Solving Humidity Calibration Challenges In Today’s Metrology Lab
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1 Solving Humidity Calibration Challenges In Today’s Metrology Lab Author/Speaker: Jeff Bennewitz Thunder Scientific Corporation 623 Wyoming Blvd. SE Albuquerque, NM 87123 U.S.A. Email: jeff@thunderscientific.com Phone: (505) 265-8701; FAX: (505) 266-6203 Introduction/Abstract Two-pressure humidity calibration technology has long been the recognized standard for on site instrumentation calibration, test and verification. The goal of this paper is to provide a tool to help you as a metrologist take advantage of the benefits of this technology so you will be able to apply the information to your daily laboratory applications. This paper will cover the operation and benefits of modern two-pressure humidity calibration systems, explain how you calibrate them, provide some sample applications and a brief history of the technology. The Importance of Reliable Humidity Calibration in Your Lab Whatever your industry, your end product is only as good as the calibration it has received. Therefore, the equipment used to calibrate or test the limits of your end product must be first, a reliable, proven technology and, second, be presented in equipment that is easy to use and also easy to validate for optimum operation parameters; preferably at your laboratory, in-house. Whether you are calibrating transducers, tools for silicon wafer production, controlling comfort levels in HVAC systems or are involved with the tight humidity management required in manufacturing moisture sensitive products such as film, semiconductors, and pharmaceuticals, you are stepping up your demand for increased reliability and accuracy in humidity measurements. Today, calibration systems are required to both obtain and maintain a 4:1 accuracy ratio. To accomplish this, humidity and dew point hygrometers must be calibrated against a source of humidity at a stable test temperature. The most accurate and reliable method of continuous humidity generation in use today for the range of ~5-98% RH is based on the “two-pressure” principle originally developed by the National Institute of Standards and Technology (NIST). The two-pressure principle is used in the most accurate on-site calibration and verification systems. These are mobile and self-contained, with an integral humidity generator that is capable of simulating a wide range of temperature/humidity values with sufficient accuracy and consistency to maintain strict 4:1 calibration ratios. In a 10-year track record, this has been the only system of its kind that can meet tight tolerance requirements. It is still the primary standard of choice for humidity calibration.

2 Understanding How the Two-Pressure RH Generator Operates Figure 1. Operational flow schematic. The best way to describe operation is by dissecting the actual operational technology of an established system. We will use the Model 2500 two-pressure humidity generator for this purpose. Model 2500 is a portable, self-contained, two-pressure humidity generator that uses compressed air of up to 175 psia (1207) kPa) provided by either a portable oil-free air compressor or other equal source and directed to a receiver. The air then passes through dual regulators, achieving regulated pressure of ~...

3 alter the humidity content of the gas. The cooling effects of expansion, while mostly counteracted by the heated valve, are fully compensated by flowing the gas through a small post-expansion heat exchanger. This allows it to reestablish thermal equilibrium with the fluid surrounding the chamber and saturator before it enters the test chamber. The final pressure PC and temperature TC of the gas are measured within the test chamber. The test chamber exhausts to atmospheric or ambient pressure and so is very near ambient pressure. A computer/controller embedded in the system controls the...

4 Conventional two-pressure generators incorporate three separate pressure transducers: one for chamber pressure, one for lower saturator pressures and one for higher saturator pressures. The problem with this approach is that at low saturator pressures, a dual-drift effect (offset drift between the chamber and low-pressure saturator transducer) can cause significant errors in the calculated RH. The generator discussed here solves this problem by using only two transducers: one low-range for chamber and lower saturator pressures, and one high-range transducer for higher saturator pressures....

5 automatically by the embedded computer, you only need to provide the three known stable references: one near the low end, one near the center, and one near the upper end of each transducer’s intended range. For a low-end temperature calibration point, you take the temperature bath to a low point, ensure stability, and then enter the value indicated by a standard or reference thermometer. Repeat this procedure at two additional points: near the middle and upper ends of the temperature range. When the three reference points have been applied, the new coefficients for each probe are...

6 This analysis was conducted to validate the accuracy of performance using temperature and pressure uncertainty calculations. These uncertainty calculations of the Model 2500 two-pressure humidity generator served to establish that the system is within the manufacturer’s stated specification and that traceability can be established with NIST. Full analytical details of the study are available on the Thunder Scientific Corporation website.3 Application Examples Portable two-pressure humidity generation calibration equipment is in heavy use in pharmaceutical, aerospace and semiconductor...

7 Portable two-pressure humidity generation calibration equipment is also a valuable tool in humidity sensor research and development, hygrometer calibration, certification, and humidity sensor original calibration certification. This technology is also critical in special long-term environmental exposure tests for weather related calibration of atmospheric and land-based humidity sensor instrument packages and for large volume humidity sensor calibration production. History of Two-Pressure Humidity Calibration Older methods of on-site verification were accomplished by either using a...