Spaceborne estimate of atmospheric CO2 column by use of the differential absorption method: error analysis

For better knowledge of the carbon cycle, there is a need for spaceborne measurements of atmospheric CO2 concentration. Because the gradients are relatively small, the accuracy requirements are better than 1%. We analyze the feasibility of a CO2-weighted-column estimate, using the differential absorption technique, from high-resolution spectroscopic measurements in the 1.6- and 2-microm CO2 absorption bands. Several sources of uncertainty that can be neglected for other gases with less stringent accuracy requirements need to be assessed. We attempt a quantification of errors due to the radiometric noise, uncertainties in the temperature, humidity and surface pressure uncertainty, spectroscopic coefficients, and atmospheric scattering. Atmospheric scattering is the major source of error [5 parts per 10 (ppm) for a subvisual cirrus cloud with an assumed optical thickness of 0.03], and additional research is needed to properly assess the accuracy of correction methods. Spectroscopic data are currently a major source of uncertainty but can be improved with specific ground-based sunphotometry measurements. The other sources of error amount to several ppm, which is less than, but close to, the accuracy requirements. Fortunately, these errors are mostly random and will therefore be reduced by proper averaging.     

Estimating random errors due to shot noise in backscatter lidar observations

We discuss the estimation of random errors due to shot noise in backscatter lidar observations that use either photomultiplier tube (PMT) or avalanche photodiode (APD) detectors. The statistical characteristics of photodetection are reviewed, and photon count distributions of solar background signals and laser backscatter signals are examined using airborne lidar observations at 532 nm using a photon-counting mode APD. Both distributions appear to be Poisson, indicating that the arrival at the photodetector of photons for these signals is a Poisson stochastic process. For Poisson- distributed signals, a proportional, one-to-one relationship is known to exist between the mean of a distribution and its variance. Although the multiplied photocurrent no longer follows a strict Poisson distribution in analog-mode APD and PMT detectors, the proportionality still exists between the mean and the variance of the multiplied photocurrent. We make use of this relationship by introducing the noise scale factor (NSF), which quantifies the constant of proportionality that exists between the root mean square of the random noise in a measurement and the square root of the mean signal. Using the NSF to estimate random errors in lidar measurements due to shot noise provides a significant advantage over the conventional error estimation techniques, in that with the NSF, uncertainties can be reliably calculated from or for a single data sample. Methods for evaluating the NSF are presented. Algorithms to compute the NSF are developed for the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations lidar and tested using data from the Lidar In-space Technology Experiment.     

Drying/hydration in cement pastes during curing

As concrete cures in the field, there is a constant competition for the mixing water between evaporation and hydration processes. Understanding the mechanisms of water movement in the drying/hydrating cement paste is critical for designing curing systems and specialized rendering materials, as well as for selecting repair materials and methodologies. In this work, X-ray absorption measurements indicate that fresh cement paste dries uniformly throughout its thickness, as opposed to exhibiting the sharp drying front observed for most porous materials. Furthermore, in layered composite cement paste specimens, water always flows from the coarser-pore layer to the finer one, both when coarser pores are produced by using an increased water-to-cement ratio (w/c) and when they are present due to using a cement with a coarser particle size distribution at a constant w/c. Conversely, no clear differential water movement is observed between layers of cement paste and mortar of the same nominal w/c. Based on the results of these experiments, drying has been introduced into the NIST CEMHYD3D cement hydration and microstructure development model, by emptying the largest water-filled pores present at any depth in the model specimen at a user-specified (drying) rate. With this addition, the CEMHYD3D model produces results in good agreement with experimental observations of both the drying profiles and the hydration kinetics of thin cement paste specimens.     

On the relation of setting and early-age strength development to porosity and hydration in cement-based materials

Previous research has demonstrated a linear relationship between compressive strength (mortar cubes and concrete cylinders) and cumulative heat release normalized per unit volume of (mixing) water for a wide variety of cement-based mixtures at ages of 1 d and beyond. This paper utilizes concurrent ultrasonic reflection and calorimetry measurements to further explore this relationship from the time of specimen casting to 3 d. The ultrasonic measurements permit a continuous evaluation of thickening, setting, and strength development during this time period for comparison with the ongoing chemical reactions, as characterized by isothermal calorimetry measurements. Initially, the ultrasonic strength-heat release relation depends strongly on water-to-cement ratio, as well as admixture additions, with no universal behavior. Still, each individual strength-heat release curve is consistent with a percolation-based view of the cement setting process. However, beyond about 8 h for the systems investigated in the present study, the various strength-heat release curves merge towards a single relationship that broadly characterizes the development of strength as a function of heat released (fractional space filled), demonstrating that mortar and/or concrete strength at early ages can be effectively monitored using either ultrasonic or calorimetry measurements on small paste or mortar specimens.     

High Count Rate Electron Probe Microanalysis

Reducing the measurement uncertainty of quantitative analyses made using electron probe microanalyzers (EPMA) requires a careful study of the individual uncertainties from each definable step of the measurement. Those steps include measuring the incident electron beam current and voltage, knowing the angle between the electron beam and the sample (takeoff angle), collecting the emitted x rays from the sample, comparing the emitted x-ray flux to known standards (to determine the k-ratio) and transformation of the k-ratio to concentration using algorithms which includes, as a minimum, the atomic number, absorption, and fluorescence corrections.This paper discusses the collection and counting of the emitted x rays, which are diffracted into the gas flow or sealed proportional x-ray detectors. The representation of the uncertainty in the number of collected x rays collected reduces as the number of counts increase. The uncertainty of the collected signal is fully described by Poisson statistics. Increasing the number of x rays collected involves either counting longer or at a higher counting rate. Counting longer means the analysis time increases and may become excessive to get to the desired uncertainty. Instrument drift also becomes an issue. Counting at higher rates has its limitations, which are a function of the detector physics and the detecting electronics.Since the beginning of EPMA analysis, analog electronics have been used to amplify and discriminate the x-ray induced ionizations within the proportional counter. This paper will discuss the use of digital electronics for this purpose. These electronics are similar to that used for energy dispersive analysis of x rays with either Si(Li) or Ge(Li) detectors except that the shaping time constants are much smaller.     

Ultrasensitive near-infrared integrated cavity output spectroscopy technique for detection of CO at 1.57 microm: new sensitivity limits for absorption measurements in passive optical cavities

A robust absorption spectrometer using the off-axis integrated cavity output spectroscopy (ICOS) technique in a passive cavity is presented. The observed sensitivity, conceptually the detection threshold for the absorption cross section (cm2) multiplied by the concentration (cm(-3)) and normalized by the averaging time, is measured to be 1.9 x 10(-12) (1/cm square root of Hz). This high sensitivity arises from using the optical cavity to amplify the observed path length in the spectrometer while avoiding cavity resonances by careful design of the spot pattern within the cavity. The instrument is ideally suited for routine monitoring of trace gases in the near-infrared region. A spectrum showing ambient carbon monoxide at 1.57 microm is presented.     

Improvement of Scanning Radiometer Performance by Digital Reference Averaging

Most radiometers utilize a calibration technique in which measurements of a known reference are subtracted from measurements of an unknown source so that common-mode bias errors are cancelled. When a radiometer is scanned over a varying scene, it produces a sequence of outputs, each being proportional to the difference between the reference and the corresponding input. The reference averaging technique presented herein employs a simple digital algorithm which exploits the asymmetry between the time-variable scene inputs and the nominally constant reference input by averaging many reference measurements to decrease the statistical uncertainty in the reference value. This algorithm is, therefore, optimized by an asymmetric chopping sequence in which the scene is viewed for more than one-half of the duty cycle (unlike the analog Dicke technique) Reference averaging algorithms are well within the capabilities of small microprocessors. Although this paper develops the technique for microwave radiometry, it may be beneficial for any system which measures a large number of unknowns relative to a known reference in the presence of slowly varying common-mode errors.     

Trace moisture detection using continuous-wave cavity ring-down spectroscopy

We have developed an instrument to measure trace concentrations of small hydride species in gases using continuous-wave cavity ring-down spectroscopy with near-infrared diode laser excitation. An rms baseline equivalent absorbance of 9.2 x 10(-11) cm(-1)/square root(n) is found, where n is the number of ring-down transients. When the 1396.376-nm absorption line of water is used, this corresponds to a noise equivalent moisture concentration in nitrogen gas of 68 pptv/square root(n). Water vapor concentration is detected over a range extending from 3 to 1000 ppbv and found to depend linearly on the concentration as determined by a calibrated commercial moisture sensor.     

Autonomous calibration method of the reference flat surface of an interferometer without using a standard flat surface

An autonomous method for calibrating the reference flat surface of an interferometer is proposed with the uncertainty analysis. The method consists of three phases; the first step is multiple rotating shifts of a specimen, the second is a linear shift, and the last is multiple rotating shifts again. The profile of the reference flat surface is basically determined by the linear shift. The linear shift errors that occurred during the linear shift are identified by the rotating shifts. The rotating shift errors caused by the rotating shifts can be compensated and the residual uncertainty can be reduced in proportion to the square root of the number of rotating shifts per one revolution. Finally, the uncertainty analysis is carried out in detail.     

Effects of cryogenic grinding on soft-tissue optical properties

Optical properties obtained from spectrophotometer measurements of reflectance and transmittance were determined for both frozen-ground and intact soft tissues. The tissues used in these experiments were calf aorta, rat jejunum, and rabbit sciatic nerve. Tissue specimens from each tissue type were frozen in liquid nitrogen and then ground with a pestle and mortar into a fine powder. A tissue paste formed once the powder returned to room temperature. The tissue paste was then sandwiched between glass slides for spectrophotometer measurements. For comparison, the optical properties of the intact specimens were also measured. Total transmission and diffuse reflection were obtained on a Varian Cary 5E spectrophotometer (400-850 nm). Absorption and reduced scattering coefficients of the tissues were determined with the Inverse Adding Doubling method. Our results suggested that within the 400-nm to 850-nm spectrum, optical properties of the ground tissue approximated intact tissue within limits of experimental error.     

Method to test rotationally asymmetric surface deviation with high accuracy

We have proposed a new absolute method to test rotationally asymmetric surface deviation. Relying on the high accuracy of Zernike polynomial fitting with least-squares algorithm for the low-frequency component and preserving the high-frequency component with the averaging method, the new method can guarantee the high accuracy of the measurement result with fewer rotational measurements compared to the traditional multiangle averaging method. It realizes a balance between the accuracy and efficiency of the measurements. It has been verified by experiments; the root mean square (rms) of residual figure between the two methods is ～0.6 nm. Meanwhile, the new method can suppress environmental noise introduced in measurement results well.     

Electrical conductivity of drying cement paste

Previous research has shown that electrical measurements can be used to monitor moisture movement inside concrete. The interpretation of these measurements is frequently based on empirical relationships between moisture changes and electrical properties of concrete. As such, these empirical relationships can limit the application of the electrical measurements to a specific material or exposure history. To facilitate the development of a general method that is applicable to a concrete member in service, this paper characterizes the electrical conduction in cement paste subjected to drying (desorption) and moisture absorption. The paper quantifies how the electrical conductivity is dependent on the volume and connectivity of the moisture inside the pores and the conductivity of pore solution. This paper also presents a procedure to quantify the contribution of the surface (solid-pore) conduction on the overall conductivity of the cement paste. The results of this investigation contribute to the development of an embedded relative humidity sensor that can be used to monitor changes in the internal humidity of concrete during its service life.     

The effects of CaCl<Subscript>2</Subscript>-blended acrylic polymer emulsion on the properties of cement mortar

Although acrylic polymer emulsions have been reported to impart many desirable attributes to cement mortar; delayed hydration, excessive air entrapment and moisture induced loss of strength have been highlighted as constraints. This paper explores the utilization of hydrated calcium chloride blended-acrylic polymer emulsion (CP) as a mitigation measure to these aforementioned drawbacks. First, the effects of 0, 0.5, 1.0 and 1.5% of CP by mass of cement on the early-age cement paste hydration and mortar flow were investigated. Thereafter, the influence of CP on the hardened porosity, moist-cured compressive strength, initial rate of capillary water absorption and rapid chloride permeability (RCPT) were evaluated. Test results indicate that the addition of CP to pastes sped up the cement hydration process, accelerating the final setting time of pastes by approximately 0.5–1.5 h as the CP content of pastes increased. Moreover, CP slightly increased the flow of fresh mortar, the hardened porosity of mortar mixtures containing 0.5 and 1.0% CP were also comparable to those of the plain reference mortar. With the exception of the 1.5% CP blended mortar, the 14 days moist-cured compressive strength of 0.5–1.0% CP blended mortar mixtures were also comparable to that of the plain reference mixture. Relative to the reference mixture, the addition of CP to mortar reduced the initial rate of capillary water absorption of mortar, with the mixture containing 1.5% CP giving a maximum reduction of 23%. Conversely, RCPT results indicate that above 0.5% CP addition level, CP generally increased the electrical conductivity of mixtures.     

Statistical treatment of photon/electron counting: extending the linear dynamic range from the dark count rate to saturation

An experimentally simple photon counting method is demonstrated providing 7 orders of magnitude in linear dynamic range (LDR) for a single photomultiplier tube (PMT) detector. In conventional photon/electron counting methods, the linear range is dictated by the agreement between the binomially distributed measurement of counted events and the underlying Poisson distribution of photons/electrons. By explicitly considering the log-normal probability distribution in voltage transients as a function of the number of photons present and the Poisson distribution of photons, observed counts for a given threshold can be related to the mean number of photons well beyond the conventional limit. Analytical expressions are derived relating counts and photons that extend the linear range to an average of ～11 photons arriving simultaneously with a single threshold. These expressions can be evaluated numerically for multiple thresholds extending the linear range to the saturation point of the PMT. The peak voltage distributions are experimentally shown to follow a Poisson weighted sum of log-normal distributions that can all be derived from the single photoelectron voltage peak-height distribution. The LDR that results from this method is compared to conventional single photon counting (SPC) and to signal averaging by analog to digital conversion (ADC).     

橡胶混凝土的抗裂性能和弯曲变形性能

通过净浆、砂浆和混凝土3 个层面的试验研究, 探讨了橡胶混凝土的抗裂性能和弯曲变形特性。根据圆环试件的开裂时间, 对3 种水灰比( 0. 3, 0. 4, 0. 5) 水泥净浆中掺入不同体积含量橡胶颗粒( 10%, 15%, 20%,30% , 50%) 后的开裂敏感性进行了对比分析。结果表明, 在水泥净浆中掺入橡胶颗粒可以显著延缓试件的开裂时间, 提高抗裂性, 延缓时间随着橡胶颗粒掺量的增加和水灰比的增大而延长。弯曲试验考察了4 个橡胶颗粒体积含量( 8%, 16%, 21. 4%, 32%) 砂浆试件和3 个橡胶颗粒体积含量( 10%, 12. 5%, 15%) 混凝土试件的弯曲变形性能。结果表明, 含橡胶颗粒的砂浆试件和混凝土试件在弯曲过程中会产生明显的塑性变形, 试件不会在承受最大荷载时产生脆性断裂, 而是经过较大的塑性变形后延性破坏。与基准试件相比, 砂浆试件和混凝土试件破坏时的极限变形值均有大幅度提高。      

线性度评价结果的不确定度

目的 介绍了用最小二乘法评价线性测量系统性度、直流增益和直流偏移几项指标时,测量结果的不确定度分析和评价过程。方法 讨论了影响评价结果不确定度的几个主要误差来源,包括信号源误差、被校系统测量误差、被校系统增益影响、量程范围内输入信号数值个数、环境影响等等,给出了减小线性度、直流增益和直流偏移几项指标评价不确定度的主要措施：（１）环境控制措施,以减少环境带来的不确定度;（２）选取高精度信号,以减少信     

The effects of pre and post construction moisture condition on the in-plane and out-of-plane strengths of brick walls

An important factor affecting the strength of a masonry wall is the capacity of the bond between the mortar and the bricks to transfer the loads within the wall. The main parameter influencing the bond strength is the moisture content of the brick units and the consistency of mortar paste at the time of construction. Another important, but little known, factor is the post-construction moisture curing of the wall. In this paper, the results of a series of standard tests, aimed at determining the effects of moisture condition on the flexural strength, direct tensile bond strength, shear bond strength, compressive strength and diagonal shear strength of brick masonry are presented. The results indicate a substantial increase in the bond strength when saturated, surface dry, brick units are used and show the favorable effects of post-construction water curing of brickwork. Also, in-plane shear and out-of-plane bending tests are carried out on a number of brick wallets and infills, constructed with different pre and post construction moisture conditions and results are compared to drive at quantitative conclusions on the effects of pre-wetting the brick units at the time of construction and post-construction moisture curing on the behavior and strength properties of brick wall construction.     

数字平均法的误差分析

文章针对以前提出的声时测量中的随机数字平均技术，给出了其理论依据，详细讨论了误差的大小，并利用概率统计理论进行了论证。同时，文中还提出均匀延时数字平均技术。在实际应用中，对该类仪器的测量误差的估计和测量次数的选取以及这类仪器的设计有着重要的指导作用。     

Error and Transient Analysis of Stepwise-Approximated Sine Waves Generated by Programmable Josephson Voltage Standards

We are developing a quantum-based 60 Hz power standard that exploits the precision sinusoidal reference voltages synthesized by a programmable Josephson voltage standard (PJVS). PJVS systems use series arrays of Josephson junctions as a multibit digital-to-analog converter to produce accurate quantum-based dc voltages. Using stepwise-approximation synthesis, the system can also generate arbitrary ac waveforms [i.e., an ac programmable Josephson voltage standard (ACPJVS)] and, in this application, produces sine waves with calculable root mean square (rms) voltage and spectral content. The primary drawback to this ACPJVS synthesis technique is the uncertainty that results from switching between the discrete voltages due to finite rise times and transient signals. In this paper, we present measurements and simulations that elucidate some of the error sources that are intrinsic to the ACPJVS when used for rms measurements. In particular, we consider sine waves synthesized at frequencies up to the audio range, where the effect of these errors is more easily measured because the fixed transition time becomes a greater fraction of the time in each quantized voltage state. Our goal for the power standard is to reduce all error sources and uncertainty contributions from the PJVS-synthesized waveforms at 60 Hz to a few parts in 10⁷ so that the overall uncertainty in an ac power standard will be a few parts in 10⁶.     

Spline function approximation for velocimeter Doppler frequencymeasurement

A spline function approximation approach for measuring the Doppler spectral peak frequency in a laser Doppler velocimeter system is presented. The processor is designed for signal bursts with mean Doppler shift frequencies up to 100 MHz, input turbulence up to 20%, and photon counts as low as 300. The frequency-domain processor uses a bank of digital bandpass filters for the capture of the energy spectrum of each signal burst. The average values of the filter output energies, as a function of normalized frequency, are modeled as deterministic spline functions which are linearly weighted to evaluate the spectral peak location associated with the Doppler shift. The weighting coefficients are chosen to minimize the mean square error. Performance evaluation by simulation yields average errors in estimating mean Doppler frequencies within 0.5% for poor signal-to-noise conditions associated with a low photon count of 300 photons/burst