Pressure Measurement Considerations Department Editor: Scott Jenkins Chemical Engineering©

Pressure measurement in the chemical process industries (CPI) is crucial to many unit operations, and selecting the most effective pressure sensors for a given situation can be complicated by a range of factors. An initial key to selection is establishing an accurate understanding of exactly what is meant when the term “pressure” is used, since there are different types. Other critical considerations include the following: media compatibility, environment, process control, electrical isolation and output signal.

Pressure types

Pressure measurements can be affected by what type of pressure sensing equipment is used, and understanding the different types of pressure is a prerequisite for selecting sensors or gages for your application. Accuracy can suffer if pressure types are misunderstood.

Differences in pressure types have everything to do with the reference point for a given pressure measurement. Here are definitions for five common pressure types:

Gage pressure

Gage pressure, the type that most people first imagine when thinking of measuring pressure, covers a positive pressure range. Its zero (reference) point is set at ambient pressure, and it is unaffected by changes in barometric pressure because the sensor is open to the atmosphere. This allows the current atmospheric pressure to be the reference against which all subsequent changes in pressure are measured. Gage pressure effectively can measure pressures below 1 psi, as well as pressures up to 200,000 psi.

Vacuum pressure

Like gage pressure, vacuum pressure’s zero point is ambient pressure, and sensors measuring it are vented — and therefore unaffected by barometric change. Since vacuum pressure refers to a negative pressure range, the distinction between vacuum and gage pressure is really a function of direction and magnitude. Sensors measuring this type are commonly used in vacuum pump systems and applications where suction is required.

Compound gage pressure
This pressure type is the combination of gage and vacuum pressure in that it involves both positive and negative pressure changes. Its zero is therefore set at atmospheric pressure, and it is vented. The value of a compound gage is seen when used in applications where the pressure fluctuates from positive to negative and vice-versa. Sensors measuring this pressure type typically do not exceed 100 psi in range.

Sealed pressure
Sealed pressure refers to a situation where the pressure sensor is not vented. This is primarily done to protect the sensor, by avoiding the introduction of moisture or dust into the sensor housing. The sensor is sealed with a pressure equal to the atmospheric pressure at the time of sealing. This pressure then becomes the reference pressure against which all pressure changes are measured. Because it is sealed, unvented pressure sensors are unavoidably affected by barometric pressure changes. It is not typically used in low-pressure applications because the barometric shift of a few psi would affect measurement accuracy significantly. However, at 1,000 psi and above, the relatively small shift would go unnoticed and can be smaller than the error band of the sensor.

In one real-world case, a sealed pressure type sensor was calibrated at a manufacturing facility in Utah and then shipped to Indiana. The atmospheric pressure differences between the locations caused the unit to fail in Indiana, while it worked properly in Utah.

Absolute pressure
Absolute pressure is used when the zero point must be set to absolute zero. To achieve this, the sensor is also sealed, but under a vacuum condition, so that air molecules are removed from the enclosure.

This then becomes the reference point and allows measurements to be made with reference to absolute zero. By definition and design, this is sensitive to barometric changes. Unlike sealed pressure, absolute pressure is often used in low-pressure applications measuring atmospheric conditions, such as in weather stations, aircraft and laboratories.

Notes
Material on pressure types was contributed by Elden Tolman, product design engineer at Automation Products Group Inc. (APG; Logan, Utah; www.apgsensors.com).