Examples of product robustness criteria for a pressure transmitter can include:

The ability to protect against process corrosion and hydrogen permeation.

The ability to handle rapid and severe over-pressure resulting from the "water-hammer" effect or improper equalizing line pressure after calibration.

The ability to protect electronics from humidity and condensation.

The necessary corrosion resistance within the field terminal electronics to protect against improper housing cover sealing of during installation.

Adequate electronics protection to prevent electrostatic (ESD) damage during handling and installation .

Adequate physical shock protection to protect against improper device handling or dropping.

One potential predictor of installed reliability is experience. In general, the fifth release of a device will be more reliable than the first. The question to the supplier, therefore, becomes: "How many thousands of devices, or devices of similar design, have been installed in similar applications?" The supplier also should be willing to provide references.

Useful references for your assessment should be, in part, based on a history of continuous improvements for product design enhancements. Minimum components of continuous-improvement programs include:

A feedback loop for communication on product reliability, including application assistance and failure analysis detection.

Documentation of any changes that affect form, fit, function, materials or performance specifications and approvals.

Design and manufacturing testing to verify any changes that affect form, fit, function, materials or significant performance criteria.

Validation of 100 percent of product to published performance specifications. Maintain, track and review data for 100 percent of units.

Random audit testing of a product from all manufacturing locations ," i.e., product inspection, performance testing, etc.

The request to the supplier, therefore, becomes: "Please provide a product quality review outlining and proving your specifications."

"Good, better, best" compares the transmitter total performance (includes reference accuracy, static pressure, temperature and drift) of one 0.1 percent and two 0.075 percent reference accuracy devices.

Repeatability and specs

How can the user quantify the impact of "real-world" sources of error? For any measurement, the accuracy and repeatability in a real-world application will usually be lower than in the laboratory. Consider, for example, a differential pressure (DP) transmitter with 0.1 percent reference accuracy installed on an orifice plate. Will the complete measurement system provide 0.1 percent, 1 percent or 10 percent flow repeatability?

The first step is to identify the factors that will cause a transmitter to be less accurate and less repeatable outside of a laboratory. Using the example of a DP flowmeter, "real-world" effects can include:

Expected ambient temperature variation.

Maximum static line pressure.

Process fluid pressure and temperature variation for gas or steam.

Line temperature variation (for liquids).

Flow turndown.

Transmitter drift over time.

The next step is to quantify the impact of these real-world conditions for the given application and transmitter of interest. For DP flow applications, reputable instrument suppliers publish specifications that allow the user to calculate and predict the impact of these and other "real-world" effects on installed flow accuracy and repeatability.

The results under "typical" installed conditions are shown in the table. Although these errors might seem small at 100 percent flow, the errors are magnified at lower flow rates for two reasons.

First, any error remains fixed over the entire transmitter range. For example, consider a 1-mile per hour (mph) error in your speedometer. At 100 mph, this is a 1 percent error; however, at 10 mph, it becomes a 10 percent error.

Second, for DP flow applications, flow is equal to the square root of DP, meaning flow errors increase exponentially at higher turndowns. For example, 1/2 flow equals 1/4 DP, 1/4 flow equals 1/16 DP, etc. Many suppliers offer software tools that allow users to quantify the installed repeatability for specific transmitters in specific, user-defined application conditions.3

Two conclusions can be made from the table:

Reference (laboratory) accuracy is a trivial component of total transmitter installed error.

Two DP transmitters with identical 0.075 percent "reference accuracies" can provide dramatically different installed accuracies. CP

Wickberg is manager of pressure applications for Rosemount Inc., a division of Emerson Process Management, Chanhassen, Minn. Contact him at eric.wickberg@emersonprocess.com.