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Calibrating Pressure Sensors for Critical Air Environments

Fri, 08/11/2006 - 5:29am

Advanced testing solution finds a home in the challenging environment of the pharmaceutical plant full of ducts, pipes and heightened demands

‘Performing the test accurately can take up to an hour with outdated systems versus the five to 10 minutes with a more advanced system.’ By Terry Troyer
It’s important to select a unit that can automatically upload results to the laboratory’s network.
Several challenges face those responsible for calibrating pressure sensors for critical air environments, including pharmaceutical plants. Pressurization equipment requires highly accurate calibrations to certify that pressure sensors are working correctly. Unfortunately, most calibrators used today are bulky, inaccurate and expensive. Testing crews drag multiple components — pressure indicator, pressure generator, voltage and current meters and data logger — to test sites, where they are plagued with ambient pneumatic disturbances that can corrupt the differential pressure readings. With them, crews are also forced to record measurements by hand. In a pharmaceutical plant, for example, rooms may be full of pipes and ductwork, and some of the pressure sensors will doubtless be in hard-to-reach places. Since the testing device will be transported up ladders and into cramped spaces, it should be small and lightweight. Typically, though, testers find themselves climbing ladders with three or four different pieces of equipment, trying to balance them as they perform sensitive tests and manually record data. A solution to this problem is a single, small and lightweight unit. There are all-in-one units that have a pressure reading device, pressure generating device and computerized test sequence with a meter to read the output from the pressure sensors. It’s important to select a unit that is automated so that results are automatically uploaded to the laboratory’s network, avoiding the task of manual entry. The ideal calibration equipment is deployable, battery powered and small enough for one person to carry easily, even up a ladder or scaffolding. Another challenge facing testers using multiple components is the detrimental effect ambient noises have on very low pressures readings. The pressure sensors have two fittings on them, a high side and a low side. To test the pressure, crews compare the pressure in one room to the pressure in an adjacent room. The pressure in the adjacent room is on one side of the pressure diaphragm and can be either on the high or low side, depending on whether it is positive or negative pressure. In addition, a pharmaceutical manufacturing room, for example, should have higher static pressure than the adjacent room to keep unwanted particles from entering. The plus side of the differential pressure transducer will be attached to the internal room that is being protected, while the low side, the low-pressure port, will be on the outside room. Sometimes, to calibrate the low-pressure sensors, technicians can just pressurize the high-pressure side and leave the other port open, but that is not always the best method. If, by chance, someone passes by while the port is open, the motion can produce turbulence that creates a testing error. The same thing can happen if someone opens a door or if a fan or blower happens to turn on while the pressure is being read; air rushes by the open pressure port and invalidates the test results. A more advanced pressure calibrator would be hooked to both sides at once to create a sealed system that is unaffected by outside disturbances. The result is a smooth, steady pressure signal and, thus, accurate testing. Most companies use micro-solenoid pressure generation and regulation, a technique that applies small pressure pulses to the positive and negative pressure test volumes to regulate the test pressure. During active pressure regulation, this system generates pneumatic noise. A better system would be to use low-differential pressure-generation technology that produces maximum pressure-setting sensitivity with minimum noise. The pressure generation should be accomplished using a piston/cylinder arrangement, whereby the differential pressure sensor under test has both high- and low-pressure ports connected to the cylinder in a push/pull configuration. As the stepper motor-driven piston advances in the cylinder, it applies positive pressure to the high port of the test pressure sensor and negative pressure to the low port. The resulting pressure-generation system is sealed and immune to the outside environmental noise and has twice the sensitivity as a single-sided piston and cylinder. The pressure generation on most calibrators is done by hand with a knob and plunger. A person cranks a piston with a knob to generate pressure and then records the data by hand. The generating of pressure by hand is open to a wide variety of problems, including over pressurizing and under pressurizing. A more advanced system can actually record the data and upload it to an electronic database. Performing the test accurately can take up to an hour with outdated systems versus the five to 10 minutes with a more advanced system. According to a representative from a major pharmaceutical company, the instability and human error associated with outdated models caused him to accept field accuracies of ۫.5 percent. His company is hoping to lower the field calibration accuracy to ۫ percent or lower with a more advanced system. The increased accuracy of more advanced documenting calibrator translates into an added quality control feature because it allows for stricter standards of safety and security, which are both important in securing critical environments. In the pharmaceutical manufacturing arena, for example, if a pressure sensor is out of calibration, the product may become contaminated. If a laboratory loses power, its fans stop blowing highly filtered air onto its process. The room’s pressure containment is breached, and the product may be compromised. The company must report this possible breach in containment to the FDA. With more advanced calibrators, the company can have quick results and make tamper-proof certification and data handling to eliminate quality control concerns. Without these automated controls, if the lab experiences power loss, the company cannot verify that the room was not contaminated and could potentially lose hundreds of thousands of dollars worth of product. Terry Troyer is the engineering project manager at Setra Systems Inc., 159 Swanson Rd., Boxborough, MA 01719, which specializes in pressure, acceleration and weight sensing devices. Troyer has a bachelor’s degree in aerospace engineering and a master’s degree in business administration and holds affiliations with the Society of Plastics Engineers, the National Society of Professional Engineers and ASHRAE. Questions about this article can be addressed to him at 978-263-1400. Additional information is available at www.setra.com.
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