A faster, easier method for intrinsically safe, differential measurements at high static pressure
Imagine this common scenario: a pressure transmitter is mounted to a pipe in a hard-to-reach location. On the previous night, a technician noticed its readings were suspect. The transmitter monitors the differential pressure across an orifice plate and its operation is critical. This pipe transports a flammable gas at 1,400 psi. If this technician were looking for a way to verify the transmitter’s reading on site, their selection of instruments for an accurate measurement would be inconvenient, heavy or fragile.
Under different circumstances, the technician might have selected from a limited number of measuring tools, including digital pressure gauges or a pressure transmitter made into a portable indicator. The limitation faced is that any devices that could handle the high static pressure in the pipe would not be certified as “Intrinsically Safe” for use in the presence of combustible gases. A chart recorder might also be used, but the accuracy would be, at best, a theoretical 0.5 percent. The remaining alternatives are lab instruments, such as deadweight testers and other secondary standards. The down side of these is that the testing may not be done at full static pressure, and either two deadweight testers or a differential deadweight tester would be required. In any case, none of these instruments could leave the lab.
The only option would be to remove the transmitter from service, which would prove to be a costly test. The technician might spend the better part of a working day driving to the test site, uninstalling the transmitter and transporting it back to the laboratory. Ideally, the transmitter should then sit overnight to reach thermal equilibrium in the lab environment. The next day, the test would be conducted, and the transmitter would need to be transported back to the site and reinstalled.
Aside from the time lost, these test conditions are less than ideal. The transmitter may have been working correctly only to sustain damage or lose its calibration during transportation. Likewise, the ambient temperature in the lab and the orientation of the transmitter during the test will also affect the accuracy of the outcome.
A Better Solution
The nVision from Ametek – Crystal Engineering offers a differential pressure function for their nVision Reference Data Recorder that offers extremely high levels of accuracy. When the nVision’s two sensors are tared against each other, readings below 3,000psi are, at most, the greater of ±0.15 psi or 0.05 percent of the displayed differential reading. Readings below 3000 psi are, at most the greater of ±0.015 psi or 0.025 percent of the displayed differential reading. The nVision’s accuracy for conventional readings from either sensor is 0.025 percent of the displayed reading, across 80 percent of its full scale. (A full accuracy chart is available by downloading the nVision Datasheet). Because it is fully temperature compensated, the nVision operates at this accuracy between -20°C to 50°C, with no additional accuracy offsets.
The nVision is a field-capable pressure/temperature/voltage/Baro recorder designed for operation in harsh environments and near materials that present a fire risk. Accordingly, the nVision is ATEX/IECEx Intrinsically Safe and certified as waterproof according to IP67. The device weighs 1.5 pounds – including its four AA alkaline batteries, two interchangeable sensors with third Baro option and protective rubber boot – and measures 7 inches x 4.5 inches x 2.5 inches.
Additionally, during recording runs in Differential Pressure mode, the nVision records the actual pressure from both its sensors and the difference between them as a third data point. The unit takes readings at pressures up to 10,000 psi as frequently as 10 times per second and stores up to 500,000 data points. While the sensors must be rated for the anticipated static pressure or higher, each pressure sensor can withstand an accidental over-pressure of, at minimum, twice its full scale rating without sustaining damage or a calibration shift. Plus, at any time, the operator may view a graph of stored data or live measurements on the nVision’s high-resolution LCD screen, or by connecting to a PC, without pausing recording during a test. Ametek – Crystal provides free software to customize the functions of the nVision and to store logged data in a neatly formatted Excel template, which the end user can use and modify.
Written by: Ametek – Crystal Engineering