Garbage in, garbage out (GIGO) may no longer be in everyone’s lexicon, which doesn’t make it any less true. Unfortunately, many control systems operate on this principle because we don’t know the quality of the signals. Calibration is how we verify the accuracy of a measurement.
There's a myth that digital- /smart- /microprocessor-based transmitters don’t need calibration because most facilities can’t equal factory calibration in the field. This may be true depending on your facility, and quite likely it's the case for the sensor mounted to the process, but you still need to make sure signals from the device represent reality. At minimum, zero and span should be verified just in case there was an error in configuration, or the device is installed in the wrong place or it was damaged during transit or installation.
A reference check is one good way to verify the integrity of a measurement without the requirements of a traceable calibration procedure. (The daily bump test many facilities require for personal gas monitors is an example of a reference check, in part because consistency can't be guaranteed.)
Handheld calibrators are the most commonly used way to verify signal integrity in the field. These units have the ability to complete a range of the most common calibrations and record the results in their memory. After a productive day, the information in the calibrator is uploaded to the associated server and software, normally via USB connection, after which the next day’s work is downloaded to the device. One advantage of intelligent devices is the ability to internally store calibration records and who performed work, thus providing a record of this information that's directly linked to the device and a backup of the information in the handheld calibrator. Intelligent devices also have the advantage of identifying themselves to the calibrator, thus reducing the chance for error.
What about those devices in difficult-to-reach locations, such as gas detectors in the peak of the roof over a compressor or wireless devices that can now be placed almost anywhere, or the device that an engineer/designer who has never been in a facility puts in the most awkward and potentially hazardous-to-reach location possible? I worked in a facility where the pump discharge pressure transmitter was placed directly over the impeller, so any time mechanical work was done on the pump, the transmitter and cabling had to be removed, requiring a scaffold for access.
Gas detector manufacturers, realizing that their devices are put in hard-to-reach locations, often support Bluetooth or infrared communications with the electronics through a window to maintain electrical classification. Other manufacturers use this same idea for communicating to electronics inside an explosion-proof enclosure.
With the increased availability of handheld tablets and smartphones suitable for use in the plant environment, access to information is being improved, but it's still mostly one-way (receiving) or as an alternate means to record what used to be done with paper as part of the field rounds. Tablets don’t have the functionality to perform a calibration without lots of specialized accessories, so that market will likely stay “safe” for a while. It would certainly be nice if handheld calibrators would connect wirelessly to record results directly in databases versus using the USB connection. That way, at least the information will be captured nearly instantly, and if necessary, changes to the day’s tasks or extra information on the device being calibrated/procedure could be downloaded without a trip back to the shop.
We have the technology to allow us to take control of the integrity of our control systems’ measurements and hence improve the control of our facilities. However, to do so, we need to realize that we want to own our measurements, sever the USB cord, and take advantage of technologies that increase confidence in our measurements, while also reducing risk and exposure of workers.
Perhaps you might want to make a 2018 New Year’s resolution that you can keep: toss the garbage signals by implementing an effective calibration and maintenance program.