Pressure sensors, calibration

Special features Special gas burette arrangement for chanisorption option Automatic pressure sensor calibrations and leaktests. Automatic gas introduction... 

Pressure sensor, can be calibrated Fig. 5.20 for precise pressure points... 

Figure 10.9 Photograph of a pressurized testing cell for calibrating pressure sensors. This unit can pressurize the cell to 8 MPa...

Knowledge of the sample pressure is essential in all high-pressure experiments. It is vital for determinations of equations of state, for comparisons with other experimental studies and for comparisons with theoretical calculations. Unfortunately, one cannot determine the sample pressure directly from the applied force on the anvils and their cross-sectional area, as losses due to friction and elastic deformation cannot be accurately accounted for. While an absolute pressure scale can be obtained from the volume and compressibility, by integration of the bulk modulus [109], the most commonly-employed methods to determine pressures in crystallographic experiments are to use a luminescent pressure sensor, or the known equation of state of a calibrant placed into the sample chamber with the sample. W.B. Holzapfel has recently reviewed both fluorescence and calibrant data with the aim of realising a practical pressure scale to 300 GPa [138]. 

Additional procedures to ensure environmental control. In addition to those procedures directly associated with the Environmental Control Program, other procedures ensure that the equipment used to collect the test data is functioning properly per manufacturing site SOP for Calibration of Biotest RCS Air Sampler and Calibration of Model FKA-Pl Differential Pressure Sensors, which provides assurance that the data collected are accurate and reliable. 

Calibration procedure for pressure sensor (specify model)... 

Verify the calibration of all instrumentation involved in monitoring the EtO cycle. Examples include thermocouple and pressure gauge calibration, gas leak testing equipment, relative humidity sensors, and gas chromatographic instrumentation. 

The most common pressure sensor for optical studies is ruby (AI2O3 Cr3+, Piermarini et al. (1975)), whose strong Ri and R2 luminescence line shifts under pressure have been calibrated up to 180 GPa at room temperature (Mao et al., 1978 Mao, 1989). At low temperatures the line position has to be corrected by a known temperature-induced shift (Noack and Holzapfel, 1979). Besides ruby also other sensors utilizing rare-earth ions have been proposed and discussed in literature (Shen et al., 1991). In most of these cases the pressure induced shifts of luminescence lines are used to determine the pressure (see sect. 4.5). 

A further requirement for a pressure sensor is the stability of the host lattice at high pressures and temperatures. This requirement strongly narrows the range of possible candidates for high-pressure sensors. From X-ray diffraction experiments under pressure, it was found that YAG is stable at least up to 69 GPa at room temperature (Liu and Vohra, 1993). This is one of the reasons why doped YAG was chosen by many researchers as a promising host for pressure calibrants. 

Figure 5. Experimental apparatus 1 - AG cylinder, 2 - electronic balance, 3 - insulated chamber, 4 - heat exchanger, 5 - thermostat, 6 - computer with software, 7, 10 - pressure sensors, 8, 16 — 19 — valves, 9 - calibrated volume, 11 - vacuum pump, 12 - hydrogen vessel, 13 - helium vessel, 14,15 - reducer, 20 - flow meter, 21,23 - thermocouples, 22 - Ians, 24 - sorbent bed, 25 - heat exchanger.

Figure 3 shows the test section and instrumentation. Ten wall temperatures on the tube external surface were measured with 0.5 mm diameter calibrated type E thermocouples electrically insulated from the aluminium. Fluid inlet and outlet temperatures were measured with 1 mm diameter calibrated type K thermocouples. Cah-bration was carried out with a Rosemount 162-CE platinum thermometer. Due to the high thermal conduchvity of the aluminium and the low thickness of the tube walls the measured temperature is very close to the wall temperature in contact with the fluid (the difference less than 0.01 K). The inlet fluid pressure was measured with a calibrated Rosemount type 11 absolute pressure sensor. Two calibrated differential pressure sensors measured the pressure loss through the test section. A Rosemount Micro-motion coriolis flowmeter was used to... 

The material of the sensing layer strongly influences the characteristics of the pressure sensor and the difficulty of the calibration process. Fig. 7.4.6 shows the change in sensitivity of two different sensing layer materials (poly-Si and thin-film NiCr metal) against temperature. Poly-Si shows a nonlinear dependence on temperature, whereas NiCr is quite linear. Also the variation of the sensitivity is much lower for thin-film NiCr. Both high linearity and low variation of sensitivity contribute to a simple and therefore cost-effective calibration concept for high-pres-... 

Process chemometrics is a field where multi-way methods were introduced more recently. A batch process gives rise to an array of batches followed over time and measured by multiple process sensors such as temperature and pressure sensors or perhaps spectroscopy, as in Figure 10.3. Multi-way component models are used for the construction of control charts and multi-way regression is used for constructing predictive calibration models. 

In the hydrolysis reaction for example, this ancillary information entails preliminary runs that establish T0 and Tx values over the temperature range to be traversed by the rampings. In adsorption studies, as a very different example, ancillary studies entail establishing the value of the monolayer. Yet again, in catalytic studies there is a need to calibrate sophisticated analytical procedures and to establish that the data will be collected in a diffusion-free regime. And in all cases there is the need to periodically calibrate flow meters, thermocouples, pressure sensors and so on. 

Other fluorescent pressure sensors have also been suggested, but are used less frequently than ruby. Wavelength calibrations with pressure have been reported for alexandrite [97,98],Sm + Y3Al50i2 (YAG) [99-101],Sm + MFCl (M = Ba,Sr) [102,103],Sm2+ SrB407 [104,105],Eu + YAG [106],Tm + YAG [107],Nd + YAIO3 [108], and V + MgO [109]. Fluorescence lifetime calibrations have also been re-... 

A working pressure sensor linked via chained calibrations to a standard liquid manometer or piston gauge maintained by an NMI... 

The main developments in experimental techniques for measuring high pressure to obtain reliable pressure sensors are extensively discussed by Decker et al. [42]. These include (1) the establishment of a primary pressure scale using a free piston gauge (2) the selection and precise measurement of identifiable phase transitions as fixed pressure points and (3) the use of interpolation and extrapolation techniques for continuous-pressure calibration based on changes in resistance, volume, or optical spectra (based on an equation of state). An alternative method of estimating absolute pressure in isotropically compressed materials is based on measurements of ultrasonic velocity [43, 44]. 

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