November 2006
  
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A Solution for Properly Calibrating Pressure Sensors for Pharmaceutical Plants
For Properly Calibrating Pressure Sensors For Pharmaceutical Plants

Terry Troyer,
Product Engineering Manager
Setra Systems

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.

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Unfortunately, most calibrators used today are bulky, inaccurate, and expensive. Testing crews drag multiple components (a pressure indicator, a pressure generator, voltage and current meters, and the data logger) to test sites, where they are plagued with ambient pneumatic disturbances which can corrupt the differential pressure readings. Crews are also forced to record their measurements by hand on clipboards. These tests waste valuable technician time, with each calibration taking up to one hour, and they open a wide margin of error with so many different components and the manual process of having a technician record numbers on a chart.

Setra 869In a pharmaceutical plant, for example, rooms may be full of pipes and ductwork, and some of the pressure sensors will doubtless be located in hard-to-reach places. The testing device, therefore, must be able to be transported up a ladder and into cramped spaces, and therefore needs to be small and lightweight. Typically, though, testers find themselves climbing ladders with three or four different pieces of equipment, trying to balance them all as they perform the sensitive tests and then, the task of manually recording the data.

A solution to this problem is for a single, small and lightweight unit to conduct all of the testing. This all-in-one unit should have an accurate pressure reading device, an accurate pressure generating device, and a computerized test sequence with a meter to read the output from the pressure sensors. The unit should also be automated so that results are automatically uploaded to the laboratory’s network, thus avoiding the cumbersome and often erroneous task of manual entry. The ideal calibration equipment is deployable, battery powered, and small enough for one person to carry comfortably, even up a ladder or scaffolding. In short, it must be able to go anywhere and perform the work of several different pieces of equipment. Single-unit calibration testing is a better choice than systems comprised of multiple components for these reasons.

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 either be on the high or the low side, depending on whether it is positive or negative pressure.

Setra 869 ProductIn addition, a pharmaceutical manufacturing room, for example, should have higher static pressure than the adjacent room to keep unwanted particles from coming in. 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.

Reliable Controls 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. The technician then records the results by hand on a clipboard. A more advanced system can actually record the data and upload it to an electronic database to eliminate the possibility of test results being compromised due to human interference. Performing the test accurately can take up to an hour with outdated systems versus the five to ten minutes with a more advanced system.

According to a representative from a major pharmaceutical company, “Due to the instability and human error associated with outdated models, we had to accept field accuracies of +/- 1.5%. We are hoping to lower the field calibration accuracy to +/- 1.0% or lower if the more advanced system proves to have long-term reliability.”

The increased accuracy of the more advanced documenting calibrator translates into an added quality control feature, as it allows for stricter standards of safety and security, which are both of the utmost importance in securing these 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, the fans stop and are no longer blowing the highly filtered air onto the process. The room’s pressure containment is then breached, and the product may be compromised. The company must report this possible breach in containment to the FDA, and must also report how they assess the situation. With more advanced units, the company can have quick results and tamper-proof the certification and data handling to eliminate any 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.

The same pharmaceutical representative stated, “We must assure that the NIST (National Institute of Standards and Technology) is at least 4X greater than the field device it is calibrating.”

With potential consequences like that, it is no wonder that many companies are switching to the more advanced documenting calibrators.

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