The measurement and calibration of very low pressures is a difficult and challenging endeavor for many industries and calibration laboratories. These very low pressures, ranging from 0 psi to approximately 1 psi, require a different measurement and calibration setup as compared to the calibration of higher pressures.
To increase the scaling of the measurements below 1 psi, other pressure units are used to describe low pressure measurements. The most common units are pascals (1 psi=6895 Pa), millibars (1 psi=69 mbar), millimeters of mercury/torr (1 psi=51.7 mmHg), or inches of water (1 psi=27.7 inH2O).
Typical Bourdon tube pressure gauges cannot usually operate in the low-pressure regions since the pressures are too low to cause movement in the tube assembly. Sensitive bellows-type mechanical gauges or electronic transducer type sensing elements are often the most widely used devices at the low pressures.
Applications for low pressure measurements include the monitoring of HVAC equipment, control of environmental and energy systems, and various research and development instrumentation.
Some of the most stringent requirements for low pressure measurements are in the medical industry, which is regulated by the FDA. Environments such as those in drug or medical device manufacturing, cleanrooms and hospital surgical and isolation rooms require low pressure medical instrumentation to maintain the positive airflow in these critical areas.
In order to maintain a dust-free and clean environment, a small positive pressure difference must be created in the room environment so that any debris is blown out of the room from the pressure when a door is opened. It is important that these systems work flawlessly to guarantee that the room is operating properly during the medical procedures or manufacturing processes taking place in them.
The FDA has tight specifications for the cleanliness of these environments and the proper calibration of the low pressure sensors ensures that they are working correctly and are within the manufacturer’s specifications.
Having such a low full-scale pressure span makes it significantly more challenging when performing calibrations on low pressure equipment. Some of the challenges are as follows:
One of the biggest issues when calibrating low pressure gauges is in the generation of the proper pressures. Low pressure measurements can be made with higher pressure sources, as in from compressed gas cylinders, however, the attached pressure regulators typically are not able to control and maintain a stable pressure at the low pressure ranges. Also, any problems with the regulator output can produce a pressure that significantly exceeds the maximum pressure rating of the equipment. The pressure sensors used in the equipment are very sensitive and even a slight overpressure can damage the sensors.
Pressure deadweight testers are typically used as a highly accurate standard for pressure calibrations, however, the lowest pressure that can be generated is determined by the mass of the piston and its area. Depending on the range of the piston used, the lowest pressure that can be generated is usually 1 to 3 psi which is too high for these applications.
For pressure generation at these low pressures, hand pumps or manifolds with fine needle controls are often used. These allow the required pressure generation without the risk of over pressurizing the equipment being calibrated.
Leaks in the calibration setup can cause a number of problems when generating low pressures. Although leaks are a minor nuisance for high pressure calibrations, even minor leaks can cause significant errors in the calibration accuracy at low pressures.
The sensors used for low pressure measurements are more sensitive to environmental influences such as vibration and temperature fluctuations due to their thinner sensing elements. They usually have reduced operating temperature ranges to ensure that environmental variations will not affect their accuracy.
Should you be calibrating your instruments in-house or outsourced? Read our guide to find out.