Terminal vs. BFSL Accuracy
Like analog pressure gauges, digital gauges measure pressure, so why do some customers prefer digital to analog? Reasons may include better accuracy, better resolution, standard features that are optional with analog gauges, added features not available with analog gauges and multi-functional capability.
As with analog gauges, all digital gauges are not created equal. What follows are guidelines to consider when selecting a digital pressure gauge.
Accuracy
Manufacturers express digital gauge accuracy any one of 3 ways:
Full-Scale Terminal Point Accuracy- all points between zero and full-scale are within the stated accuracy of the gauge. This accuracy allows for zeroing of the gauge at start-up to eliminate any sensor offset. The accuracy of a gauge with full-scale terminal point accuracy includes hysterisis, linearity and repeatability.
When process temperature error is a concern, look for a gauge with full-scale terminal point accuracy and a gauge that features total error band (TEB). A gauge with TEB accuracy is calibrated to the stated manufacturers accuracy which includes hysterisis, linearity, repeatability and temperature. When a temperature band, e.g., 0° to 150° F (-18°C to 63°C) is specified as part of the accuracy, the gauge will retain its accuracy throughout the stated temperature band without accuracy degradation..
Best Fit Straight Line (BFSL) Accuracy- linearity error is minimized by best fitting a straight line to a non-linear curve. Gauges calibrated using BFSL may have a zero offset at calibration that must be maintained to ensure accuracy throughout the range.
There are two issues with BFSL accuracy:
- re-zeroing of the gauge may invalidate the published accuracy
- a zero offset at start-up may be the result of either BFSL or zero drift
When digital gauge specifications do not specify how accuracy is being measured, i.e., terminal point, BFSL or percent of reading, the gauge is usually calibrated using the BFSL method
Percent of Reading Accuracy- this accuracy is calculated by multiplying the published accuracy times the pressure being read. Accuracy (in psi) changes as the pressure reading changes. Manufacturers express this accuracy as a percent of span, usually either 20% to 100% of span or 0% to 20% of span. When the 20% to 100% of span accuracy is stated, the manufacturer states a second, lesser accuracy (for 20% to 100% of span).
Disadvantages to a percent of reading gauge include:
- gauge accuracy changes throughout the range of the gauge. A single gauge may not meet customer accuracy requirements as pressure increases or decreased. The user is required to ‘do the math’ to determine gauge accuracy
- most field calibration ‘standards’ are percent of span devices. When the digital gauge is used as a gauge master, a separate operating procedure must be developed to add a percent of reading device
- When temperature is included as part of a percent of reading gauge, the temperature span, e.g., 14° F to 122°F (-10° to 50°C) may be less than offered with a full-scale terminal accuracy gauge that includes total error band (TEB). An example of a gauge with TEB full scale terminal point accuracy is 0°F to 150°F (-18°C to 63°C).
Resolution
Gauge resolution involves the users ability to ‘read the gauge’.
Analog Gauges: are read by viewing the gauge pointer in relationship to the graduations on the dial of the gauge. A reading is taken where the gauge pointer meets the closest gauge graduation. The reading taken is somewhat subjective and is based on the readers’ judgement of where the pointer is in relationship to the closest dial graduation. One reader of the gauge may perceive the pointer at, for example, 15.5-psi whereas the next reader may perceive the gauge pointer as reading 15.7 psi.
Digital Gauges: Since the pressure of a digital gauge is displayed digitally rather than with a pointer and dial (analog gauges), all persons reading a specific gauge pressure will report the same reading.
- Digital gauge specifications also differ in the number of digits displayed, the more digits, the better the resolution. For example, a 3.5 digit display on a 200 psi gauge at full-scale will read 200, a 4 digit display will read 200.0 and a 5 digit display will read 200.00.
- Character display height should be considered when purchasing a digital gauge. The larger the characters, the more readable the gauge will be.
- Some digital gauges offer a percent of range bar graph to further identify the current pressure displayed.
Gauge Footprint
The overall size of the gauge should be considered prior to purchase of a digital gauge. Digital gauges serve many different markets. These gauges can be used as masters to calibrate/check calibration of other gauges, they can be used in the customers process or be used on OEM equipment. To minimize the overall footprint and maintain maximum readability, consider a gauge where the sensor resides internal to the gauge case. This best protects the sensor and provides the most efficient use of space.
Standard Features
No one digital gauge manufacturer offers the same standard features. Depending on the manufacturer some offer a max/min feature standard to allow for the last minimum and maximum pressures read by the gauge. A backlite may also be a standard feature.
Some feature-rich digital gauges offer a Menu driven display. Some of the features that may be contained in the menu are multiple units of measure, a recalibration mode, a password protect menu feature, dampening and update rate to solve process pressure pulsation issues and multiple languages.
Multi-Functional Capabilities
Unlike analog gauges, some digital gauges are available with multi-functional features. These features may include local indication, 4-20Ma output, line power and switches. Digital gauges with these features offer superb value. Multi-functional capability of this digital gauge saves the customer money, time and space. This value-rich multi-functional feature allows for one digital gauge to replace up to 3 instruments; a pressure gauge, a transducer and a switch(es). Save on installation costs, instrument cost, additional pipe-cut-out(s) and space.
