for such a monitoring plan.

Can a mechanical gauge be used?

Yes, for a point requiring local observation or periodic manual recording. A point requiring alarms, historical records or central-system integration should normally use an electronic transmitter or digital gauge.

Can the same gauge measure room pressure and HEPA-filter pressure?

Only where its range, accuracy and overpressure capability are suitable. HEPA-filter pressure loss is normally higher than room-to-room pressure and usually requires a different measurement range.

Is a calibration certificate sufficient for acceptance?

No. Acceptance should also verify installation location, port connections, tubing integrity, local indication, the signal loop, alarms and data displayed by the central monitoring system.

ISO 14644 differential pressure solutions from VCR Cleanroom Equipment

VCR Cleanroom Equipment supplies and supports the selection of differential pressure gauges, transmitters and monitoring solutions for ISO Class 5, ISO Class 6, ISO Class 7 and ISO Class 8 cleanrooms, Clean Booths, Pass Boxes, Air Showers, Dispensing Booths, HEPA Boxes and HVAC systems.

VCR can assist with defining the measurement purpose, range, accuracy, 4–20 mA output, Modbus communication, alarm functions, calibration requirements and acceptance documentation for each cleanroom project.

Hotline: 090.123.9008
Email: [email protected]
Website: Vietnam Cleanroom