Performing Under Pressure
How Physical Properties Affect Units of DP

David W. Spitzer, P.E.

Pressure can be defined as the force acting on a surface divided by the area of the surface. This seems rather straightforward, but common practices can make expressions of pressure confusing and can result in measurement errors.
Many pressure units are commonly used in industry, such as pounds per square inch (PSI) and kilograms per square centimeter (kg/cm2). Note that the first unit (PSI) is a force divided by an area, while the second unit (kg/cm2) is a mass divided by an area. In a strict sense, the second unit (kg/cm2) is not a unit of pressure — but we use it anyway.
Now consider the units of differential pressure that are commonly used to calibrate differential-pressure transmitters — inches (or millimeters) of water column. The use of these units dates back to the days when U-tubes were used to calibrate differential-pressure transmitters. Certain heights of water were used to simulate the desired calibration pressures on the differential-pressure transmitter. Consequently, it was convenient to express the calibration as the height of water in the U-tube. This practice is still prevalent today even though few suppliers and users calibrate differential-pressure transmitters with a U-tube.
Neither inches nor millimeters of water column is a force divided by an area — it is a height of a particular liquid. Interestingly, changing the temperature of the water while maintaining the same height will change the density of the water and the force exerted on the transmitter. Hence, the pressure that the transmitter measures will vary with the water temperature — even if the height of the water column remains the same. Therefore, the temperature of the water must be defined in order to infer a particular pressure.
The reference temperature of the water must be defined to ensure the proper pressure is applied to the transmitter during calibration. Common reference temperatures include 4 C, 15 C, 20 C, 60 F, and 68 F. The most common reference temperatures for calibration are 4 C and 20 C. Water at 4 C is 1.0018 times denser than water at 20 C, so the calibration error introduced by calibrating using the incorrect reference temperature is approximately 0.2 percent that corresponds to a flow calibration error of approximately 0.1 percent. This may not seem like much, but it biases annual invoicing of $10 million by approximately $10,000 per year.
The solution to this problem is relatively straightforward and low-cost — calibrations of differential-pressure transmitters should be calculated and expressed in pressure units that are not dependent upon physical properties, such as kPa or mbar.

David W. Spitzer, P.E., is a regular contributor to Flow Control. He has more than 30 years of experience in specifying, building, installing, startup, and troubleshooting process control instrumentation. He has developed and taught seminars for over 20 years and is a member of ISA and belongs to the ASME MFC and ISO TC30 committees. Mr. Spitzer has written a number of books concerning the application and use of fluid handling technology, including the popular “Consumer Guide” series, which compares flowmeters by supplier. Mr. Spitzer is currently a principal in Spitzer and Boyes LLC, offering engineering, product development, marketing, and distribution consulting for manufacturing and automation companies. He can be reached at 845 623-1830.


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