Measuring very low-pressure in controlled environments like data centers, isolation rooms, labs, and operating rooms, for example, is challenging because they must maintain a very specific and very controlled atmosphere. If these environments are compromised in any way, it puts the people and the equipment you are protecting at risk. That's why you must ensure your pressure measurement instrumentation meets the specified guidelines for accuracy and reliability.
As a product manager at Ashcroft, I have 26 years of industry experience, including the past eight years working exclusively on low-pressure sensing technology. In this article, you will see how we define 'very low' pressure, see examples of controlled environments where low-pressure measurements are required, and learn how pressure is measured in these applications. You will also gain a better understanding of the impact temperature has on pressure measurement and what to look for in a pressure measurement instrument to ensure your controlled environment stays secure.
When you are done reading, you will also be directed to additional articles and resources that may be of interest for other questions you may have.
What is the definition of 'very low' pressure?
While 'low pressure' can be defined as anything below 15 psi, 'very low' pressure is often compared to the gentle flap of a butterfly wing. For a more scientific definition, let's start with an example. Imagine the pressure exerted by a one-inch of water column. Now divide that by 100. The result is 0.01 inches of water. That is the difference in pressure between protective environment (PE) rooms, as recommended by the Centers for Disease Control (CDC), to prevent the release of pathogens and contaminants into the environment. A PE is a specialized, patient-care area, typically found in hospital isolation rooms or cleanrooms.
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.
What are the challenges of measuring very low-pressure in controlled environments?
As mentioned earlier, several industries have low-pressure applications that require precise pressure measurements and monitoring. Here are two examples of critical room applications and the challenges they encounter.
1. Isolation rooms
The first critical room example is an isolation room in a hospital. The biggest challenge for these environments is keeping the differential pressure (DP) as low as 0.01 inches of water to prevent contaminant infiltration into the surrounding hallways. With such a tight tolerance of a low DP required, the DP measuring instrument controlling the air handling equipment must be extremely accurate, sensitive and stable.
Looking closer at an isolation room, this type of critical environment requires negative differential pressure to prevent an airborne bacteria or virus from leaving the room and infecting staff, other patients or visitors. For this reason, a dead-ended type transmitter is recommended over alternate technologies so that pathogens cannot escape from the room.
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. Cleanroom.
Cleanrooms are another example of a critical environment. The main difference between this and isolation rooms is that cleanrooms require a positive differential pressure working environment that is at a higher level than surrounding rooms to prevent air or contaminants from entering the cleanroom from the hallway.
In other words, if you open a door from the cleanroom into the hallway, the air should flow out of the room into the hallway; no air or contaminants should enter the room from the hallway.
Figure 3. Cleanroom example.
There are also a few challenges that exist in both isolation rooms and cleanrooms, including head effects, space constraints, length of tubing and the type of sensor technology used.
How temperature affects the output of a pressure measurement instrument.
In cleanroom environments, pressure transducers can be mounted in plenum areas, which can contain fans, ducts and other heating equipment. This equipment can cause a temperature effect (increase) on the pressure tubing installed in that area which can have a direct effect on the output of the transducer.
In the illustration below, you will notice that pressure transducers are located in the upper-right zone (a temperature-controlled area) while the fan motor is positioned in the upper-left zone. As the fan motor begins operation, the temperature will begin to rise, and the air within the pressure tubing heats up.
Figure 4. Temperature effects on pressure transducers.
In this example, the transducer is installed in a temperature-controlled room, which is 68 °F. While the pressure tubing located 5 meters away is seeing an air temperature of 95 °F (due to the fan motor), the resulting temperature differential is what we call the “head” effect. This happens when the density of air changes with temperature.
To minimize the temperature effects on the pressure instrument, you can run the pressure tubes together to ensure they are kept at the same temperature. In addition, you can insulate the tubes or keep them far away from heat sources. For more information on pressure tubing, read Can Pressure Tubing Length Affect My Low-Pressure Transducer?
The challenges of measuring air flow.
Building systems, critical room control and critical variable air volume (VAV) applications each have requirements for air-flow measurement. Some of the challenges related to these measurements include high static pressures and the flow versus pressure calculation.
Air flow in a duct is commonly measured using a pitot tube. Measuring air flow requires that the pressure instrument used with a pitot tube, be extremely sensitive, repeatable and stable.
A pitot tube is a tube inside of a tube that measures the differential between the dynamic and the static air in a duct. The inner tube monitors the dynamic air flow while the outer tube monitors the static air in the duct. This process uses the Bernoulli equation shown below to calculate the air flow.
Figure 5. Example of how air flow is measured using the Bernoulli equation. 
The DP (differential pressure) across the pitot tube is proportional to the velocity of the air in the duct squared. Meaning when the velocity increases by a factor of 2X, the pressure increases by a factor of 4X. The measurement challenge is the wide span of flow rate, high static pressures and the accuracy and repeatability needed to maintain the system.
What to look for in a pressure sensor for very low-pressure environments.
As we discussed throughout this article, the pressure instrument you select for controlled environments must be extremely accurate, reliable and sensitive. Two examples of sensors that meet these criteria are the Ashcroft® CXLdp pressure transducer and the Ashcroft® GXLdp pressure transducer, both with Si-Glas™ technology. This silicon MEMS sensor combines the high sensitivity of a variable capacitance transducer with the repeatability of a micro-machined, single-crystal silicon diaphragm.
The Si-Glas™ sensor is composed of sputtered metals and glass molecularly bonded to silicon. There are no epoxies or other organics in the sensor to contribute to drift or mechanical degradation over time. Using a silicon diaphragm-type sensor ensures inherent stability, high sensitivity, superior long-term repeatability and good accuracy.
Figure 6. Si-Glas™ Variable Capacitance Sensor.
Ready to learn more?
Now that you know how we define 'very low' pressure, where low-pressure measurements are required, and what to look for in a pressure transducer, you may still have questions. As a next step, you can read these other articles or talk to one of our industry experts for answers.
- Choosing a Pressure Transducer for Critical Environments
- How Does Media Temperature Affect Pressure Transducer Performance?
- How Much Do Pressure Transducers Cost? (6 Factors Impacting Price)
In the meantime, download our eBook to read about pressure instruments for critical environments.