Are you getting your money's worth?
August 2012, Maintenance, Test & Measurement, Calibration
In this buyers’ guide, the late Tom Halaczkiewicz, president of Crystal Engineering, explains the important features and specifications to look for when purchasing pressure measurement devices. Some of Tom’s designs are used by NASA, the International Space Station and top race car teams.
With new technology always under development, shopping for the right pressure gauge grows more confusing by the month. It is easy to get lost in the details and end up with a gauge that does not meet your needs.
* Are you getting your money’s worth?
* Are you really getting the quality instrument you need?
* Will the gauge perform as you expect?
When you work with high pressure or flammable materials, you want a gauge that helps you avoid the occasional human error. Some errors threaten your safety, while others threaten the quality of your results. To keep you safe, your gauge should:
* Warn you when you have exceeded full scale pressure.
* Provide a generous safety factor for accidental overpressure.
* Limit the zero range (the amount of live pressure you can clear from the gauge).
* Always display live pressure.
* Stop displaying pressure if the sensor sustains damage.
While no tool can keep you completely safe, you should look for these features in your pressure gauge to help you prevent common and avoidable mistakes. You also need to avoid mistakes that change the results of your test. To help prevent measurement error, your gauge should:
* Provide a simple interface.
* Clearly indicate the units you are working in.
* Offer a way to limit features and units to those the user needs.
* Stand up to dropping and rough treatment without sustaining damage.
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 multi-function calibrators might use a menu system or multi-language operation, a simple pressure gauge should have neither.
Because digital gauges use electronic firmware – rather than mechanical parts – 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 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 perfect for this application.
Long-term data logging – a digital pressure gauge capable of long-term data-logging is an excellent replacement for an analog chart recorder in many situations. For optimal data-logging functionality, a gauge should also be rated Intrinsically Safe and IP67 (waterproof in 3 feet of water), to withstand harsh environments. The recorded data should be easily exportable.
Customisable 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 you to define custom pressure units and deactivate any pressure units you do not want to use at the moment.
Interface with a computer – if you need 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 confusing. One of the major differences is whether they offer ‘of reading’ or ‘of span’ accuracy.
When manufacturers define their accuracy as ‘% of span’, they are describing the accuracy as a percentage of the gauge’s full scale. For example, a 100 bar gauge with an 0,1% of span accuracy would be accurate to ±0,1 bar across its entire range. By convention, a gauge specified as a 0,1% gauge is implied to be a 0,1% of span gauge.
When manufacturers define their accuracy as ‘% 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 bar would be accurate to ±0,1 bar at that pressure. At 50 bar, the same gauge would have an accuracy of ±0,05 psi.
This last example demonstrates why only high-end digital gauges can offer % of reading accuracy. Specifications with % of span are actually a legacy from mechanical gauges, whose resolution was limited by how closely manufacturers could print the graduations on their displays. Today, digital pressure gauges that can display readings with sufficient resolution across their entire range use % of reading specifications.
In some gauges, the least significant digit does not change in increments of 1 as you would expect. It may increment by 2s, 3s, or even 5s. This is due to inadequate resolution of the analog 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 four-digit display might read ‘2000 units’. Once the pressure drops below 1000, it adds a decimal place, to show ‘999.9 units’. In addition to being difficult to use at 1000 units, the gauge’s specification may include a least significant digit which varies with pressure. You will need to know where those changes occur.
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, 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 you work in every day.
For outdoor use, your pressure gauge needs 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.
The factory calibration is an opportunity for a manufacturer to prove the performance claims made in their specification. You should look carefully at your gauge’s first calibration and the specifications for recalibration.
Calibration interval – shorter calibration intervals allow manufacturers to improve their basic accuracy and 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 1 year between calibrations to manage their 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 if you could calibrate your own gauge with the right equipment. If the gauge requires factory calibration, your calibration laboratory cannot help you. Also, check the instructions for calibration. Some gauges require that you apply, for example ‘precisely 37.5% of full scale’, and many more similar points to adjust the gauge. If you use a deadweight tester, this is probably not easy to do.
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 – this can be difficult to determine. You cannot 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, 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 screw-drivers 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 causes readings to be unstable until the strain gauge reach 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.
Your gauge enclosure should be compatible with hydraulic and other fluids you use. 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 you drop the gauge. You will not find this in the brochure or the operator’s manual, but it is quite likely someone could drop your gauge.
Find out if the gauge’s battery life meets your needs, and whether it uses standard batteries. Lithium batteries are great 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 surprisingly difficult to reassemble. In some designs, you need to be careful not to damage the sensor or its cable when replacing batteries.
Finally, make sure you 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 you drop it? Can you read this gauge in direct sunlight or from a distance? Does your gauge have a built-in backlight? Actually trying the gauge can answer all of these questions, and should be part of your evaluation before you buy it.
For more information contact Paul Dhooge, Blanes Instruments, +27 (0)11 425 1465, firstname.lastname@example.org