Differential pressure measurement in cleanrooms
01 January 2023

Differential pressure measurement in cleanrooms

Rooms for critical processes i.e. cleanrooms as well as mini environments demand precise environmental monitoring technology and methods in order to operate consistently within specifications. This is not just about meeting standards, but more importantly about the quality of your products e.g. pharmaceutical medicines, semiconductor chips, or manufacturing processes. Robust and highly accurate differential pressure instruments from Novasina address pressure measurement challenges and help with the smooth operation of your cleanroom facilities.

There are multiple standards that can be mentioned in connection with cleanrooms. While VDI 2083-19 addresses the tightness of containments, other standards state clear differential pressure difference requirements, like the DIN EN ISO 14644-3/4 cleanroom classification, VDI 2083 (3) or GMP Annex 1 of 5-20 or 5-15 Pa.

The primary approach to ensure the protection of people, products, and manufacturing environment is to create pressure cascades between rooms. This is intended to ensure that air always moves from a cleanroom to a non-controlled room. Additional techniques that are used to maintain cleanrooms include: point- and surface air extraction, clean air hoods, laminar flow workspaces, air curtain, filter fan units, mini environments, Restricted Access Barrier Systems (RABS), safety work benches for biology or similar, glove boxes, clean machines, isolators and other containments.

Measurement requirements in cleanrooms

According to DIN EN ISO 14644-3, the measurement requirements of minimal differential pressure must be between 0 to 50 Pa. To be able to successfully make this measurement, instruments with low measurement uncertainty are required for cleanrooms. Novasina provides a specific product line with an automatic mechanical zero point which makes it possible to measure very small pressure differences without being affected by the usual long term drift due to aging or mounting positions.

Due to the need to ensure that there always is an overpressure to ensure proper air flow, most instruments sold into cleanrooms are bi-directional, showing a range from e.g. -25 to +25Pa. With this measurement range, it can be documented for critical productions that room pressure did not drop below 0 Pa differential pressure between contamination-classes. In such cases, the low uncertainty provided by the mechanical auto-zero is critical.

Measurement methods in cleanrooms

There are two main measurement methods being used to track differential pressure in cleanrooms. One is the so-called “static”-Method and the other is the “dynamic” differential pressure method which works on the thermal mass flow principle. This measurement technology consists out of a heating element which is placed between two temperature sensitive resistors. When air flow occurs, an elevated temperature profile occurs on one of the resistors based on the direction of air flow. The static measurement principal follows the strain gauge method. The principal behind this method is that pressure is converted to force as a diaphragm is stretched and this is measured by a piezo-resistive MEMS sensor. Both methods have their pros and cons and the technology has evolved over the years. Novasina, as a Swiss sensor manufacturer, has multiple years of experience with both methods.

Static method in cleanrooms

The current standard detection component for the static method is a high accurate piezo-resistive element. However, due to aging of membranes, not all static measurement transmitters can offer a stable long term reading. This is why pressure as well as mounting position will have an affect onto the diaphragm, resulting in an error of up to several Pascal over time. More importantly, the zero point stability can be affected resulting in drifts of 1 to 2 Pascal within one year. Novasina solved this issue with the use of highest quality sensing technology as well as electronics. To ensure zero point stability, an automatic mechanical zero point is taken every few hours for the bi-directional instruments. To accomplish this important zeroing, a solenoid valve that switches to the same pressure onto both sides of the diaphragm is needed and will offer the best accuracy and long term stability even after years of operation.

Dynamic method in cleanrooms

While static methods are the most commonly used, older product lines used the dynamic measurement principal. However, the required tube length compensation for the correct reading proved to be a challenge. That said, the ongoing trend of miniaturization might reduce possible tube length effects to negligible levels, making dynamic methods a possibility again. At Novasina, evolving technology within our core competencies is regularly evaluated. Pending an evaluation of the technical and commercial potential of dynamic methods, future dynamic differential pressure measurement product lines might be reintroduced.

Cleanroom related filter and flow monitoring

The differential pressure drop measured across a filter of an air conditioning system allows conclusions about the degree of contamination and helps to set maintenance intervals. For filter monitoring a less accurate differential pressure instrument can be used as it is not necessary to determine such small differences.

Differential pressure can also be used for flow monitoring. The flow velocity due to a pressure drop over a restricting orifice or membrane diffuser can be calculated. A first calibration might be required, but after that, it is a good indicator for the flow velocity in m/s.