Differential Pressure Gauge for an ISO Class 7 Cleanroom

ISO Class 7 cleanrooms are used for controlled processes requiring defined airborne-particle conditions. The room may serve as a main production area, a background for a higher-class clean zone, a material-preparation area, an assembly room, a testing space or an intermediate environment between the cleanroom and external areas.

Maintaining the ISO Class 7 condition requires the HVAC system to control filtration, supply and return airflow, airflow direction, room leakage and pressure relationships with adjacent spaces.

A differential pressure gauge helps operators confirm segregation, identify HVAC deterioration and obtain information for maintenance, investigations and cleanroom-performance assessment.

ISO 14644 does not prescribe one gauge model, one measurement range or one fixed pressure difference for every ISO Class 7 cleanroom. Instrument requirements should reflect the intended use, HVAC arrangement, background environment, airflow direction and monitoring plan.

What is an ISO Class 7 cleanroom?

ISO Class 7 is an airborne-cleanliness classification defined by ISO 14644-1. The standard classifies cleanrooms and clean zones according to airborne-particle concentration at specified particle sizes. Differential pressure, air-change rate and installed filter type do not directly determine the ISO classification.

A positive-pressure room equipped with HEPA filters cannot be confirmed as ISO Class 7 until airborne-particle classification has been performed using an appropriate particle-counting method.

Differential pressure is a supporting environmental parameter. It does not replace airborne-particle measurement.

An ISO Class 7 installation may be configured as:

• An independent production cleanroom.

• A background room for ISO Class 5 or ISO Class 6.

• A room containing a Clean Booth or Mini-environment.

• A material- or component-preparation area.

• A contamination-sensitive assembly room.

• A room containing localized unidirectional airflow equipment.

• An intermediate space between an airlock and a higher-class room.

• A room supplied through an AHU, HEPA Boxes or FFUs.

Each arrangement may require a different pressure relationship, number of monitoring points and monitoring method.

Why is differential pressure important?

Differential pressure establishes the intended direction of air movement between spaces.

In a conventional positive-pressure arrangement, the ISO Class 7 room is maintained above the pressure of a corridor or lower-cleanliness area. This helps limit the ingress of less-controlled air through construction leakage and door gaps.

When a door opens, the temporary airflow direction may also reduce the entry of particles, fibres, microorganisms or other contaminants.

Where ISO Class 7 provides the background for a Clean Booth, LAF unit, Mini-environment or ISO Class 5 zone, stable background pressure may support the performance of the localized clean-air system.

Some applications require negative pressure to contain powders, chemicals, biological agents or hazardous materials. In these installations, the pressure gauge confirms the inward airflow direction required for containment.

Positive pressure is therefore not universally correct. The pressure arrangement should satisfy the product-protection, personnel-protection and environmental-containment objectives of the process.

Does ISO 14644 require 10 Pa for ISO Class 7?

ISO 14644-1 classifies cleanrooms according to airborne-particle concentration, while ISO 14644-2 establishes minimum requirements for a monitoring plan based on parameters that measure or affect airborne-particle concentration. The ISO classification itself does not establish a universal 10 Pa requirement for ISO Class 7 rooms.

The required pressure should be based on:

• Cleanliness and function of adjacent spaces.

• Intended airflow direction.

• Leakage through panels, ceilings and doors.

• Door gaps and other openings.

• Supply, return and exhaust airflow.

• Door-opening frequency.

• Personnel and equipment loads.

• Product-protection requirements.

• Containment requirements.

• Cross-contamination risk.

• HVAC balancing and performance results.

A facility may use 5 Pa, 10 Pa, 15 Pa, 20 Pa or another justified value where design and testing confirm that the intended airflow direction is maintained.

Two adjacent ISO Class 7 rooms may also use a pressure difference to separate processes, control material movement or reduce cross-contamination.

The target value should be defined in the URS, HVAC drawings, pressure-cascade diagram and acceptance criteria.

Does ISO Class 7 require unidirectional airflow?

The ISO Class 7 designation does not automatically require unidirectional airflow.

ISO 14644-3 provides test methods for both unidirectional and non-unidirectional airflow cleanrooms in as-built, at-rest and operational occupancy states.

Many conventional ISO Class 7 rooms use mixed or non-unidirectional airflow with ceiling supply and appropriately positioned return-air grilles. A local ISO Class 5 zone may nevertheless be created within the room using an LAF unit, FFU system, Clean Booth or Mini-environment.

Differential pressure measurement does not replace other performance tests such as:

• Airborne-particle classification.

• Airflow-volume or velocity measurement.

• Airflow-uniformity testing.

• Airflow visualization.

• HEPA-filter integrity testing.

• Recovery testing.

• Room-leakage assessment.

A pressure gauge confirms the pressure relationship at the measuring point. It does not demonstrate the complete airflow behaviour within the cleanroom.

Identify the correct measurement duty

Typical ISO Class 7 pressure-measurement points include:

• ISO Class 7 versus ISO Class 8.

• ISO Class 7 versus a corridor.

• Cleanroom versus an airlock.

• ISO Class 7 versus an unclassified area.

• Clean Booth versus an ISO Class 7 background.

• Mini-environment versus the production room.

• Negative-pressure room versus an adjacent area.

• Across a HEPA or ULPA filter.

• Across AHU pre-filters or intermediate filters.

• On a Pass Box, Air Shower, Dispensing Booth or other cleanroom device.

Room differential pressure and filter differential pressure have different purposes.

Room pressure confirms the direction of airflow between spaces. Filter differential pressure indicates changing resistance and filter operating condition. These duties normally require different ranges, alarm settings and evaluation criteria.

Select the correct measuring range

The measurement range should match the design operating pressure.

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

For pressure between an ISO Class 7 room and an adjacent space, 0–25 Pa or 0–50 Pa may be considered where these ranges match the design. They are engineering examples rather than mandatory ISO 14644 values.

A bidirectional range such as ±25 Pa or ±50 Pa may be appropriate where:

• Pressure reversal is possible.

• Both positive and negative pressure must be monitored.

• The room has multiple operating modes.

• Containment is required.

• Reference pressure may vary.

The final range should consider the target value, alert level, action limit, door-opening transients and maximum credible pressure.

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

Accuracy and zero stability

ISO 14644 does not specify one accuracy class for all ISO Class 7 differential pressure gauges.

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

Where a room operates at 15 Pa and has a 10 Pa action limit, the combined error of the sensor, signal transmission, input module and display system should be small enough to distinguish reliably between the two conditions.

Instrument assessment should include:

• Accuracy within the working region.

• Repeatability.

• Zero drift.

• Long-term stability.

• Temperature and humidity effects.

• Overpressure resistance.

• Display resolution.

• Calibration uncertainty.

• Zero-adjustment method.

Zero stability is especially important for low-range sensors. A shift of only a few pascals can represent a significant percentage of the room-pressure value.

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 is simple, requires no electrical supply and provides an immediate visual indication.

A basic mechanical gauge normally cannot:

• Record data automatically.

• Generate remote alarms.

• Store event time stamps.

• Display pressure trends.

• Communicate with a BMS or EMS.

• Detect signal failure.

An electronic transmitter or digital gauge is generally more suitable where the pressure relationship directly affects ISO Class 7 control, supports a higher-class clean zone or requires automatic alarms.

ISO 14644-2 requires the monitoring plan to consider parameters that measure or affect airborne-particle concentration. Monitoring frequency should therefore be established through the monitoring plan and risk assessment rather than by the ISO Class 7 designation alone.

Output signals and central-system integration

Electronic instruments may provide:

• 4–20 mA.

• 0–10 V.

• Modbus RTU.

• Modbus TCP.

• BACnet.

• Alarm relays.

• Ethernet communication where available.

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

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

The system should also identify:

• Sensor power loss.

• Open signal wiring.

• Out-of-range signals.

• Frozen readings.

• Communication failure.

• Sensor or zero-adjustment faults.

An instrument failure should not produce a false normal indication.

Establishing ISO Class 7 pressure alarms

Alarm limits should not be copied from another room or automatically set at 10 Pa.

A practical alarm strategy normally includes:

• Target operating value.

• Normal operating range.

• Alert level.

• Action limit.

• Alarm delay.

• Reset hysteresis.

• Defined operator response.

Where a room normally operates at 15 Pa and has a 10 Pa action limit, the alert level may need to be above 10 Pa so deterioration is identified before control is lost.

An alarm delay can reduce nuisance events caused by brief door openings. An excessive delay may conceal a genuine loss of segregation.

Alarm settings should be established after HVAC balancing and review of operational data. The alarm should be challenged by creating an unacceptable pressure condition or simulating the corresponding input signal rather than merely changing a software setpoint.

Suitability for the ISO Class 7 environment

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

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

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

For electronics and semiconductor applications, additional considerations may include:

• Particle-shedding materials.

• Chemical emissions.

• Electrostatic-discharge requirements.

• Airborne molecular contamination.

• Corrosion resistance.

• Process-chemical compatibility.

• Ionic or metallic contamination risk.

ISO 14644-4 addresses the cleanroom process from requirements and design through construction and start-up. Instrument location, functionality and acceptance criteria should therefore be defined during the URS or design stage.

Pressure-tap location and tubing

The pressure tap should measure representative room static pressure.

It should not be installed:

• Directly in a supply-air jet.

• In a high-velocity airflow region.

• Too close to a return-air grille.

• Directly at a door gap.

• Near a fan or pressure disturbance.

• Where it may be obstructed.

• Where liquid may accumulate.

The high- and low-pressure ports must be connected correctly. Reversed connections will produce an incorrect sign or airflow direction.

Tubing should be leak-tight and protected against kinking, crushing and liquid accumulation. Both lines should be clearly identified to prevent incorrect reconnection during installation or maintenance.

Where a reference port is located in a ceiling void or wall cavity, the designer should verify that this space represents the intended reference pressure.

Calibration requirements

The instrument should be calibrated before use and at approved intervals.

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

• The normal operating region.

• Alert level.

• Action limit.

• The upper part of the working span where relevant.

The reference instrument should have suitable accuracy and metrological traceability.

The calibration certificate should state individual test results, errors, acceptance criteria and calibration uncertainty where applicable.

Calibration frequency should reflect measurement criticality, manufacturer recommendations, environmental conditions and historical drift.

Where an instrument is found out of tolerance, the facility should assess the effect on previous pressure data and on the ISO Class 7 condition during the affected period.

Complete 4–20 mA loop verification

Where the official reading is taken from a BMS or EMS, the transmitter calibration certificate alone does not verify the complete measurement system.

Loop testing should confirm:

• Reference pressure.

• Local transmitter display.

• Output current.

• PLC analogue-input value.

• HMI, BMS or EMS indication.

• Relay status.

• Alarm operation.

• Historical data record.

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

Acceptance testing in cleanroom occupancy states

ISO 14644-3 provides test methods supporting the assessment of unidirectional and non-unidirectional cleanrooms in as-built, at-rest and operational states.

Differential pressure acceptance testing should consider:

• Stable HVAC operation.

• Door opening and closing.

• Pressure recovery time.

• Personnel and equipment effects.

• Airlock operation.

• Fan failure or reduced airflow.

• Sensor-signal loss.

• Power failure and restoration.

• Alarm operation.

• Agreement between local and central values.

The required occupancy state and acceptance criteria should be specified in the URS, design documents or acceptance plan before testing.

Required technical and acceptance documents

The ISO Class 7 differential pressure 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.

• Inspection and calibration procedure.

• Maintenance history.

• Setpoint-change records.

• Impact assessment following an out-of-tolerance result.

ISO 14644-5:2025 requires an operations control programme covering personnel, material entry and exit, cleaning, maintenance and monitoring. The instrument should therefore remain controlled throughout the cleanroom lifecycle rather than only during initial handover.

Common mistakes

Common errors include:

• Assuming ISO Class 7 always requires 10 Pa.

• Selecting an excessively wide range.

• Using one range for room and HEPA-filter pressure.

• Reviewing only percentage-of-full-scale accuracy.

• Ignoring zero drift.

• Positioning the pressure tap beside a supply diffuser.

• Reversing the high- and low-pressure ports.

• Failing to check tubing leakage.

• Calibrating the transmitter without checking the loop.

• Failing to challenge alarms under operating conditions.

• Failing to determine recovery after door opening.

• Assuming pressure monitoring replaces particle classification.

FAQ: ISO Class 7 differential pressure gauges

Must an ISO Class 7 room maintain 10 Pa?

No. ISO 14644 does not define one universal pressure difference for every ISO Class 7 room. The value should be based on airflow direction, background cleanliness, HVAC configuration, manufacturing application and risk assessment.

Does ISO Class 7 require continuous pressure monitoring?

The ISO class alone does not determine monitoring frequency. The monitoring plan should define the frequency according to the effect of differential pressure on cleanroom performance. Critical points or background rooms supporting a higher-class clean zone are often suitable for continuous monitoring.

Can a mechanical gauge be used?

Yes, for local observation or periodic manual recording. A point directly affecting the ISO Class 7 condition should normally use an electronic transmitter with alarms and signal output.

Does a differential pressure gauge prove ISO Class 7 compliance?

No. ISO Class 7 is determined by airborne-particle concentration testing under ISO 14644-1. Differential pressure is a supporting parameter used to maintain the controlled condition.

Can a room-pressure gauge measure HEPA-filter pressure?

Only where its range, accuracy and overpressure capability are suitable. Filter pressure loss is generally higher than room-to-room differential pressure and normally requires a separate range.

ISO Class 7 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 7, ISO Class 6 and ISO Class 5 cleanrooms, Clean Booths, Mini-environments, FFUs, LAF units, 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 project.

Hotline: 090.123.9008
Email: [email protected]
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