Calculated Calibration Factors Research Articles

Numerical study on the effect of circumferential position of ultrasonic transducers on ultrasonic cross-correlation flowmeter performance under asymmetric air flow profile

There are several ultrasonic flowmeter technologies such as transient time, cross-correlation, Doppler and etc. The performance of different types of ultrasonic flowmeters under disturbed flow profile due to elbow at upstream has been well studied recently except the cross-correlation type. The ultrasonic cross-correlation flowmeter (UCCF) has different mechanism of flow measurement in comparison with other types of ultrasonic flowmeters that strongly requires its own studies. In case of disturbed flow profile in pipe, due to asymmetrical velocity profile, the UCCF accuracy and required calibration factor are strongly dependent on the circumferential position of ultrasonic transducers on the pipe. Study on UCCF transducers circumferential position and specifying transducers proper position could incredibly improve the UCCF accuracy. The effect of transducers circumferential position on the accuracy in both single path and multipath has not been investigated for a UCCF so far. In this work, Computational Fluid Dynamics (CFD) simulations were conducted using turbulence model of RSM (Reynolds Stress Model) Stress-ω to simulate the asymmetrical air flow profile inside the elbow and the connected straight pipe in order to study the effect of circumferential position of transducers on calibration factor for special type of ultrasonic flowmeters ,UCCF, at different Reynolds numbers and various locations of flowmeter, for the first time. Moreover, the effects of Reynolds numbers and the flowmeter distances from the elbow on the calibration factor were analyzed. Finally, based on calculated calibration factors at different transducer circumferential positions, the best transducers arrangement at different flowmeter locations were proposed for single and multipath cross-correlation flowmeter. Also, in multipath ultrasonic cross correlation flowmeter, the optimum number of transducers were investigated.

Novel coincidence setup using indigenously developed portable USB gamma spectrometer and associated analysis software

We present the development of a compact gamma spectrometer that is also capable of coincidence measurements, by connecting two units by a cable. The spectrometer consists of a CsI scintillator mated to a PIN photo-diode as the detector, a charge sensitive pre-amplifier, a shaping amplifier, and a 1024 bin Multi-Channel analyser. The assembly fits on a single PCB of dimensions 55mm x 105mm enclosed in a compact aluminum box. The USB powered plug and play design of the spectrometer makes it a highly portable surveying tool. While the pre-amplifier and shaping amplifier circuits are derived from schematics available in the open source domain, the MCA has been custom built. The data acquisition, analysis cum visualization software has been written in Python and is open sourced. A dual parameter list mode MCA capable of recording time stamped energy data from each of the individual gamma spectrometers including identification of coincident events was also developed for energy-gated coincidence measurements, and algorithms to visualize heat maps and surface plots of the obtained list mode data have been incorporated into the software.An FWHM of 81.2 keV was measured for the 1332 keV peak of 60Co. Two point calibration using the 1173 keV and 1332 keV peaks was carried out. The calculated calibration factors are applied to the 661.7 keV peak of 137Cs. The measured peak is at 661.3 keV, corresponding to 0.1% accuracy. However, a full-range linearity testing with a commercial pulser showed a non-linearity with a maximum value of 0.6%. The dead time of the electronics was measured using a pulse generator, and was found to be less than 20 microseconds. Photopeak efficiency of the detector measured with the 661.7 keV peak of 137Cs was found to be around 29%.In a novel approach, the MCA firmware has been modified to demonstrate non-energy gated gamma-gamma coincidence with no additional circuitry. The coincidence filtering has been verified with a 1μC 22Na source using a timing window of 400 ns.

Detector calibration and measurement issues in multi-color time-resolved laser-induced incandescence

Time-resolved laser-induced incandescence is used to infer the size distribution of gas-borne nanoparticles from time-resolved pyrometric measurements of the particle temperature after pulsed laser heating. The method is highly sensitive to aspects of the measurement strategy that are often not considered by practitioners, which often lead to discrepancies between measurements carried out under nominally identical conditions. This paper therefore presents a well-documented calibration procedure for LII systems and quantifies the uncertainty in pyrometric temperatures introduced by this procedure. Calibration steps include corrections for: (1) signal baseline, (2) variable transmission through optical components, and (3) detector characteristics (i.e., gain and spectral sensitivity). Candidate light sources are assessed for their suitability as a calibration reference and the uncertainty in calculated calibration factors is determined. The error analysis is demonstrated using LII measurements made on a sooting laminar diffusion flame. We present results for temperature traces of laser-heated particles determined using two- and multi-color detection techniques and discuss the temperature differences for various combinations of spectral detection channels. We also summarize measurement artifacts that could bias the LII signal processing and present strategies for error identification and prevention.

Absolute isotope ratios – Analytical solution for the determination of calibration factors for any number of isotopes and isotopologues

Absolute isotope ratios cannot be determined directly by mass spectrometry. The measured ion intensities are biased by many effects like mass fractionation or discrimination and amplifier gain. To correct the ion intensities, calibration factors are needed. These calibration factors can be derived by comparing the known isotope ratios of a reference material and its measured intensity ratios, provided that these materials have been characterized traceable to the International System of Units (SI). As mentioned above, absolute isotope ratios are not directly available but are necessary for calibration. Thus, if there is no well-characterized reference material or it is out of stock, another solution for this problem must be found. Nier [Nier, 1950] already presented a solution for this problem through gravimetrically prepared mixtures back in 1950. Mana and Rienitz [Mana and Rienitz, 2010] derived an analytical solution for the needed calibration factors for a system of three isotopes from this idea. Until then, calibration factors could only be determined iteratively. This paper shows how calibration factors can be derived from gravimetrically prepared mixtures for a system of any number of isotopes. In contrast to iterative methods, the method presented here is not affected by possible poor convergence or local minima. The subsequent uncertainty propagation is much more straightforward being another big advantage. Two examples are given to demonstrate the overall validity of this approach. In the first example, this approach is applied to the seven-isotope system of mercury. In the second, using carbon dioxide, it is shown that this method can also be applied to a system of twelve isotopologues. Additionally, we present an EXCEL® application GIMiCK, which allows every user to calculate calibration factors for any number of isotopes and isotopologues.

Simulation of internal contamination screening with dose rate meters

Assessing the intake of radionuclides after an accident in a nuclear power plant or after the intentional release of radionuclides in public places allows dose calculations and triage actions to be carried out for members of the public and for emergency response teams. Gamma emitters in the lung, thyroid or the whole body may be detected and quantified by making dose rate measurements at the surface of the internally contaminated person. In an accident scenario, quick measurements made with readily available portable equipment are a key factor for success. In this paper, the Monte Carlo program Visual Monte Carlo (VMC) and MCNPx code are used in conjunction with voxel phantoms to calculate the dose rate at the surface of a contaminated person due to internally deposited radionuclides. A whole body contamination with 137Cs and a thyroid contamination with 131I were simulated and the calibration factors in kBq per µSv/h were calculated. The calculated calibration factors were compared with real data obtained from the Goiania accident in the case of 137Cs and the Chernobyl accident in terms of the 131I. The close comparison of the calculated and real measurements indicates that the method may be applied to other radionuclides. Minimum detectable activities are discussed.

Energy dependence and angular dependence of an optically stimulated luminescence dosimeter in the mammography energy range.

This study aimed to investigate the energy dependence and the angular dependence of commercially available optically stimulated luminescence (OSL) point dosimeters in the mammography energy range. The energy dependence was evaluated to calculate calibration factors (CFs). The half‐value layer range was 0.31–0.60 mmAl (Mo/Mo 22–28 kV, Mo/Rh 28–32 kV, and W/Rh 30–34 kV at 2‐kV intervals). Mo/Rh 28 kV was the reference condition. Angular dependence was tested by rotating the X‐ray tube from −90° to 90° in 30° increments, and signal counts from angled nanoDots were normalized to the 0° signal counts. Angular dependence was compared with three tube voltage and target/filter combinations (Mo/Mo 26 kV, Mo/Rh 28 kV and W/Rh 32 kV). The CFs of energy dependence were 0.94–1.06. In Mo/Mo 26–28 kV and Mo/Rh 28–32 kV, the range of CF was 0.99–1.01, which was very similar. For angular dependence, the most deteriorated normalized values (Mo/Mo, 0.37; Mo/Rh, 0.43; and W/Rh, 0.58) were observed when the X‐ray tube was rotated at a 90° angle, compared to 0°. The most angular dependences of ± 30°, 60°, and 90° decreased by approximately 4%, 14%, and 63% respectively. The mean deteriorated measurement 30° intervals from 0° to ± 30° was 2%, from ± 30° to ± 60° was 8%, and from ± 60° to ± 90° was 40%. The range of energy dependence in typical mammography energy range was not as much as that in general radiography and computed tomography. For accurate measurement using nanoDot, the tilt needs to be under 30°.

Estimation of mean pore-size using formation evaluation and Stoneley slowness

Continuous estimation of pore-size can be very helpful to use in optimization of well completion and effective production of hydrocarbon reservoirs. This paper shows the application of two new empirical models derived of the Kozeny-Carman equation to estimate the mean pore-size. The models are: The formation evaluation model, and the Stoneley slowness model. The main objective of this work is to give a general framework to develop the number of methods that provide a continuous estimation of formation mean pore-size along the well.In the final form of the derived equations, the calibration factors for minerals were the only constants that had to be determined using a calibration method. Therefore, the proposed models were calibrated with core data using the least squares method. Fortunately, all the required core data were accessible to be used for the calibration process. Moreover, the effects of some parameters such as the shape factor were ignored. The shape factor value is influenced by the pore shape. The calibrated models were then used to re-compute the mean pore-size in the logged intervals. The results showed that, in many points, there was a relatively low difference between the estimated mean pore-size and the core pore-size. According to the calculated calibration factors for two methods, illite had a very negative effect upon the pore-size values among all the minerals. The proposed models could be used as an index for determining the permeable and non-permeable layers. As a result of the calibration of the proposed models (particularly for the Stonely model), the RMSE values calculated for the permeabilities of less than 100 mD were lower than that of the permeabilities of greater than 100 mD. The least RMSE values were calculated for the permeabilities ranged in 10–100 mD. Moreover, the simultaneous assessments of the presented pore-size models, and comparing their results with the core data, helped us to evaluate the correctness of the calculated pore-size as well as the permeability.

A novel approach for absolute radar calibration: formulation and theoretical validation

Abstract. The theoretical framework of a novel approach for absolute radar calibration is presented and its potential analysed by means of synthetic data to lay out a solid basis for future practical application. The method presents the advantage of an absolute calibration with respect to the directly measured reflectivity, without needing a previously calibrated reference device. It requires a setup comprising three radars: two devices oriented towards each other, measuring reflectivity along the same horizontal beam and operating within a strongly attenuated frequency range (e.g. K or X band), and one vertical reflectivity and drop size distribution (DSD) profiler below this connecting line, which is to be calibrated. The absolute determination of the calibration factor is based on attenuation estimates. Using synthetic, smooth and geometrically idealised data, calibration is found to perform best using homogeneous precipitation events with rain rates high enough to ensure a distinct attenuation signal (reflectivity above ca. 30 dBZ). Furthermore, the choice of the interval width (in measuring range gates) around the vertically pointing radar, needed for attenuation estimation, is found to have an impact on the calibration results. Further analysis is done by means of synthetic data with realistic, inhomogeneous precipitation fields taken from measurements. A calibration factor is calculated for each considered case using the presented method. Based on the distribution of the calculated calibration factors, the most probable value is determined by estimating the mode of a fitted shifted logarithmic normal distribution function. After filtering the data set with respect to rain rate and inhomogeneity and choosing an appropriate length of the considered attenuation path, the estimated uncertainty of the calibration factor is of the order of 1 to 11 %, depending on the chosen interval width. Considering stability and accuracy of the method, an interval of eight range gates on both sides of the vertically pointing radar is most appropriate for calibration in the presented setup.

An improved method of Lambertian CCD-camera radiation flux measurement based on SMARTS (simple model of the atmospheric radiative transfer of sunshine) to reduce spectral errors

A Lambertian CCD-camera method is convenient to measure concentrating radiation fluxes, where a crucial factor, a calibration factor, always varies with spectra and brings errors. In this paper, a new calibration method is proposed based on spectral normalization calculation and tries to reduce spectral errors in Lambertian CCD-camera measurement. The calibration factor for AM1.5 is standardized over a transmittance range by matching gray values of photos to readings of calorimeter. A spectrum is calculated by SMARTS (simple model of the atmospheric radiative transfer of sunshine) according to the local time, latitude and longitude. A calibration factor is adjusted by calculated spectral offsets accordingly. Therefore an absolute radiation flux distribution is obtained by a gray value captured by the CCD-camera without calorimeter. Calculated results indicate that spectral irradiance between 700 and 800 nm dominates gray values on the target for solar radiation flux measurement. The offsets are increasing continuously from AM1 to AM5, which are validated by experimental results. The difference between measured and calculated calibration factors is 11%, which fits to the results of error estimate. These indicate that the improved method was feasible and reliable to measure concentrating radiation fluxes easily.

SU-E-T-58: Specific Patient Exit Dose Verification Base on Treatment Planning System and Experimental Data

Purpose: In our center we have developed an exit dose verification technique enable us to check two dimensional patient exit dose at EPID level. This system works base on collected patient portal dose images. A series of pre calculated calibration factors which depends on patient thickness, field size and air gap between patient and detector were developed. Methods: Two dimensional transmitted portal dose images (PDI) utilized to compare them with predicted portal dose maps calculated using treatment planning system (TPS).This study investigates the effect of phantom thickness, air gap size between the phantom and the EPID, field size on portal transmitted dose images. A model performed to find the optimal calibration conditions, for which the field size dependence minimized. To validate the model two phantom models including: a cubic solid water and a patient model phantoms were used. To determine the calibration parameters of EPID model, several measurements have been performed using a two dimensional ionisation chamber detectors array (Matrixx by IBA) and the results were compared with those calculated dose images using treatment planning system (Pinnacle3 V9). For the acquisition of portal images of IMRT fields, an in-house software (MATLAB) was used gamma method to compare portal dose images and predicted portal dose maps. Results: The EPID model succeeded in predicting the EPID response for various field sizes and offsets within 3% of measurements. This method is still in early stages, but for the first few anal cancer IMRT patients evaluated an average of 93% of points passed the criteria of gamma method with the criterion of deviation of 5% and 5mm. Conclusions: Although we are in developing phase, the result showed that it could be used for clinical routine patient dose verificatio.

Optimizing the calibration and interpretation of dynamic ocular force measurements

Ophthalmodynamometric studies can provide useful clinical information regarding glaucoma, cerebrospinal fluid pressure, and vascular disease but are affected by variable reproducibility and unknown calibration of force in terms of intraocular pressure (IOP). The aim of this study was to calculate calibration factors and identify key design principles using three different types of ophthalmodynamometers. We constructed a modified ophthalmodynamometer named OcuDyn using a large contact lens inside a ring force transducer with continuous signal acquisition and averaging. OcuDyn and Sisler ophthalmodynamometers were applied to isolated pig eyes which were cannulated and connected to a fluid reservoir and a pressure transducer to measure induced IOP at increasing force application. Using these two devices and a Meditron ophthalmodynamometer, we measured the minimum ophthalmodynamometric force (ODF) required to induce pulsation in the hemi-veins and diastolic retinal artery of each eye in glaucoma patient volunteers and family members. Blood pressure was measured with the sphygmomanometer cuff held at eye level. In pigs, the relationship between induced IOP and ODF was strong (minimum r > 0.98, p < 0.001). In humans, the pressure increment (blood pressure - baseline IOP) correlated highly with human arterial ODF (r = 0.83, n = 75, p < 0.001, mean slope 0.32). Mean coefficients of variation were 12.4% (n = 66) for veins and 5.6% (n = 39) for arteries in diastole; these results did not differ significantly with those from the Meditron but were significantly lower than results with the Sisler (20%, p = 0.0014). Linearity between induced IOP and ODF is strong, suggesting that calibration of 0.32 mmHg/g for OcuDyn or using two point blood pressure measures allows for valid interpolation. The better optics of the Meditron and OcuDyn result in more repeatable end-point determination and outweigh benefits conferred by signal averaging or slit lamp mounting. We calculate calibration factors of 0.89 mmHg/arbitrary unit for Meditron and 0.82 mmHg/g for Sisler. These factors may be useful in the estimation of ocular vascular, orbital, and cerebrospinal fluid pressures.

Monte Carlo modeling of beta-radiometer device used to measure milk contaminated as a result of the Chernobyl accident

This paper presents results of Monte Carlo modeling of the beta-radiometer device with Geiger–Mueller detector used in Belarus and Russia to measure the radioactive contamination of milk after the Chernobyl accident. This type of detector, which is not energy selective, measured the total beta-activity of the radionuclide mix. A mathematical model of the beta-radiometer device, namely DP-100, was developed, and the calibration factors for the different radionuclides that might contribute to the milk contamination were calculated. The estimated calibration factors for 131I, 137Cs, 134Cs, 90Sr, 144Ce, and 106Ru reasonably agree with calibration factors determined experimentally. The calculated calibration factors for 132Te, 132I, 133I, 136Cs, 89Sr, 103Ru, 140Ba, 140La, and 141Ce had not been previously determined experimentally. The obtained results allow to derive the activity of specific radionuclides, in particular 131I, from the results of the total beta-activity measurements in milk. Results of this study are important for the purposes of retrospective dosimetry that uses measurements of radioactivity in environmental samples performed with beta-radiometer devices.

Calculation of 18F, 99mTc, 111In and 123I calibration factor using the penelope ionization chamber simulation method

A new method using the Monte Carlo code PENELOPE for ionization chamber simulation has already been successfully used for calculating calibration factors needed for the measurements of radionuclides with photon emission (2003, Appl. Radiat. Isot., to be published). This work has been continued at (Laboratoire National Henri Becquerel) in order to calculate the calibration factors for radionuclides with short half-lives used in medical services. Activity measurements of 18F, 99mTc, 111In and 123I using the calculated calibration factors were obtained with standard uncertainties equal to 0.6% for 18F, 99mTc and 1.5% for 111In and 123I.

Respiratory inductance plethysmography in healthy infants: a comparison of three calibration methods.

Respiratory inductance plethysmography (RIP) measures respiration from body surface movements. Various techniques have been proposed for calibration in order that RIP may be used quantitatively. These include calculation of the proportionality constant of ribcage to abdominal volume change (K). The aims of this study were to 1) establish whether a fixed value of K could be used for calibration, and 2) compare this technique with multiple linear regression (MLR) and qualitative diagnostic calibration (QDC) in normal healthy infants. Recordings of pneumotachograph (PNT) flow and RIP were made during quiet (QS) and active sleep (AS) in 12 infants. The first 5 min in a sleep state were used to calculate calibration factors, which were applied to subsequent validation data. The absolute percentage error between RIP and PNT tidal volumes was calculated. The percentage error was similar over a wide range of K during QS. However, K became more critical when breathing was out of phase. A standard for K of 0.5 was chosen. There was good agreement between calibration methods during QS and AS. In the first minute following calibration during QS, the mean absolute errors were 3.5, 4.1 and 5.3% for MLR, QDC and fixed K respectively. The equivalent errors in AS were 11.5, 13.1 and 13.7% respectively. The simple fixed ratio method can be used to measure tidal volume with similar accuracy to multiple linear regression and qualitative diagnostic calibration in healthy unsedated sleeping infants, although it remains to be validated in other groups of infants, such as those with respiratory disease.

An ionization chamber as a secondary standard for activity

A commercially-available radionuclide calibrator is used to maintain the unit of activity in the Slovak Institute of Metrology (SMU) and to check and verify activity meters in the field of nuclear medicine. The calibration of the instrument is described. The long-term stability is checked at regular intervals by means of 137Cs and 226Ra reference sources. A slow decrease of efficiency of about 2% per year has been observed, which has to be corrected for. An energy-dependent efficiency curve of the ionization chamber is established by transfer of calibration factors from a similar instrument at the Physikalisch-Technische Bundesanstalt (PTB). The curve has been checked by measurements with activity standards of 60Co, 54Mn, 22Na, 85Sr, 65Zn, 88Y, 57Co, 139Ce, 137Cs, 125I, 241Am, 210Pb and 109Cd. The established curve permits one to calculate calibration factors for additional short-lived radionuclides used in nuclear medicine. An overall uncertainty was obtained for activity measurements with calculated calibration factors under standard measurement conditions of better than 1.8%.

Optimizing the two-coil mutual inductance measurement of the superconducting penetration depth in thin films

When a pair of coils is positioned on opposite sides of a superconducting thin film, measurement of their mutual inductance may in principle be used to infer the penetration depth λ in the superconductor. We have studied how to optimize this measurement with respect to coil design, and have found that the approach that has been generally used is far from the optimum. Useful simplifications to the expression relating mutual inductance to penetration depth are derived. An analysis of the sources of uncertainty in determining λ is presented. For an optimized coil set, the major source of uncertainty often is uncertainty of the thickness of the film. The sensitivity to changes in λ is also studied; it is shown that this can approach 1 pm for a typical high temperature superconductor sample. Finally, it is shown that the analysis may be extended to normal metal films, with the skin depth playing a role similar to that of the penetration depth in superconductors. Measurement of a high conductivity normal metal foil can be used to check calculated calibration factors for a coil set or to determine the skin depth.

Calculation of Calibration Factors and Layout Criteria for Gamma Scanning of Waste Drums from Nuclear Plants

This paper present a procedure to calculate calibration factors for converting the measured gamma rate of waste drums into activity content and a layout and free release measurement criterion for waste drums. A computer program is developed that simulates drum scanning technique, which calculates calibration factors and eliminates laborious experimental measurements. The calculated calibration factors exhibit good agreement with experimentally determined values. By checking the calculated calibration factors for trial equipment layouts (including the waste drum and the scanning facility) using the layout and free release measurement criterion, a layout can be achieved that clearly determines whether there can be free release of a waste drum.

Single-position calibration of the respiratory inductance plethysmograph.

Since we intended to use the respiratory inductance plethysmograph (RIP) to study breathing patterns in a single body position, we developed a method to calibrate the RIP in one position (seated) and verified the stability of this calibration procedure during a 60-min period. The subject breathed spontaneously through a pneumotachograph for 1 min during all calibration and verification runs. All inspiratory data from the abdomen and rib cage RIP transducers was analyzed using multiple linear regression analysis to calculate calibration factors for the transducers. Eight normal subjects were studied, and the stability of calibration at 20, 40, and 60 min was determined. The correlation coefficients were all greater than 0.94. The mean slope and mean intercept describing the relationship of the RIP volume to the pneumotachograph volume for all calibration and verification runs were 0.995 +/- 0.074 and 0.012 +/- 0.018 liter, respectively. This calibration method allows a spirogram to be generated from the RIP signals. Therefore, this technique may be valuable in further investigating resting breathing patterns in humans.

Methylene unit calibration for analysis of organic acids by gas--liquid chromatography/mass spectrometry.

We describe a method for automatic computation of "methylene unit" retention-time indices in real time on a computer-controlled gas chromatograph/mass spectrometer system. The calculation is based on the exponential relationship between the methylene unit values of hydrocarbon reference standards and the oven temperature at their elution. Using this mathematical relationship, we calculated calibration factors, which eliminates the need to include a set of methylene unit reference standards with each run and simplifies and speeds identification of unknown peaks in chromatograms. Long-term stability and good precision of the calibration factors were observed during the six-month period of study.

COMPARISONS BETWEEN RELATIVE GRAVITY MEASUREMENTS WITH THE CAMBRIDGE PENDULUM APPARATUS AND WORDEN GRAVITY METERS IN NORTH AMERICA, SOUTH AFRICA, AUSTRALIA, NEW ZEALAND, AND GREAT BRITAIN

Summary The invar pendulums of the Cambridge pendulum apparatus are ferromagnetic and their periods are affected by the Earth's magnetic field. Before 1952, the pendulums were shielded from the Earth's field by a mu-metal screen; since 1952 Helmholtz coils have been used to annul the Earth's field. To see if there is any discrepancy between results with the two systems, pendulum observations made with the screened system in South Africa, Australia, New Zealand, and Great Britian, and with the Helmholtz coil system in North America, have been compared with gravity meter observations made by Dr. G. P. Woollard and his collaborators. The observations, in three groups-South Africa; Australasia and Great Britain; and North America-have been adjusted by the method of least squares, a calibration factor for the gravity meter being calculated for each group. There is no significant difference between the factors so calculated, and accordingly no detectable difference between the two pendulum systems. The calculated calibration factors of Dr. Woollard's gravity meters IOE and 41 b are 0.1 per cent greater than those used in calculating the values published in 1952 and 1953 (Woollard, Harding, Muckenfuss, Bonini, and Black, 1952; Woollard, Bonini, Black, Rose, and Ault, 1953). The pendulum observation between Cambridge and Melbourne has a residual of nearly 1.2 mgal; otherwise the pendulum observations in South Africa and Australasia form a satisfactory basis for gravity surveys there. Two pendulum observations between Ottawa and Washington differ by 2.8 mgal and the differences of gravity between Ottawa, Washington, and Teddington (NPL) may be uncertain by about I mgal.