Vacuum Comparison Standard for the Calibration of Absolute Gauges (0.65 Pa to 130 kPa)
The Vacuum Comparison Standard (VCS) is an automated pressure transfer standard capable of absolute-mode gauge calibrations in the range of 0.65 Pa to 130 kPa. Automation provides many advantages, but the prominent benefits are reduced turnaround time and lower cost. This new service offers customers a calibration option other than the NIST Ultrasonic Interferometer Manometers (UIMs), which are primary standards with the lowest stated uncertainty in the world, but require significant time and money to operate. Additionally, most commercial gauges do not have the resolution/accuracy/stability to fully utilize the advantages of the UIMs. NIST designed and constructed the VCS to provide calibrations to meet customer requirements for lower cost, faster turnaround calibrations with direct NIST traceability.
The VCS is a portable Transfer Standard Package (TSP) which can disseminate pressure at significantly lower uncertainties than commercially available transfer standards. Although higher than the UIMs, the TSP uncertainty is sufficient to calibrate 90% of commercial gauges. At the heart of this standard is a set of two Capacitance Diaphragm Gauges (CDGs) along with a Resonance Silicon Gauge (RSG) enclosed in a temperature controlled enclosure (see photo).
The crucial advantage of the NIST TSP is the ability to exploit the advantages of both gauge types. CDGs offer unbeatable resolution at lower pressures (typically 1 part in 106 of full scale range), but are subject to up to 0.5% uncertainty (k=2) due to long term stability (drift uncertainty). However, the short term stability (< 8 hours) is excellent (less than 0.01%). RSGs offer drift uncertainties of 0.01% (k=2), but don’t have the resolution or low full scale ranges of CDGs. Because of these characteristics, it is possible to calibrate the CDGs just before use and wind up with a very low uncertainty measurement. The software takes advantage of this by performing a pre-calibration adjustment to correct the CDG drift against the RSG. This is an entirely automated and in-situ self-calibration to adjust the CDG calibration function just before customer data is collected. The CDG calibration function is gauge dependent and is usually a second to fourth order fit and while overall shifts of this function do occur in time (drift), most CDG drift can be accounted for because the shape of the function is usually very stable. The TSP is recalibrated against the UIM regularly to reassess the calibration function over time.
Additionally, both CDG and RSG gauges see significant noise reduction by placement in a temperature controlled enclosure and ensuring they remain fixed (level). For temperature control, NIST implemented a commercial portable cooler with a built in Peltier heating/cooling unit. By using a feedback loop with a Platinum Resistance Thermometer (PRT) and a NIST designed electronics circuit, the temperature in the enclosure is controlled to ±10 mK. The cooler is mounted to a flat plate which is leveled to within 100 μRad. By ensuring the gauges are calibrated and leveled in the same manor, the random component of uncertainty is below 0.01 % (k=2) at the lowest pressure and is negligible at higher pressures.