How to measure very low-pressure in controlled environments

Measuring very low pressure in controlled environments requires highly accurate, stable differential pressure instrumentation capable of detecting changes as small as 0.01 inches of water column. In spaces like isolation rooms, cleanrooms, laboratories and data centers, even the slightest pressure variation can compromise safety, sterility or system performance.

Engineers responsible for these environments face a difficult challenge: maintaining extremely tight pressure tolerances while accounting for temperature changes, tubing layout, space constraints and airflow dynamics. When pressure control fails, contaminants can migrate, pathogens can spread or sensitive processes can be disrupted.

In this article, we’ll define what “very low” pressure means, examine common controlled-environment applications, explore measurement challenges and outline what to look for in a pressure sensor designed for these critical spaces.

What is 'very low' pressure? 

Very low pressure typically refers to differential pressures measured in fractions of an inch of water column—often as low as 0.01 in. H₂O.

While “low pressure” can generally describe anything below 15 psi, controlled environments operate in a much smaller and more sensitive range. To understand how small 0.01 inches of water column really is, consider this:

• It represents the Center for Disease Control and Prevention (CDC) recommended pressure difference for protective environment (PE) rooms used to prevent contaminant release.

• It is approximately half the atmospheric pressure difference between a 5-foot and 6-foot person standing side by side (about 0.018 in. H₂O).

• It is comparable to a very subtle disturbance in air.

These minimal pressure differences are critical in hospital isolation rooms and cleanrooms, where airflow direction determines whether contaminants are contained or allowed to migrate.

To demonstrate how low 0.01 inches of water measurement is, take a look at the picture of a 5 ft. man and a 6 ft. man in Figure 1 below. The difference in height between these two men breathing the same air is 1 ft, with an atmospheric pressure difference equal to 0.018 inches of water.  That small differential pressure (DP) is almost double what the CDC recommends for critical room control.

Figure 1. Differential pressure example.

Why is measuring very low pressure challenging?

Measuring very low pressure in controlled environments is difficult because the allowable differential pressure tolerance is extremely small, and even minor environmental influences can affect the output of the instrument.

Several industries rely on precise low-pressure measurement and monitoring. Two common examples, hospital isolation rooms and cleanrooms, illustrate why stability and sensitivity are essential.

1. Isolation rooms require negative differential pressure to contain contaminants

This is critical to prevent airborne bacteria or viruses from leaving the room and infecting staff, patients or visitors. In an isolation room at a hospital, for example:

• Room pressure must remain slightly lower than the hallway.

• Differential pressure setpoints can be as low as 0.01 in. H₂O.

• The instrument controlling the air handling system must be highly accurate, sensitive and stable.

A dead-ended type transmitter is often recommended in these applications to prevent pathogens from escaping through the measurement device itself.

In this isolation room application, we are measuring the difference in air pressure between each room and the reference point, in the corridor (or hallway). One challenge is that a small product footprint is often required to fit within the designed space.

As you can see in the image below, pressure transducers can be mounted within each room, but they can also be mounted remotely for easy monitoring from a separate location.

Figure 2. Isolation room example 

2. Cleanrooms require positive differential pressure to prevent contamination

Cleanrooms operate on the opposite principle: they maintain positive differential pressure so that contaminants cannot enter from surrounding areas.

When a cleanroom door opens:

• Air should flow outward into the hallway.

• No air or contaminants should enter the controlled space.

Although the pressure polarity differs from isolation rooms, the measurement challenges remain similar:

• Very low differential pressure ranges

• Tight control tolerances

• Long-term sensor stability requirements

Both applications also face additional challenges, including head effects, tubing layout constraints and proper sensor technology selection.

Figure 3. Cleanroom example

How do temperature differences impact pressure measurement accuracy?

Temperature changes can affect very low-pressure measurement by altering air density inside pressure tubing, creating what is known as a “head” effect.

In many building and cleanroom applications, pressure transducers may be mounted in plenum spaces that contain fans, ducts and heating equipment. These components can create localized temperature increases that affect the air inside the pressure tubing.

For example, in the illustration below you will notice:

• The pressure transducers are located in the upper-right zone (a temperature-controlled area).

• The fan motor is positioned in the upper-left zone near a fan motor.

•  As the fan motor begins operation, the temperature will begin to rise, and the air within the pressure tubing heats up. 

When measuring extremely low differential pressure, even small temperature differences can introduce measurable error.

To minimize temperature affects you can:

• Run pressure tubing (for high and low ports) together
• Keep tubing exposed to similar temperatures
• Insulate tubing where possible
• Avoid routing near heat sources

Managing these factors helps preserve measurement accuracy and system stability. For more information on pressure tubing, read Can Pressure Tubing Length Affect My Low-Pressure Transducer? 

Figure 4. Temperature effects on pressure transducers

What are the challenges of measuring airflow in ducts?

Airflow measurement in controlled environments relies on differential pressure instruments that are sensitive enough to detect small pressure changes across a pitot tube.

A pitot tube consists of:

• An inner tube that measures dynamic pressure

• An outer tube that measures static pressure

The difference between these two pressures is used in the Bernoulli equation to calculate air velocity.

A key consideration is that differential pressure is proportional to the square of velocity. If velocity doubles, the differential pressure increases by a factor of four. This relationship creates several measurement challenges:

• Wide operating flow ranges

• Relatively high static pressures compared to small DP values

• The need for repeatable, stable measurements

Because airflow control is directly tied to pressure measurement accuracy, sensor performance plays a critical role in maintaining environmental control.

Figure 5. Example of how air flow is measured using the Bernoulli equation.

Factors to consider when selecting a differential pressure sensor for controlled environments

A pressure sensor used in very low-pressure environments must provide high sensitivity, repeatability and long-term stability while minimizing drift.

Important performance characteristics include:

• High accuracy at low differential pressure ranges

• Strong repeatability

• Minimal temperature sensitivity

• Long-term output stability

• Robust sensor construction

Silicon MEMS-based variable capacitance sensors, such as those used in Ashcroft® CXLdp Pressure Transducer, DXLdp Pressure Transducer and GXLdp Pressure Transducer with Si-Glas™ technology, combine the high sensitivity of variable capacitance measurement with the repeatability of a micro-machined, single-crystal silicon diaphragm.

Because the sensing element is composed of silicon, sputtered metals and glass molecularly bonded together—without epoxies or organic materials—the design helps reduce mechanical degradation and long-term drift.

In controlled environments where differential pressures are measured in hundredths of an inch of water column, sensor stability is not optional—it is essential.

Figure 6.  Si-Glas™ Variable Capacitance Sensor 

Ready to learn more?

Now that you understand how very low pressure is defined, why controlled environments require such tight tolerances and how temperature and airflow influence measurement accuracy, you may be interested in the related resources listed below.  

If you have specific application questions, contact us and our pressure measurement specialists are available to help you evaluate the right sensing technology for your controlled environment.

In the meantime, download our guide to learn about pressure instruments designed for critical environments.