Calibration Drift Research Articles

On-line instrumentation of cryogenic systems and plant to an accuracy of 0.01% using cold electronics

The measurement of control parameters, such as temperature, pressure, fluid velocity, mass flow, fluid density and liquid level, is limited in accuracy by degradation of analogue signals passing from the cold sensors to ambient-temperature data handling equipment. Recent developments at Southampton show how significant improvement in accuracy can be achieved to the point where the normal expectation of accuracy for sensor-generated data is 0.01%, limited only by calibration errors and drifts. This accuracy can be achieved using CMOS-based hybrid circuits, in which the analogue signals from the sensors are converted to digital signals at the cold end before transmission without degradation.

Some Factors Affecting In Situ Measurement Using the Cambridge Self-Boring Pressuremeter

Abstract Research presented in this paper was conducted to assess the influence of environmental and physical factors on measurements made with the self-boring pressuremeter (SBPM). It was found that temperature changes during deployment and long-term instrument calibration drift can result in misinterpretation of in situ soil properties. It was also observed that neglecting to consider the sequential order of strain arm lift-off and radial confinement in both membrane calibration and data reduction can have an effect on interpretation of horizontal stresses, particularly in soft clays. Control experiments were conducted to quantify the effect of these factors. Based on the results of these experiments, a method for assessing excess pore pressure at the beginning of a test using the standard Cambridge SBPM transducers was developed. This method incorporates findings from temperature behavior tests. In addition, a modified method of membrane stiffness correction is proposed which includes consideration of sequential order of lift-off and confinement effects.

Implementation of self-organizing neural networks for visuo-motor control of an industrial robot

The implementation of two neural network algorithms for visuo-motor control of an industrial robot (Puma 562) is reported. The first algorithm uses a vector quantization technique, the ;neural-gas' network, together with an error correction scheme based on a Widrow-Hoff-type learning rule. The second algorithm employs an extended self-organizing feature map algorithm. Based on visual information provided by two cameras, the robot learns to position its end effector without an external teacher. Within only 3000 training steps, the robot-camera system is capable of reducing the positioning error of the robot's end effector to approximately 0.1% of the linear dimension of the work space. By employing adaptive feedback the robot succeeds in compensating not only slow calibration drifts, but also sudden changes in its geometry. Hardware aspects of the robot-camera system are discussed.

Calibration of the NOAA AVHRR Shortwave channels using split pass imagery: I. Pilot study

A method of deriving in-flight calibration factors for Channels 1 and 2 of the AVHRR radiometer is described. The technique utilizes the dual views of a selected area of the ocean obtained from consecutive satellite overpasses, coupled with a radiative transfer model whose aerosol properties are constrained by surface measurements. The method is applied to a sample NOAA 11 data set obtained about 18 months after launch, and the sensitivity of the recovered calibration drift factors to uncertainties in the data and model parameters is examined. Best estimates of the calibration factors, defined as the fractional sensitivity relative to the preflight calibration, are 0.75 ± 0.04 in Channel 1 and 0.76 ± 0.06 in Channel 2. The Channel 2 result is subject to additional error due to uncertainty in the water vapor loading, and observational constraints on the water vapor profile will be necessary to ensure the success of the method in this channel.

An Intelligent Chilled Mirror Humidity Instrument

An intelligent, chilled mirror humidity instrument has been designed for use on buoys and ships. Our design goal is for the instrument to make high-quality dewpoint temperature measurements for a period of up to one year from an unattended platform, while consuming as little power as possible. The instrument uses a General Eastern Dew-10 chilled mirror sensor and is controlled by an onboard digital processor that is programmable in BASIC. Communications to an external logger are provided by an RS-232 compatible interface. The housing is made of PVC and is approximately 76 cm long × 11.5 cm in diameter; the complete instrument weight is 5.8 kg. Energy consumption is typically 800 J per measurement; standby power consumption is 0.05 W. A series of dockside tests have been carried out to evaluate the long-term accuracy and reliability of the D10IQ. As a standard, we used an EG&G 200M Dewtrak chilled mirror instrument that was cleaned manually at frequent intervals. We found that the mean difference between the D10IQ and EG&E 200M was roughly 0.9°C, which is within the expected error of our calibration procedure, and that the standard deviation of the difference was about 0.8°C. The variance is contributed equally by a fast time scale random fluctuation and a 5- to 10-day period variation. For our purpose it is most significant that there was very little calibration drift of the D10IQ so long as the mirror reflectance stayed above a well-defined and readily monitored threshold value. Thus it appears that the D10IQ can provide a fairly reliable and accurate means for measuring humidity from unattended platforms.

Valid: A generally applicable expert system-based program for the quantitative validation of the results of analytical methods

Wolters, R., Van den Broek, A.C.M. and Kateman, G., 1990. VALID: A generally applicable expert system-based program for the quantitative validation of the results of analytical methods. Chemometrics and Intelligent Laboratory System, 9: 143–175. A system has been developed with which a quantitative validation of the results of an analysis can be achieved, for a specific application of an analytical method. The validation system, VALID, consist of an expert system-based program, in which knowledge about method validation is linked to the necessary (calculation) procedures and (mostly statistical) tests. The knowledge is represented in the system by means of a set of rules. The control of VALID takes place by a consultation of this rule set. Using this means of control renders the system suitable for a wide variety of analytical methods. There is no dependence on any particular analytical technique. During the course of a VALID run the reliability of the analysis system is estimated by means of the criterion of maximum total error, which incorporates both the random error and the systematic error of the results. Furthermore, a number of error sources are checked, such as the calibration procedure, interferences, matrix effects, and drift. The program provides the user with experimental designs and interprets the results of the experiments. Finally, the estimated reliability of the analytical results is compared to the demands specified by the user, which results in a judgement on the suitability of the technique. The performance of the validation system is illustrated by its application to the determination of cadmium in soil samples by electrothermal atomic absorption spectrometry.

PSYCHROMETRIC MEASUREMENT OF SOIL WATER POTENTIAL

A commercially available thermocouple psychrometer sample changer (Decagon SC-10A) was used to measure the water potential of field soils ranging in texture from sand to silty clay loam over a range of {minus}0.5 to {minus}20.0 MPa. The standard error of prediction based on regression statistics was generally between 0.04 and 0.14 MPa at {minus}5 MPa. Replacing the measuring junction of the unit changed the calibration slightly; however, it did not significantly alter measurement accuracy. Calibration curves measured throughout a year of testing are consistent and indicate no systematic drift in calibration. Most measurement uncertainty is produced by shifts in the intercept in the calibration equation rather than the slope. Both the variability in intercept and the regression error seem to be random. Measurements taken with the SC-10A show that water potential in both sand and silt loam samples removed from 1.5-MPa pressure plates was often 0.5 to 1.0 MPa greater than the 1.5-MPa applied pressure. Limited data from 0.5-MPa pressure plates show close agreement between SC-10A measurements and pressure applied to these more permeable plates.

Drift in Energy Calibration of Energy Dispersive X-ray Fluorescence Analyzers and Its Correction

AbstractIt is to be expected that drift will occur in the energy calibration of Energy Dispersive X-Ray Fluorescence Analysis (EDXRFA) spectrometers. The error thus caused in the measured x-ray intensity is evaluated in this paper. In order to minimize the error, two steps are necessary. The first is to make a new energy calibration to follow the trend of the drift. The second step involves moving the regions of the characteristic peaks to accommodate the drift. Software has been developed to accomplish both steps automatically.

Two-barrel bile-acids-sensitive microelectrodes based on liquid ion exchanger.

Several liquid membrane microelectrodes sensitive to bile acids (two barrel, tip diameter about 0.5 micron) are described. The results of different liquid ion exchangers such as Aliquat 336/decanol, trioctylmethylammonium/decanol, hexadecyltrimethylammonium/decanol, benzyldimethylhexadecylammonium/decanol, hexadecyltributylammonium/5% hexachlorobenzene + 0.5% bromoacetanilide in o-dichlorobenzene are compared with each other, and the better one among them is the mixture of benzyldimethylhexadecylammonium cholate/decanol with hexadecyltributylammonium taurocholate/5% hexachlorobenzene + 0.5% bromoacetanilide in o-dichlorobenzene because of its quicker response time and low drift. The calibration curves, slopes, test limits, selective coefficients, drifts, and response times of the various bile-acids-sensitive microelectrodes in different calibration solutions were demonstrated and compared with each other.

A vector space solution to incipient sensor failure detection with applications to automotive environments

An efficient method of sensor diagnosis in electronically controlled IC engines is demonstrated. The approach taken is based on the failure detection and isolation (FDI) theory, and the resulting diagnostic systems can be implemented in on-board digital computers of passenger vehicles. The proposed diagnostic system, referred to as the binary phase detection filter (BPDF), uses a pair of failure-sensitive filters. Based on the reference model of an internal combustion engine, BPDF not only isolates the failed sensor, but also estimates the associated amount of failure. A wide variety of sensor failures, including soft partial types (e.g. calibration or bias drift), can be detected using BPDF. >

New generation CTD system (conductivity-temperature-depth sensor)

A microprocessor-based CTD (conductivity-temperature-depth) sensor unit is described in which the microprocessor is used to vary the analog-to-digital conversion process to achieve 16-bit resolution at a measurement rate of 500 measurements per second, or 18-bit resolution at 30 measurements per second. Software control of resolution, sampling rate, and sampling sequence permit the sensor unit to be software-configured for a wide variety of missions. Pressure and conductivity sensors are expected to substantially improve the system accuracy. The conductivity cell design has six electrodes which are all relatively remote from the external measurement volume (approximated by a 2-cm diameter sphere). This so called recessed-electrode concept is expected to eliminate calibration drift due to contamination of the electrodes and the insulating surfaces of the cell, thus ensuring excellent long-term stability.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Optimum frequency of calibration monitoring

This paper develops an algorithm by which to compute the optimal frequency of calibration monitoring to minimize the total cost of analyzing a set of samples and the required calibration standards. Optimum calibration monitoring is needed because of the high cost and calibration drift of the analyzing equipment. Gas chromatographs with mass spectroscopy equipment give trace analyses with previously unattainable precision, accuracy, and speed of turn-around when properly calibrated and systematically monitored. Calibration monitoring is done by including a known standard every fixed number of samples. Currently, this fixed number, or calibration monitoring frequency, is arbitrarily chosen; however, this paper gives a method to compute the optimal calibration monitoring frequency so that the total analyses costs for both samples and standards will be minimized.

Analysis of the marine sediment reference material PACS-1 by inductively coupled plasma mass spectrometry

The determination of sixteen trace elements (V, Cr, Co, Ni, Cu, Zn, As, Sr, Mo, Cd, Sn, Sb, Hg, Tl, Pb and U) in the new marine sediment reference material PACS-1 by a combination of external calibration and isotope dilution strategies is described. The use of the background ion 40Ar + 2 as an internal standard for multielement analysis was evaluated and found to be most effective when concentrations of concomitant elements are high enough to cause a large change in sensitivity or to induce a severe calibration drift. For most of the analyses reported here, concomitant effects were not large enough for this approach to result in a dramatic improvement in precision or accuracy.

The historical temperature series of Bologna (Italy): 1716?1774

The Bologna meteorological record is one of the oldest and most complete among the series already collected for Europe. Data from the regions south of the Alps being extremely sparse, this station is of great importance for all climatic reconstructions dating as far back as the beginning of the 18th century. The focus here is on the temperature series, because it presents some a priori problems with respect to the series analysis itself. These problems relate to the homogeneity of the temperature record, which is affected by the use of different thermometers within the record, the statistical reconstruction of the thermometric scales, and the drift in calibration of one of these instruments. After correcting for these heterogeneities, the temperature series indicate a thermal behaviour in agreement with other historical European sources. The Bologna series is characterised by a warm period during the first decades of the 18th century, with large decreases in temperature in 1740 and 1742, and a positive trend from 1742 to the end of the record. The behaviour of the Bologna temperature series is in good agreement with the series for Central England, De Bilt (Netherlands) and Padua (Italy).

An Assessment of the Long-Term Performance of Gas Continuous Emission Monitoring Systems

A field test program was conducted to evaluate the long-term performance of several gas continuous emission monitoring systems (CEMS).* This paper presents the results gathered on the long-term accuracy and calibration drift characteristics of ten CEMS installed at scrubber-controlled, coal-fired power plants. The program involved periodic accuracy audits and a review of available calibration drift data for selected CEMS. Accuracy audit results show that both SO2 and NOx CEMS are capable of providing accurate data on a long-term basis. However, frequent audits are necessary in order to verify the performance of an individual CEMS. The results of the calibration drift data evaluation show that despite infrequent occurrences of excessive drift, CEMS operated with a significant bias for extended time periods simply because corrective action was not taken in a timely manner.

The Determination of Boron by Inductively Coupled Plasma Atomic Emission Spectroscopy. Part 1: Method Development

Abstract The use of inductively coupled plasma atomic emission spectroscopy (ICP-AES) for trace determination of boron was investigated. Usable spectral lines occur both in the normal air and vacuum ultraviolet regions of the spectrum. Optimization of instrumental parameters and a study of interferences were carried out. Good sensitivities were achieved, however, an unexpected memory effect in the torch-nebulizer system and calibration drifts were experienced. These problems, together with line and back-ground interferences emphasize the need for careful investigation when developing new analytical schemes with ICP-AES.

Monitoring of transcutaneous carbon dioxide tension.

The authors studied the analytic and clinical characteristics of transcutaneous carbon dioxide monitoring in acutely ill NICU infants. Most infants were premature. The authors used both developmental and commercially available equipment. Monitoring was performed at temperature settings of 44 degrees C. With commercially available equipment, it was found that the least-squares linear regression relationship (r = 0.86, Sy.x = 12.9 torr, n = 100 patients) between transcutaneous PCO2 (y) and arterial PCO2 (x) was: y = -2.8 + (1.86)(x) (in torr). Calibration drift was unacceptably large in about 10% of the 500 monitoring sessions performed. Therefore, the authors conclude that transcutaneous PCO2 monitoring is most appropriate for following short-term trends in PCO2 over a period of one to three hours. Continuous transcutaneous PCO2 monitoring was most useful clinically after extubation when the objectives were careful observation for changes in PCO2 and prevention of reintubation.

Mechanical and physiological calibration of four cycle ergometers

Mechanical and physiological calibrations were performed on four research-grade cycle ergometers. Ten subjects rode each ergometer twice in a randomized testing order. The subjects pedaled at 60 rpm for 5 min at each of three power outputs, i.e., 49, 98, and 147 W. Heart rate, metabolic, and perceptual data were obtained each minute. Prior to and immediately following these test rides, mechanical calibrations were obtained in duplicate. From the mechanical calibrations, Ergometer A was approximately 10% below actual values at each power output. Ergometer B demonstrated a variable error, with the largest percentage and absolute errors occurring at the lower power outputs. Ergometers C and D generally demonstrated less than a +/- 3% error. Following the physiological calibration, Ergometer B exhibited a substantial drift in calibration, while Ergometers A, C, and D maintained their original calibration. The physiological data supported the mechanical calibration, and Ergometer B demonstrated a substantial drift between the first and second trials and produced substantially different results compared with the other three ergometers. Ergometers A, C, and D demonstrated acceptable consistency in results both within trials and among ergometers. These results demonstrate the importance of proper calibration and of understanding the calibration characteristics of ergometers selected for research purposes.

Skin-surface CO 2 measurement in sick preterm and term infants

To evaluate the clinical usefulness of a heated CO2 electrode at 43 degrees C for measurement of skin surface PCO2, we made 157 simultaneous determinations of PSCO2 and arterial PCO2. Arterial samples were drawn from an umbilical artery catheter, and the PaCO2 was measured within two minutes and was corrected for body temperature. We obtained two to 43 paired samples per infant from 22 infants (gestational age = 27 to 40 weeks) during the first 12 days of life. Over a four-hour period there was no significant drift of the electrode calibration and no significant change in PSCO2-PaCO2 (delta). Over the range PaCO2 26 to 72 torr, PaCO2 was linearly related to PSCO2 (PaCO2 = 0.37 PSCO2, + 17.46) r = 0.79). To pursue the factors which determine delta, we performed step-wise multiple regression analysis and found that the only significant independent variables were PSCO2 and postnatal age (delta = 0.79 CO2 [torr] - 1.2 age [days] - 23.9 [r = 0.93]). Insignificant independent variables were blood pressure, gestational age, hematocrit, weight at time of study, and drug therapy.

Hot-film sensors calibration drift in water

The calibration drift due to contamination of hot-film and fibre probes in water has been investigated and a method for correcting the relation between Reynolds and Nusselt numbers is advanced. The results obtained with two different sensor configurations demonstrate the generality of the model. Measurements of mean velocity and longitudinal pressure gradient in a plane mixing layer confirm the validity and applicability of the method.