With new technology always under development, shopping for the right pressure gauge grows more confusing by the month. It’s easy to get lost in the details and end up with a gauge that doesn’t meet the requirements. In this buyer’s guide, Tom Halaczkiewicz, president of Crystal Engineering, explains the important features and specifications to look for and understand when purchasing pressure measurement devices.
Error Prevention: When working with high-pressure or flammable materials, it is important to select a gauge that helps avoid the occasional human error. Some errors threaten safety, while others threaten the quality of results. To ensure safety, the gauge should:
Issue a warning when full-scale pressure has been exceeded.
Provide a generous safety factor for accidental overpressure.
Limit the zero range (the amount of live pressure that can be cleared from the gauge).
Always display live pressure.
Stop displaying pressure if the sensor sustains damage.
While no tool can ensure complete safety, end-users should look for the above features in a pressure gauge to help prevent common and avoidable mistakes. At the same time, it is also important to avoid mistakes that change the results of a test. To help prevent measurement error, the gauge should:
Provide a simple interface.
Clearly indicate the current units being displayed.
Offer a way to limit features and units only to those the user needs.
Stand up to dropping and rough treatment without sustaining damage.
Ease of Use: A good pressure gauge should offer the simplest interface possible for all its features. Generally speaking, you should not need a manual to use the device. While some multifunction calibrators might use a menu system or multilanguage operation, a simple pressure gauge should have neither.
Customizable Functions: Because digital gauges use electronic firmware – rather than mechanical parts – to manage their functions, they have great potential for customization. Here are some of the useful custom features offered by quality pressure gauges on the market.
Pressure Safety Valve Testing: In order to test pressure safety valves effectively, a pressure gauge needs to use an especially high read rate for capturing the moment a safety valve opens. Some gauges operate at this read rate constantly, while others use a special mode to increase the read rate for this purpose.
Pressure Switch Testing: Recent legislation requires pressure switch testing in certain applications like compressor stations. This legislation requires the maintenance of a permanent testing record. A special function that records data to the pressure gauge as it reads is a good fit for this application.
Long-Term Data Logging: A digital pressure gauge capable of long-term data logging is a good replacement for an analog chart recorder in many situations. For optimal data logging functionality, a gauge should also be rated Intrinsically Safe and IP-67 (waterproof in three feet of water) to withstand harsh environments. The recorded data should be easily exportable to Excel, csv, or text files.
Customizable Units: Some users need pressure readings that relate directly to their application – feet of seawater or ft-lbs, for example. A good digital pressure gauge will allow the user to define custom pressure units and deactivate any unnecessary pressure units.
Interface with a Computer: If the user needs a device for continuous logging or real-time data acquisition, some pressure gauges come with an RS-232 or USB connector and the software to report directly to your computer.
“Of Reading” vs. “Of Span” Accuracy: The true accuracy of a pressure gauge under operating conditions can be difficult to determine, in part because manufacturers’ specifications are often very confusing. One of the major differences between brand names is whether they offer “of reading” vs. “of span” accuracy.
When manufacturers define their accuracy as “percent of span,” they are describing the accuracy as a percentage of the gauge’s full scale. For example, a 100-PSI gauge with a 0.1 percent of span accuracy would be accurate to +/-0.1 PSI across its entire range. By convention, a gauge specified as a 0.1 percent-gauge is implied to be a 0.1 percent of span gauge.
When manufacturers define their accuracy as “percent of reading,” they are describing the accuracy as a percentage of the reading currently displayed. For example, a gauge with 0.1 percent of reading accuracy that displays a reading of 100 PSI would be accurate to +/-0.1 PSI at that pressure. At 50 PSI, the same gauge would have an accuracy of +/-0.05 PSI.
This last example demonstrates why only high-end digital gauges can offer percent of reading accuracy. Specifications with percent of span are actually a legacy from mechanical gauges, whose resolution was limited by how closely manufacturers could print the graduations on their dials. Today, digital pressure gauges that can display readings with sufficient resolution across their entire range use percent of reading specifications.
Resolution: In some gauges, the least significant digit does not change in increments of one as you would expect. It may increment by twos, threes or even fives. This is due to inadequate resolution of the analog to digital converter, and is especially noticeable on ranges such as millimeters of mercury or, in some cases, on metric scales like kPa.
Another problem with some digital gauges is the “floating point decimal.” Gauges with insufficient resolution may add a decimal place depending on the pressure displayed. For example, a gauge with four-digit display might read “2,000 PSI.” Once the pressure drops below 1,000, it adds a decimal place, to show “999.9 PSI.” In addition to being difficult to use at 1,000 PSI, the gauge’s specification may include a “least significant digit,” which varies with pressure. You will need to know where those changes occur.
Temperature Effects: If you work outdoors, your readings may be less accurate than you think. Temperature should not affect your pressure gauge. Any specification that indicates a narrow operating temperature band, for example, “from 18 C to 28 C” implies this gauge has inadequate temperature compensation. What looks like a small accuracy adder for every increment of temperature rapidly overwhelms the basic specification of the gauge at the temperatures they work in every day.
For outdoor use, pressure gauges need active temperature compensation built into its operation. To accomplish this, the manufacturer would monitor temperature internally, with correction algorithms to adjust pressure measurements for temperature changes.
|A percent of reading gauge has better accuracy than a percent of full-scale gauge at any pressure below full scale.|
Manufacturer’s Calibration: The factory calibration is an opportunity for a manufacturer to prove the performance claims made in the gauge”s specification. Before you buy, ask for a sample of the gauge”s factory calibration certificate to see the quality of calibration being performed. Also, check what the specifications for recalibration require.
Calibration Interval: Shorter calibration intervals allow manufacturers to improve their basic accuracy. A gauge can advertise fantastic performance if the calibration interval is 90 days or less. With the exception of industries with federally mandated 90-day calibration cycles, most businesses plan for one year between calibrations to manage re-certification costs.
Calibration Certificate: This should be free, because modern instruments are manufactured using NIST-traceable, automated calibration equipment. Look for calibrations from an ISO 17025-accredited calibration lab.
Calibration: Check to see if it’s possible to calibrate the gauge in-house. If the gauge requires factory calibration, a calibration lab cannot help. Also, check the instructions for calibration. Some gauges require, for example, “precisely 37.5 percent of full scale,” and many more similar points to adjust the gauge. If using a deadweight tester, this is probably not easy to do (e.g., 37.5 percent of a 30 PSI gauge is 11.25 PSI).
When you know what to look for, you can tell in advance which gauges can stand up to the elements you work in.
Welded Sensor: It’s not generally possible to tell whether a sensor is fully welded by looking at the gauge, so check with the manufacturer. Non-welded sensors use o-rings or even thread tape inside the gauge, and these designs sometimes indicate “not for oxygen service.” O-rings can degrade, and both o-rings and thread tape have the potential for leaks, especially if anything other than air or nitrogen is used.
Moisture: If the specification warns you to use clean, dry air, this sensor does not have an isolating diaphragm and is not suitable because moisture or liquid water will eventually cause sensor failure.
Diaphragm: Look for sensors with isolating diaphragms that are gas/liquid compatible and protected from damage. Good designs use a filter or a very small pressure port opening to keep small screwdrivers or cotton swabs away from the sensor.
Sensor Technology: Currently, piezoresistive sensors with oil isolation provide the best combination of performance and value. They are highly repeatable and handle overpressure conditions well. Sensors employing bonded strain gauges or thin film strain gauges rely on the deformation of a metal diaphragm. This makes them similar to mechanical pressure gauges, in that overpressure can cause a permanent shift in calibration.
Also, some designs in low-cost digital gauges should only be used with liquids. Rapid changes in pressure cause readings to be unstable until the strain gauge reaches thermal equilibrium, up to a minute later. To test for this, zero the gauge, apply full- scale pressure using air or nitrogen, then vent the gauge and see if it returns to zero.
Enclosure: The gauge enclosure should be compatible with hydraulic and other fluids typically used. Solvents and hydraulic fluids can attack polycarbonates, so look for a metal enclosure. For service near saltwater, marine-grade materials are optimal. Most, if not all gauges, use liquid crystal displays. If the gauge does not have a hard plastic or glass window, dropping a tool onto the display will destroy the LCD.
Also, ask what happens when the gauge is dropped. This information is generally not in the brochure or the operator’s manual, but gauges are often dropped, so this information is important to know.
Batteries: Find out if the gauge’s battery life meets the application need and whether it uses standard batteries.
Lithium batteries are an excellent technology, but a 9V lithium is expensive and difficult to find. Many gauges require a degree of disassembly to change the batteries and are often surprisingly difficult to reassemble. In some designs, you need to be careful not to damage the sensor or its cable when replacing batteries.
Finally, it is critically important to take the time to actually test and use a gauge. Spec sheets and brochures only tell part of the story. Is it easy to use? How good is the zero stability? How repeatable is it? Does it drift? What happens when it’s dropped? Can you read the gauge in direct sunlight or from a distance? Does the gauge have a built-in backlight? Actually trying the gauge can answer all of these questions and should be part of the evaluation process before you buy it.
Tom Halaczkiewicz is the president of Crystal Engineering Corporation. A graduate of California Polytechnic State University, Mr. Halaczkiewicz has been designing and manufacturing silicon pressure-sensor-based calibrators and gauges since the early 1980s. Some of his designs are or have been used on NASA Space Shuttle flights, the International Space Station, and by top racecar teams.