Description:
There is a need to measure pressure accurately. Chemical manufacturers need process sensors that are able to monitor changes in manufacturing systems. These systems need to have a low uncertainty and high sensitivity to change. Often accurate pressure measurements in the chemical manufacturing industry are necessary to keep manufacturing processes safe. Sometimes, these changes affect other parameters, such as a flow measurement, and are necessary to maintain good manufacturing processes. At NIST, the need for highly accurate pressure measurements is prominent when determining the thermophysical properties of fluids, especially because these measurements lead to the development of theoretical models for industry. NIST researchers have developed methods to achieve better-than-quoted uncertainty in today’s pressure transducers through good practices, but the market is still limited. In order to meet the high standards NIST has for metrology measurements, we seek a high temperature in situ pressure sensor that achieves better resolution than the current pressure transducers available today.
The goal of this SBIR subtopic is to develop an in situ pressure sensor for fluid systems that operate up to 200 ˚C and pressures up to 7 MPa. Here, “in situ” means that the sensor is either attached directly to the system being measured (e.g., attached to a standard pipe fitting) or in very close proximity to the system; in either case it would be at the same temperature as the system. The sensor shall have remarkable temperature stability, control over drift, a small wetted volume (less than 5 mL), and be manufactured from materials that are highly corrosion resistant. On the market today, there are sensors that can reach the desired temperature of 200 ˚C, but these sensors often have large volumes or high uncertainty. The desired pressure sensor is expected to have a small volume to allow for easy coupling to a variety of precision measurement systems at NIST [1,2] as well as being able to act as a sensor in the chemical industries.
NIST envisions at least two basic design approaches, and either would be acceptable, as would other proposed designs. In the first approach, the sensor would directly measure the pressure and transmit a signal to a control computer. In the second basic approach, the sensor would measure the difference between the pressure of the fluid system and a reference pressure. In this embodiment, the reference pressure would be that of an inert gas or a hydraulic fluid, which would then be measured by a conventional pressure sensor that would be located remotely, e.g., at ambient temperature.
NIST is interested in a system with the following performance metrics:
· The pressure sensor shall operate in thermostated conditions of –70 ˚C to 200 ˚C.
· The pressure measurement must reach equilibrium in a reasonable time (< 5 minutes).
· The uncertainty in pressure must be better or equal in pressure to current pressure transducers, i.e., 0.7 kPa or 0.01% of range. The uncertainty shall include all effects, including hysteresis with increasing or decreasing pressures, compensation for temperature over the full operating range, and drift in the zero point.
· The measurement pressure range shall be ambient up to 7 MPa. The sensor shall provide a direct pressure measurement up to 7 MPa or be able to withstand differential pressures up to 7 MPa (if a differential pressure measurement).
· Electronic signals (both raw signals and computed pressure) shall be accessible to the user through a standard interface (e.g., USB, IEEE-488, or RS-232).
· Any electronic coupling within the thermostated area (i.e., wiring, connectors) shall be temperature compatible up to 200 ˚C. Other components (such as read-out electronics may operate at room temperature.
· All wetted parts of the sensor shall be fabricated of corrosion-resistant materials.
· The sensor should be able to be calibrated and maintain its calibration with minimal drift. It is desired that NIST scientists should be able to calibrate it at regular intervals.
· Drift: The sensor must meet the uncertainty specification with calibration intervals of no more than 4 months.
· Hysteresis: Any hysteresis associated with changes in temperature or pressure must fall within the overall uncertainty specification.
· The internal volume shall be less than 5 mL.
· The overall size of the sensor within the thermostated zone shall be 1 L or less. Electronics, however, may reside outside of this area.
Phase I expected results:
Provide a complete design of the pressure sensor. It is expected that CAD drawings of the sensor will be produced. It is also expected that theory and calculations relevant to the sensor function will be explained and provided. The awardee shall address their expected values for each one of the metrics above and describe how they designed their sensor to meet them in their final report for Phase I.
Phase II expected results:
Construct a fully-functional, tested prototype. The prototype shall be cycled over the full range of temperature and pressure at least 10 times to demonstrate the stability and performance metrics and the data from these tests shall be provided. Documentation on drift, stability and chemical stability shall also be made available. Each metric in the bulleted list above must be measured and addressed. The prototype will be made available to NIST for testing prior to the end of the SBIR Phase II award.
NIST staff is willing to participate in discussions and provide input on the awardee’s design during the development process through email, teleconference or face-to-face visits. NIST is also willing to do testing, although the awardee will not be able to count this testing towards the testing requirements of Phase II. NIST scientists may be available for demonstration of the device at the awardee’s home site, if desired.
References:
[1] Outcalt, S. L. and Lee, B.-C. “A Small-Volume Apparatus for the Measurement of Phase Equilibria”, J. Res. Natl. Inst. Stand. Technol. 2004, 109 (6), 525−531.
[2] McLinden, M. O. and Lösch-Will, C. “Apparatus for wide ranging, high-accuracy fluid (p, ρ, T) measurements based on a compact two-sinker densimeter”, J. Chem. Thermodyn. 2007, 39, 507-530.
