In a cleanroom, monitoring the particle concentration alone is not sufficient. Temperature, relative humidity and the pressure difference to adjacent rooms also influence process stability, product quality and contamination control.
The three measured variables fulfil different functions. Temperature and humidity determine the room climate and influence electrostatic charging, material behaviour, the risk of condensation and working conditions, among other factors. Differential-pressure measurement, on the other hand, monitors whether the intended airflow between the cleanroom, airlock and adjacent room is maintained.
A single humidity sensor therefore does not constitute a complete cleanroom-monitoring system. Reliable monitoring requires suitable temperature/humidity sensors, differential-pressure transmitters, alarm functions and traceable measurement-data storage to be planned together.
This article explains how the measured variables interact, where sensors should be positioned and what must be considered when defining limits, calibration and documentation.
Table of contents
- Which measured variables are important in a cleanroom
- Distinguishing cleanroom classification from climate limits
- Correctly assessing relative humidity
- Why temperature and humidity should be measured together
- Dew point and risk of condensation
- Pressure cascade between cleanrooms and airlocks
- The correct sensor position
- Defining limits and alarms
- Data loggers and continuous monitoring
- Calibration and functional testing
- Typical installation and measurement errors
- Practical example: Humidity fluctuation in a material airlock
- Which information is required for selection
- Which measuring instruments / products are suitable?
- Conclusion
- Frequently asked questions about cleanroom climate monitoring
Which measured variables are important in a cleanroom
A cleanroom is primarily classified according to its particle concentration. However, the climate parameters must be controlled in a way that continuously supports the intended process and the required level of cleanliness.
| Measured variable | Typical function | Possible consequence of deviation |
|---|---|---|
| Temperature | Stable process and working conditions | Material expansion, altered process conditions or comfort problems |
| Relative humidity | Protection against drying, condensation and electrostatic charging | ESD risk, corrosion, product changes or condensation |
| Differential pressure | Maintaining the intended airflow direction | Ingress or egress of particles and contaminants |
| Dew point | Assessment of the condensation risk | Moisture formation on cold surfaces or components |
The measured values should not be considered in isolation. If relative humidity increases, for example, this may be caused by an additional moisture load. However, it may also result solely from a decrease in room temperature.
Distinguishing cleanroom classification from climate limits
A common misconception is that a particular cleanroom class automatically specifies a fixed temperature or humidity range. However, air-cleanliness classification primarily describes the permissible concentration of airborne particles.
The permissible climate and pressure values are derived from the specific application. The following factors must be considered, among others:
- sensitivity of the product or process
- requirements concerning personnel and protective clothing
- risk of electrostatic discharge
- microbiological and chemical risks
- heat and moisture loads caused by equipment and personnel
- cleaning and disinfection procedures
- requirements resulting from qualification and risk assessment
A pharmaceutical installation may require different limits from a cleanroom used for electronics manufacturing or optical components. General guideline values must therefore not be adopted without an application-specific assessment.
Correctly assessing relative humidity
Relative humidity describes the degree to which the air is saturated in relation to its temperature-dependent maximum water-vapour capacity. It is stated as a percentage of relative humidity.
A value of 50% RH does not mean that the air consists of 50% water. It means that, at the current temperature, the air contains approximately half of the maximum possible amount of water vapour.
Humidity that is too low can promote electrostatic charging in electronics and semiconductor manufacturing. This increases the risk of ESD damage to sensitive components.
Humidity that is too high, on the other hand, can promote corrosion, microbial growth, changes in hygroscopic materials or condensation on cold surfaces.
The permissible humidity depends on the process. A sensor must therefore not only cover the expected range, but must also provide the required accuracy and long-term stability.
Why temperature and humidity should be measured together
Relative humidity is directly dependent on temperature. If air cools while its water content remains unchanged, its relative humidity increases. If it warms up, the relative value decreases.
A humidity value without a corresponding temperature value can therefore only be interpreted to a limited extent.
For example, if a cleanroom is cooled from 22°C to 19°C overnight, the relative humidity can increase even though no additional moisture has entered the room. A humidity alarm on its own could then incorrectly be interpreted as a dehumidification problem.
Combined temperature/humidity sensors are advantageous for such applications because both values are measured at the same location and at the same time.
Where high accuracy is required, it must nevertheless be checked whether heat from the transmitter housing or adjacent devices influences the temperature sensor. A heated probe frequently measures relative humidity that is too low.
Dew point and risk of condensation
The dew point is the temperature to which air would have to be cooled for it to become saturated with water vapour and for condensation to begin.
It is particularly relevant when cold surfaces are present in the cleanroom, for example:
- cooling lines
- evaporators or cooling coils
- metal enclosures with a cooled interior
- materials from a cold storage area
- external walls or poorly insulated components
If the surface temperature falls below the dew point of the room air, condensation may form even though the relative humidity in the centre of the room remains within the intended limits.
For critical applications, it is therefore useful to monitor the temperature of particularly cold surfaces in addition to the room climate, or to define a sufficient margin between the dew point and the lowest surface temperature.
Pressure cascade between cleanrooms and airlocks
Differential-pressure measurement monitors the pressure difference between two rooms. A cleanroom with a higher classification is frequently operated at a slight positive pressure relative to a less clean adjacent room.
When small leaks or doors are opened, air therefore preferably flows from the cleaner area towards the less clean area. The pressure difference thus supports the intended contamination-control concept.
When hazardous, toxic or biologically contaminated substances are present, however, a negative-pressure concept may be required so that no contaminated air escapes to the outside.
The decisive factor is therefore not the highest possible positive pressure, but the correct airflow direction in accordance with the plant and protection concept.
A differential-pressure transmitter has a positive and a negative connection. One is connected to the monitored cleanroom and the other to the reference room. If the connections are reversed, the device displays an inverted sign or an incorrect condition.
With very small pressure ranges, the zero point, mounting position, hose routing and pressure-tapping points are particularly important. Even small deviations can represent a significant proportion of the measuring range.
The correct sensor position
A sensor should represent the actual room condition. An easily accessible location is not automatically a suitable measuring point.
Temperature and humidity sensors should not be installed directly:
- in the direct supply-air stream
- above heaters or heat-generating machinery
- next to doors and airlock openings
- on cold external walls
- in the immediate vicinity of humidifiers
- behind panels without sufficient air exchange
In larger or thermally non-uniform rooms, a single sensor may not be sufficient. During qualification, the temperature and humidity distribution should be investigated so that representative and unfavourable positions can be identified.
The pressure-tapping points must be positioned so that no direct airflow enters the connections. Draughts, doors or supply-air outlets can otherwise generate dynamic pressure components and distort the differential-pressure measurement.
Hoses must not be kinked, blocked or leaking. With a wall penetration, it must be clearly documented which connection is assigned to which room.
Defining limits and alarms
A target range, warning threshold and action limit should be defined for each measured variable.
A warning threshold provides an early indication of an emerging deviation. The action limit identifies a condition that triggers a defined response or investigation.
The following factors must be considered when defining the limits:
- measurement uncertainty of the sensor
- normal fluctuations during operation
- door openings and airlock cycles
- start-up time of the ventilation system
- permissible process limits
- consequences of a brief exceedance
An alarm should not be triggered by every brief door opening. On the other hand, an alarm delay that is too long can conceal an actual ventilation-system failure.
Delay times and acknowledgement rules must therefore be justified and documented. Critical pressure differences should be monitored and recorded continuously.
Data loggers and continuous monitoring
For quality-relevant cleanroom monitoring, an occasional local indication is frequently insufficient. The measured values must be stored over time and evaluated in a traceable manner.
Depending on the application, a suitable monitoring system should provide the following functions:
- combined recording of temperature, humidity and differential pressure
- synchronised timestamps
- configurable warning and action limits
- alarms for sensor or communication faults
- trend display and data export
- user and change management for regulated applications
- secure storage and data backup
Analogue signals such as 4–20 mA or 0–10 V can be integrated easily into PLCs and building-management systems. Digital interfaces such as Modbus, on the other hand, allow several measured variables and additional status information to be transmitted over a shared connection.
For 4–20 mA measuring chains, the UPS4E loop calibrator can be used during commissioning. It allows the sensor current, PLC scaling and alarm limits to be tested independently of one another.
Calibration and functional testing
Humidity sensors can drift due to dust, cleaning agents, condensation or chemical exposure. Differential-pressure sensors can also develop a zero-point error due to mounting-position effects, ageing or contaminated pressure lines.
The calibration interval should be defined on a risk-based basis. Influencing factors include:
- process criticality
- required accuracy
- results of previous calibrations
- environmental and cleaning exposure
- possibility of an intermediate check
- manufacturer recommendations
During humidity calibration, several points within the actual working range should be checked. A check carried out only under normal room conditions may fail to reveal a scaling error at the limits of the permissible range.
Differential-pressure transmitters should be checked for zero after installation and after a sufficient warm-up period. Both pressure connections must be exposed to the same pressure for this purpose.
In addition to calibrating the sensor, the complete measuring chain must be tested. A correctly calibrated sensor provides little benefit if the PLC input is scaled incorrectly or the alarm limit has been configured incorrectly.
Typical installation and measurement errors
| Error | Possible effect | Better approach |
|---|---|---|
| Humidity sensor installed directly in the supply-air stream | The measured value represents the supply air rather than the room | Derive a representative room position from the qualification |
| Sensor installed next to a heat source | Temperature too high and relative humidity too low | Maintain sufficient distance from lamps, displays and machinery |
| Only relative humidity monitored | Temperature-related changes are interpreted incorrectly | Record temperature and humidity together |
| Absolute pressure confused with differential pressure | The pressure cascade between rooms is not monitored | Use a separate differential-pressure transmitter |
| Positive and negative connections reversed | Incorrect sign or reversed alarm evaluation | Label the pressure lines clearly |
| Leaking or kinked pressure hoses | Values that are too low, delayed or unstable | Inspect hoses regularly and route them in a protected manner |
| Limits defined without considering measurement uncertainty | False alarms or undetected deviations | Take sensor error and process fluctuations into account |
| Calibration performed only on the sensor | Errors in the PLC, scaling or alarm system remain undetected | Test the complete measuring chain |
Practical example: Humidity fluctuation in a material airlock
Repeated humidity alarms occur in a material airlock connected to a production cleanroom. The cleanroom itself shows stable values, while the airlock briefly exceeds the warning threshold during certain shifts.
Insufficient dehumidification is initially suspected. However, a combined evaluation of temperature, humidity, differential pressure and door status reveals a different relationship.
During frequent material transfers, both doors are opened at short intervals. At the same time, the differential pressure temporarily drops significantly. Warmer, more humid air from the adjacent room enters the airlock.
In addition, the humidity sensor is installed directly next to the door and therefore responds particularly strongly to the incoming airflow. It does not represent the average condition within the airlock.
The sensor is moved to a representative position determined during qualification. The door control is adjusted so that sufficient time is allowed for the pressure and climate conditions to stabilise.
After the modification, the recorded data still shows brief changes, but no further impermissible exceedances. At the same time, the differential-pressure trend confirms that the intended pressure cascade is re-established reliably after each airlock cycle.
The example shows why humidity alarms should not be assessed without considering temperature, pressure and the operating condition.
Which information is required for selection
At least the following information is required for a suitable sensor and monitoring solution:
- industry and cleanroom application
- cleanroom class or GMP area
- intended temperature and humidity ranges
- required accuracy and measurement uncertainty
- pressure cascade and required differential-pressure range
- room size and number of measuring points
- wall, duct or remote-probe mounting
- cleaning and disinfection agents
- required output signals or bus interfaces
- alarm and documentation requirements
- calibration and validation requirements
For existing installations, the room layout, ventilation diagram, pressure cascade and previous trend records are particularly helpful.
Which measuring instruments / products are suitable?
The humidity sensors and dew-point sensors category includes room, duct and process sensors for relative humidity, temperature and, depending on the version, additional calculated humidity variables.
Humidity and temperature sensors for HVAC applications
The humidity and temperature sensors for HVAC applications are suitable for combined measurement of both climate variables.
Depending on the version, various analogue signals are available, including 0–10 V and 4–20 mA. For a quality-critical cleanroom, it must be checked whether the accuracy, calibration capability and documentation meet the application-specific requirements.
IPTF500 for monitoring environmental conditions
The IPTF500 combines temperature, relative humidity and absolute pressure. Interfaces such as Modbus RTU, Ethernet or M-Bus allow integration into higher-level monitoring systems.
However, the measured absolute pressure represents the barometric ambient pressure. A suitable differential-pressure transmitter is additionally required to monitor the pressure difference between two cleanrooms.
IDPS 300 differential-pressure transmitter
The IDPS 300 multi-range differential-pressure transmitter is intended for dry, non-aggressive gases and compressed air and can be used to monitor small pressure differences.
During selection, the measuring range, zero point, overload capacity, output signal and mounting position must match the pressure cascade.
Data loggers and portable reference measurements
The data loggers category contains systems for time-based recording and the documentation of climate parameters.
A portable climate measuring instrument can additionally be useful for qualification, troubleshooting and comparative measurements. It does not replace stationary monitoring, but enables an independent check of sensor positions and room distributions.
ICS Schneider Messtechnik assists with the selection of humidity/temperature sensors, differential-pressure transmitters, data loggers and interfaces. The sensor technology, calibration strategy, alarm system and data acquisition should be considered as one complete system.
Conclusion: Cleanroom climate can only be assessed reliably by combining several measured variables
A humidity sensor is an important component of cleanroom monitoring, but is not sufficient on its own. Relative humidity must always be considered together with temperature.
The dew point supplements the assessment when cold surfaces or temperature-sensitive products are present. Differential-pressure measurement additionally monitors whether the intended airflow direction between the cleanroom, airlock and adjacent room is maintained.
Generally applicable temperature and humidity limits cannot be derived from the cleanroom class alone. The values must be appropriate for the product, process, personnel and regulatory environment.
The significance of the measurement depends strongly on the sensor position. Supply air, doors, heat sources and cold external surfaces can create local conditions that do not represent the room as a whole.
For quality-relevant applications, measured values should be stored continuously, alarm limits should be defined in a traceable manner and sensors should be calibrated regularly.
Only the combined assessment of temperature, humidity, differential pressure and operating condition enables reliable root-cause analysis and documentable cleanroom monitoring.
Frequently asked questions about cleanroom climate monitoring
Which humidity is permissible in a cleanroom?
There is no universally applicable value for all cleanrooms. The permissible range is derived from the product, process, ESD risk, personnel and regulatory requirements.
Does the ISO cleanroom class specify temperature and humidity?
No. The classification primarily describes the particle concentration. Climate and pressure limits must be defined for the specific application.
Why must temperature and humidity be measured together?
Relative humidity changes with temperature. Without a temperature value, a change in humidity can easily be interpreted incorrectly.
Where should the humidity sensor be installed?
At a representative position away from direct supply air, heat sources, doors and local moisture sources. The exact position should be derived from the room qualification.
What is the difference between absolute pressure and differential pressure?
Absolute pressure is measured relative to a vacuum. Differential pressure describes the pressure difference between two rooms or measuring points and is decisive for the cleanroom pressure cascade.
Why is differential pressure monitored in a cleanroom?
It indicates whether the intended airflow direction between areas with different cleanliness or protection functions is maintained.
Is a higher cleanroom pressure always better?
No. The pressure must match the protection concept. When hazardous substances are present, a controlled negative pressure may be required instead of positive pressure.
When is the dew point relevant?
When cold surfaces, cooling lines or temperature-controlled products are present. If their temperature falls below the dew point, condensation may form.
How often must cleanroom sensors be calibrated?
The interval is defined on a risk-based basis according to the process criticality, required accuracy, environmental exposure and previous calibration results.
Is a local display sufficient?
Usually not for quality-relevant monitoring. Trends, alarms and deviations should be stored with timestamps and evaluated in a traceable manner.
What should be checked first when a humidity alarm occurs?
Temperature, sensor position, ventilation status, door openings and differential pressure should be considered together before a humidification or dehumidification fault is assumed.
Which information does ICS Schneider require for selection?
The application, measuring ranges, accuracy, pressure cascade, number and position of the measuring points, output signals, alarm requirements and calibration and documentation requirements are required.