Pressure Measurement & Control


Choosing the right pressure gauge for the task

November 2014 Pressure Measurement & Control

According to Lanes Instruments, shopping for the right pressure gauge grows more confusing by the month because new technology is constantly under development. So, it is easy to get lost in the details and end up with a gauge that does not meet the requirements. This buyer’s guide explains the important features and specifications to look for and understand when purchasing pressure measurement devices.

Performance

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, users should not need a manual to use the device. While some multifunction calibrators might use a menu system or multi-language operation, a simple pressure gauge should have neither.

Customisable functions – because digital gauges use electronic firmware to manage their functions, they have great potential for customisation. 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 rate constantly, while others use a special mode to increase the read rate for the 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 analogue chart recorder in many situations. For optimal data logging functionality, a gauge should also be rated Intrinsically Safe and IP67 (waterproof in three feet of water) to withstand harsh environments. The recorded data should be easily exportable to Excel, csv, or text files.

* Customisable units: some users need pressure readings that relate directly to their application. 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.

Accuracy

‘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 sometimes confusing. One of the major differences between brand names is whether they offer ‘Of Reading’ or ‘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% of span accuracy would be accurate to ±0,1 psi across its entire range. By convention, a gauge specified as 0,1%-gauge is implied to be a 0,1% 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% 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 (twice as accurate).

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 reading 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 analogue to digital converter, and is especially noticeable on ranges such as millimetres 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 a four-digit display might read ‘2,000 psi.’ Once the pressure drops below 1000, it adds a decimal place to show ‘999,9 psi’. In addition to being difficult to use at 1000 psi, the gauge’s specification may include a least significant digit that varies with pressure. Users need to know where those changes occur.

Temperature effects: for outdoor work, readings may be less accurate than is desirable. Temperature should not affect your pressure gauge. Any specification that indicates a narrow operating temperature band, for example, ‘from 18 to 28°C’ implies this gauge has inadequate temperature compensation. What looks like 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 their operation. To accomplish this, the manufacturer would monitor temperature internally, with correction algorithms to adjust pressure measurements for temperature changes.

Manufacturer’s calibration: the factory calibration is an opportunity for a manufacturer to prove the performance claims made in the gauge’s specification. Before buying, 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 the recalibration requirements.

* 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 is 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% 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% of a 30 psi gauge is 11,25 psi).

Construction

Welded sensor: it is 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. 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 to use clean, dry air, this sensor does not have an isolating diaphragm and may not be 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 9 V 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. Specification 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 form part of the evaluation process before buying it.

For more information contact Paul Dhooge, Blanes Instruments, +27 (0)11 425 1465, paul@blanes.co.za



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