The Challenges of Maintaining and Improving the Uncertainty of an Industrial Humidity Calibration Laboratory

摘要

The paper describes the issues associated with maintaining a busy industrial humidity calibration laboratory and the challenges faced in trying to maintain the laboratory measurement uncertainty whilst achieving a throughput of thousands of sensors per year. The humidity calibration facility of Michell instruments is essentially split into two parts - a UKAS (EA) accredited laboratory for high level measurements with direct traceability and audit path to NPL and NIST standards, and a commercial laboratory providing lower level tertiary calibration of tens of thousands of dewpoint sensors per annum. The UKAS section focuses on excellence rather than on volume - hence the processes and procedures are largely manual and quite time-consuming, whilst the commercial section is there to handle large volumes of sensors automatically and with the minimum of human intervention. The two therefore require very different approaches in terms of equipment, procedures and analysis and the uncertainty levels achieved reflect the type of operating model used in each case. Humidity calibration systems used by secondary laboratories have tended to be constructed either as clones of National Standards, using a two-pressure or two-temperature generation method, or as simple divided flow systems utilising calibrated vertical tube flow meters. The former are very expensive to produce and have certain limitations in terms of usability. The latter are cheaper to produce, but also suffer from inflexibility and difficulty in automation. Furthermore, these systems tend to offer varying flow rates dependent on the generated humidity level. The humidity calibration system described in this paper provides accurate and highly repeatable humidity generation using a combination of liquid and gas mass flow controllers. It allows automated use through the integration of a precision chilled mirror dewpoint hygrometer that provides both the control feedback to the generator and traceability to National Humidity Standards. The paper describes the two processes, provides a practical consideration of the component uncertainties and explores ways in which these uncertainties can be refined and minimised through improved procedures, better equipment and careful operation. Also provided are detailed calculations of the liquid and gas mass flow ratios used to derive appropriate humidity levels in the measurement chamber. A novel technique to ensure sensitivity and stability of the generated humidity is described, along with the techniques employed to ensure homogeneity of the humidified air. The paper also describes the physical design and construction challenges that were overcome in producing a fully integrated system. An uncertainty budget for the whole system is provided, indicating the key contributory factors and suggesting ways in which the measurement uncertainty can be minimised. This paper was presented at the 2009 NCSL International Workshop & Symposium held at the San Antonio Convention Center July 30, 2009.