Independent Calibration Method Research Articles

Developments and Progress in Non-contact Eye Tonometer Calibration

Abstract This paper focuses on the problematic of intraocular pressure (IOP) measurements, performed by non-invasive methods. More specifically, the devices that are connected with the presented finding are non-contact tonometers that use concentrated air stream and optical sensors to determine the IOP within a human’s eye. The paper analyzes various influential factors that have an effect on the determination of the IOP values originating from the patients themselves and from the non-contact tonometer devices. The paper furthermore elaborates on the lack of independent methods of calibration and control of these devices. In order to fill this gap a measurement standard device that is capable of calibrating and testing these devices with traceability to the basic SI unit is presented. A detailed characterization and the determination of the expected uncertainty of the device are provided. By introducing an independent and traceable calibration method and control of non-contact tonometers into the clinical practice, the reliability of the measured IOP that is the primary indicator of glaucoma can be improved.

Study of full-link on-orbit geometric calibration using multi-attitude imaging with linear agile optical satellite.

We conducted a systematic investigation into independent on-orbit geometric calibration, with the aim of applying it to linear agile optical satellite (AOS). Using a combination of multi-attitude images, a complete full-link independent calibration method chain was achieved, in which both the internal and external systematic parameters could be calibrated using the self-constraint of these images, as distinct from the absolute constraints identified from ground calibration sites. In accordance with the capacity for restraint associated with the self-constraint of the images and the geometric characteristics of the systematic errors in the imaging model, the calibration parameters that were deemed suitable for mathematical estimation under this self-constraint, as well as capable of compensating for the systematic errors, were determined by two equivalent compensations. Subsequently, a stepwise calibration for the estimation of external and internal parameters was conducted, where the corresponding points, matched from two separate combinations of multi-attitude images, were applied to the external and internal calibrations, respectively. With an aided elevation, the optimal calibration parameters were achieved under these conditions without the use of a ground control point (GCP). Finally, a set of innovative experiments were conducted on rigorously simulated data to verify the theoretical accuracy and feasibility of this method. The experimental results indicated that the method could achieve an overall theoretical accuracy of around 0.002 arc seconds, and showed good geometric consistency for all charge-coupled device (CCD) detectors.

Evaluation of the IAGOS-Core GHG package H<sub>2</sub>O measurements during the DENCHAR airborne inter-comparison campaign in 2011

Abstract. As part of the DENCHAR (Development and Evaluation of Novel Compact Hygrometer for Airborne Research) inter-comparison campaign in northern Germany in 2011, a commercial cavity ring-down spectroscopy (CRDS) based gas analyzer (G2401-m, Picarro Inc., US) was installed on a Learjet to measure atmospheric water vapor, CO2, CH4, and CO. The CRDS components were identical to those chosen for integration aboard commercial airliners within the IAGOS (In-service Aircraft for a Global Observing System) project. Since the quantitative capabilities of the CRDS water vapor measurements were never evaluated and reviewed in detail in a publication before, the campaign allowed for an initial assessment of the long-term IAGOS water vapor measurements by CRDS against reference instruments with a long performance record (Fast In-situ Stratospheric Hygrometer (FISH) and CR-2 frost point hygrometer (Buck Research Instruments L.L.C., US), both operated by Research Centre Jülich). For the initial water calibration of the instrument it was compared against a dew point mirror (Dewmet TDH, Michell Instruments Ltd., UK) in the range from 70 000 to 25 000 ppm water vapor mole fraction. During the inter-comparison campaign the analyzer was compared on the ground over the range from 2 to 600 ppm against the dew point hygrometer used for calibration of the FISH reference instrument. A new, independent calibration method based on the dilution effect of water vapor on CO2 was evaluated. Comparison of the in-flight data against the reference instruments showed that the analyzer is reliable and has a good long-term stability. The flight data suggest a conservative precision estimate for measurements made at 0.4 Hz (2.5 s measurement interval) of 4 ppm for H2O < 10 ppm, 20 % or 10 ppm (whichever is smaller) for 10 ppm < H2O < 100 ppm, and 5 % or 30 ppm (whichever is smaller) for H2O > 100 ppm. Accuracy of the CRDS instrument was estimated, based on laboratory calibrations, as 1 % for the water vapor range from 25 000 ppm down to 7000 ppm, increasing to 5 % at 50 ppm water vapor. Accuracy at water vapor mole fractions below 50 ppm was difficult to assess, as the reference systems suffered from lack of data availability.

Accuracy assessment of aerosol source apportionment by dual wavelength photoacoustic measurements

Aerosol source apportionment is a major challenge for air quality monitoring. Due to the frequently observed correlation between the chemical properties of aerosol particles and the wavelength dependence of their optical absorption coefficient, the photoacoustic (PA) method (similarly to alternative optical absorption measurement techniques) offers the unique possibility of real time source apportionment under certain circumstances. However, so far, the selection of lasers into a multi-wavelength PA system for source apportionment was based only on rule of thumb type estimations (like “one should use the highest available light powers with the widest possible wavelength separation”). Here we introduce a simulation method to estimate the accuracy of source apportionment and to be used for the optimised selection of the light sources of a dual wavelength PA system in cases, when light absorbing carbonaceous aerosol (LAC) originates from two distinct sources and dominates the atmospheric aerosol load. The method has two phases and it can be applied either on actual measurement results or on simulated datasets. In the first “calibration” phase the parameters of a non-linear calibration curve are determined, which describes the correlation between the ratio of optical absorption coefficients (OACs) measured photoacoustically at two wavelengths and the ratio of the source specific segregated mass concentrations (mass concentration ratio, MCR) measured by an independent calibration method. During the second “measurement” phase these calibration parameters are used to estimate the MCRs solely from the photoacoustically measured OAC ratios. The relative uncertainty of this estimation is used to grade dual wavelength PA systems. Executions of this simulation on different MCR, OAC datasets have proved that with a fairly simple PA system (which consists of a 405 and a 1064nm diode laser having modulated light power of 150 and 300mW, respectively) MCR determination is feasible with at least 10% relative accuracy whenever the mass concentration of LAC exceeds 1µg/m3.

A positioning and orientation method based on the usage of INS and single-beam Lidar

A positioning and orientation technique for land vehicle based on the integration of INS and single-beam Lidar was presented. First, the definition of the reference coordinates of inertial measurement unit (IMU), vehicle and Lidar was presented, and an independent calibration method for parameters such as installation angles was established. Error model of the single-beam Lidar was deduced based on the defined coordinates and the error model and the combined measurement model of the integrated system were established. Kalman filter was applied to realize the integrated positioning and orientation based on INS and single-beam Lidar. Finally, the numerical simulation and vehicle test were carried out and the correlated results were presented to demonstrate the validity of the positioning and orientation method proposed in this work.

Design of MFL Inspection Device for Underground Oil Tanks Based on Independent Calibration Method

The underground oil tanks are usually ellipsoid shaped and horizontal placed, in order to inspect the underground oil tanks using MFL method, a magnetization structure contains three sub parts was designed for curvature plates, the impact by the uneven placement sensors was analyzed, a new channel independent calibration method was raised. The calibration method, as well as the digital signal processing method was programmed in the inspection software, then a motor drive walking device was designed, at last the inspection device was established. Through comparison of channel independent and identical calibration methods, the experiment result using channel independent calibration method has better precision of about 1/3 discovery rate.

Polar mesosphere summer echo strength in relation to solar variability and geomagnetic activity during 1997–2009

Abstract. This paper is based on measurements of Polar Mesosphere Summer Echoes (PMSE) with the 52 MHz radar ESRAD, located near Kiruna, in Northern Sweden, during the summers of 1997–2009. Here, a new independent calibration method allowing estimation of possible changes in antenna feed losses and transmitter output is described and implemented for accurate calculation of year-to-year variations of PMSE strength (expressed in absolute units – radar volume reflectivity η). The method is based on radar-radiosonde comparisons in the upper troposphere/lower stratosphere region simultaneously with PMSE observations. Inter-annual variations of PMSE volume reflectivity are found to be strongly positively correlated with the local geomagnetic K-index, both when averaged over all times of the day, and when considering 3-h UT intervals separately. Increased electron density due to energetic particle precipitation from the magnetosphere is suggested as one of the possible reasons for such a correlation. Enhanced ionospheric electric field may be another reason but this requires further study. Multi-regression analysis of inter-annual variations of PMSE η shows also an anti-correlation with solar 10.7 cm flux and the absence of any statistically significant trend in PMSE strength over the interval considered (13-years). Variations related to solar flux and K-index account for 86% of the year-to-year variations in radar volume reflectivity.

Validation of primary hydrophone calibrations by inter‐laboratory comparisons and by independent calibration methods

A description is presented of two approaches which may be used to validate primary calibration methods for hydrophones and transducers. Firstly, a comparison may be made with another independent absolute calibration method, preferably one based on a different physical principle (and therefore with few common sources of uncertainty). Secondly, an inter‐laboratory comparison of calibrations may be undertaken between different institutes operating at a similar level. This paper describes the results of such exercises for free‐field calibration of hydrophones in the range from 1 kHz to 500 kHz. Firstly, two independent calibration methods are compared: the three‐transducer reciprocity method and a method based on optical interferometry. The differences observed in the results are typically less than 0.5 dB, which is of the same order as the overall uncertainties of each of the methods. Secondly, the results are shown of a recent international comparison of hydrophone calibrations involving institutes from Canada, China, Germany, Russia, South Africa, UK, and USA. Here, the agreement was generally within quoted uncertainties, the results generally lying within a ±0.5 dB band for frequencies up to 300 kHz. A discussion is given of the general sources of uncertainties in the calibrations.

Anisotropy, cracking, and shrinkage of vertisol samples Experimental study and shrinkage modeling

Measuring and modelling the soil shrinkage curve (ShC) has generated increased interest, but uncertainty in shrinkage curve parameters has not been documented, and the most recent models have not been compared. This study was performed on 11 core samples collected in the same horizon of a vertisol. The objectives of the study were to (i) characterize the anisotropy of soil samples shrinkage by comparison of vertical, horizontal and volume shrinkage, (ii) analyse the impact of shrinkage anisotropy on fitted shrinkage parameters, (iii) determine uncertainty in shrinkage parameters, and (iv) compare the plasma pore volume as modelled by the clay-paste model (CP) to the plasma pore volume as estimated with the soil shrinkage XP model. The coefficients of variation of the different ShC parameters were smaller than 12% and 3% for water contents and volumes, respectively. Cracking led us to exclude some vertical ShC for XP model fitting, as the ShC departed strongly from the S-Shape model. The vertical geometry factor was smaller than 3 and the horizontal geometry factor was larger than 3 in average. The geometry factor values were changing with water content; they deviated widely from 3 at water saturation, but remained constant and close to 3 in between air entry (AE) and maximum swelling (MS). Cracking and anisotropy had no impact on the XP fitted shrinkage parameters. They were not significantly different between one-dimensional (1-D) and volume measurements. The results of our study suggest that parameters from 1-D shrinkage might be adequate to describe three-dimensional shrinkage. The XP plasma pore volumes at shrinkage limit (SL) were in the range of mercury porosimetry values, the average fitted plasma volume was only 1.7% larger than the median measured value. Direct calibration of CP model from independently measured parameters gave water contents at SL and AE one order of magnitude smaller and two times larger than the fitted values, respectively. But when the AE point water content was set equal in the CP and the XP model, the two modelled plasma ShC were almost identical, with differences depending only on the small difference between fitted and measured plasma volume at SL. Thus, the CP model could be used instead of XP-plasma model to describe the plasma ShC, provided that AE points were identical. This is, however, contradictory with the proposed independent calibration method. The present results are encouraging regarding ShC determination and the comparability of shrinkage modelling approaches. Further comparison of the models requires additional experimental information on the soil plasma shrinkage, especially on MS point.

The controlling chemistry of surface deposition from sodium and potassium seeded flames free of sulfur or chlorine impurities

Sodium and potassium salt deposition have been studied in a series of propane and hydrogen flames free of sulfur or halogen impurities. With the collection probe in the 400 to 800 K range, samples of pure carbonate are observed and more importantly the rates of, for example, sodium carbonate deposition measured in terms of alkali metal are identical to those previously reported for sodium sulfate formation and also those observed for dominant NaCl deposition. Moreover, the behavior of Na 2CO 3 deposition mirrors exactly that of Na 2SO 4 in this temperature range. It shows a corresponding first order dependence on flame total sodium concentration, a zero order dependence on flame carbon, an insensitivity to fuel type, equivalence ratio, flame temperature, flow rate, probe material, or the nature of the sodium speciation in the flame, be it atomic or the hydroxide, or the state of the flame equilibration. A constant rate of deposition between 330 and 800 K conveys formation kinetics with a zero activation energy and that the surface accommodates atomic sodium equally well, be it below or above its dew point temperature and also at a seemingly approximately equal rate to that of flame NaOH. The fact that Na 2CO 3 cannot exist in the gaseous state in a flame finally proves irrefutably that these alkali deposition processes producing sulfate, carbonate or halide salts are heterogeneous in nature. The high collection efficiencies of the surface for alkalis have been confirmed by a further independent new calibration method for flame total alkali content. Also deposition rates are seen to be extremely similar in C 3H 8 /O 2 flames heavily diluted with either He, Ne, or Ar and also in a very fuel rich H 2 or D 2 flame. As with sulfate deposition, the rate of deposition is predominantly controlled by the actual flux of alkali in the flame gases that are intercepted by the collection probe. Moreover, there is an insensitivity to probe geometry and the nature of the flame flowfield, be it laminar or turbulent. The theoretical understanding of the complex boundary layer penetration and deposition mechanism is still inadequate in explaining these observations. The most intriguing results and differences from sulfate deposition have been observed on probes at lower temperatures (330–370 K). Although the formation of NaHCO 3, and more so KHCO 3, was expected to compete with that of their carbonates, in the case of sodium under fuel lean conditions only a small competing contribution of NaNO 3 formation was noted. This was very marginal for fuel rich conditions. However, with potassium the effects were enhanced and KNO 3 competes significantly with K 2CO 3 under fuel lean conditions. However, in fuel rich flames an unexpected dominant formation of potassium oxalate, K 2C 2O 4, was observed, along with some K 2CO 3 and a small amount of KHCO 3. Thermodynamic expectations in this lower temperature regime tend to suggest nitrate>bicarbonate>carbonate>oxalate. This is our first clearly observed non-equilibrium deposition behavior where the flame begins to display a pivotal role in controlling the surface molecular distribution. It also raises the possibility that low temperature surfaces in flames may be a new route for synthesizing certain thermodynamically metastable materials.

Charged device model metrology: limitations and problems

The inconsistent readings of various charged device model (CDM) test heads indicates severe metrology problems exist. Test head-to-test head response times vary by factors of two to three and no independent calibration method exists. CDM waveforms depend upon the total measurement system. This paper discusses the problems and methods necessary to the accurate capture of CDM waveforms.

Applicability of the Compound Independent Calibration Method for the Chromatographic Quantitation of Trihalomethanes with Atomic Emission Detection

The applicability of the compound independent calibration method for the chromatographic analysis of four trihalomethanes (THM) after detection with a Microwave Induced Helium Plasma Atomic Emission Detector (MIP) was studied. The approach used was based on comparison of calibration curves constructed for the THM after monitoring the MIP signal at a carbon emission line and a chlorine emission line. The results of these studies also allowed evaluation of the usefulness of the MIP detector for the analysis of the THM at typical concentration levels of environmental concern.

Reflectance- and radiance-based methods for the in-flight absolute calibration of multispectral sensors

Variations reported in the in-flight absolute radiometric calibration of the Coastal Zone Color Scanner (CZCS) and the Thematic Mapper (TM) on Landsat 4 are reviewed. At short wavelengths these sensors exhibited a gradual reduction in response, while in the midinfrared the TM showed oscillatory variations, according to the results of TM internal calibration. The methodology and results are presented for five reflectance-based calibrations of the Landsat 5 TM at White Sands, NM, in the period July 1984 to November 1985. These show a ±2.8% standard deviation (1 σ) for the six solar-reflective bands. Analysis and preliminary results of a second, independent calibration method based on radiance measurements from a helicopter at White Sands indicate that this is potentially an accurate method for corroborating the results from the reflectance-based method.