Control Engineering

Link This | Email this | Blog This | Comments (0)

Can you recommend a portable instrument to measure room air pressure?
May 26, 2008

The full text of the question is: Could you recommend some handheld instruments that can easily measure and indicate room air pressure (positive or negative, and how much)?

The short answer is “No.” As a journalist I can’t recommend products. Also, I would need more information to make a selection for you. I can, however, suggest how to go about finding the instrument you need.

You are, of course, looking for a barometer. There are a number of technologies used to create barometers, ranging from the original mercury barometer invented by Evangelista Torricelli in the 17th Century to miniaturized units fabricated using microelectromechanical system (MEMS) techniques. All barometers are specialized adaptations of dual-port differential-pressure gauges. The adaptation invariably consists of closing off one port to create a reference-pressure chamber. The instrument then reports the difference between the pressure at the open port and that in the closed chamber.

Differential pressure displaces fluid in a hydrostatic gauge (manometer) and a diaphragm in an aneroid gauge.

There are really only two basic pressure-gauge types: hydrostatic and aneroid. Hydrostatic guages — generally referred to as manometers — consist of a U-shaped tube partially filled with fluid of known density. Any pressure difference between the two sides (arms) of the tube registers as a difference in the height of fluid in the arms. Aneroid (meaning “without fluid”) uses a flexible membrane (diaphragm) in place of the fluid. The degree to which the diaphragm flexes signals the pressure difference.

Converting a differential pressure gauge to a barometer is a simple matter of closing off one port, which becomes a reference-pressure chamber. In the manometer, the reference pressure is a vacuum.

QA26MAY08a2: 

Wall-hanging and desktop barometers are available in almost any hardware or department store. Such instruments are actually quite sensitive due to the small range of natural atmospheric pressure variations — usually 0.5 in/Hg above or below the average of 30 in/Hg (or ±1.7%) according to the Challenger Building, Tips, and Technique web site. (This website is associated with the vendor of Challenger homebuilt ultralight aircraft.) Their accuracy depends on how accurately you zero them in to begin with. Linearity and calibration slope are, however, not guaranteed.

If atmospheric pressure is an important input for controlling your process, you probably want something a bit more reliable. The best place to start looking is the Internet. Googling “barometer” turned up over 10 million hits. Narrowing the search to “handheld barometer” reduced that to 155,000. The top hits were distributors who cater to backpackers and boaters.

Two promising vendors, however, were more scientifically oriented: the Control Company, and Edmund Scientifics. The Control Company offering is said to have ±0.75% NIST-traceable accuracy and sells for $160.50. The Edmund Scientifics unit looks suspiciously similar, but does not claim to be traceable and sells for $129.95.

Remember that just because a unit looks the same as a traceable unit doesn’t mean that it’s traceable, too. To claim traceability, the unit must be part of a calibration program that involves a chain of calibration reaching back to the National Institute of Standards and Technology (NIST). When purchasing such a unit, you should receive a certificate evidencing this traceability chain.

Often distributors special order units marked with the distributor’s logo from a manufacturer. Special-order instruments from the same manufacturer for different distributors usually look the same except for the logo. They are likely to be the same physically, too. One, however, may order instruments run through a NIST-traceable calibration program, while the other may not. 

My Google search turned up a third vendor, Omega Engineering (my old standby for this type of sensor), which offers a unit for which it claims very high accuracy (±0.02%) and NIST traceability — as well as a nose-bleed-inducing $2,385 price tag. You pay for what you get with scientific instruments!

To choose among these and other offerings, you need to consider your application requirements. How much accuracy do you need? How portable does the unit really have to be and why? How do you plan to record the measurements?

For example, when playing around with the wind tunnels at Arizona State University, I used a mercury barometer affixed to a wall. I’d have to dig out my notes to quote its calibration specifications, but let’s just point out that it took into account variations in the local gravitational field. The unit sure wasn’t portable, but I didn’t really need it to be. I just wrote the reading down in a research notebook, and keyed it into the data acquisition program for each test run.

There are any number of reasons why you might really need portability. Perhaps there are several separate locations within your facility that need comparative barometric measurements. Maybe you plan to check barometric-pressure sensors that are part of your process control at several locations around your facility. Maybe you want to lock the instrument in a safe location when not in use.

All these factors need to be considered carefully when choosing an instrument.

Posted by Charlie Masi on May 26, 2008 | Comments (0)