Response of the measuring instrument

The actual reactant concentration, in the reactor at any time t is given by Cr, but owing to the slow response of the measuring instrument, the measured concentration, shown by the instrument. Cm, lags behind Cr, as indicated in Fig. 2.9. 

The linear model is correct (i.e., the response of the measuring instrument does indeed vary linearly with concentration). 

During measurements of the ionization current with an electrometer, one would like to know the response of the measuring instrument if the signal from the ionization chamber changes. Assume that the current of the chamber changes suddenly from a value of to The response of the electrometer is obtained by considering the equivalent electronic circuit of Fig. 5.10, shown in... 

When following potential changes, eg electrochemical cell capacitance charging or discharging, ac impedance measurements, or electrochemical noise measurements, the bandwidth response of the measuring instrument may limit the application. 

The actual analogue values we need to measure reflectance are given on the next page as 7.8.30. as follows. Note that the optical response curves of the measuring parts, i.e.- the non-linearity of the source and detector, are now corrected in the response of the overall instrument. 

Sensitivity is a significant characteristic in all scientific disciplines which have to do with measurements. Sensitivity is defined from the viewpoint of instrumental measuring as the change in the response of a measuring instrument divided by the corresponding change in the stimulus (ISO... 

The linearity of a method is defined as its ability to provide measurement results that are directly proportional to the concentration of the analyte, or are directly proportional after some type of mathematical transformation. Linearity is usually documented as the ordinary least squares (OLS) curve, or simply as the linear regression curve, of the measured instrumental responses (either peak area or height) as a function of increasing analyte concentration [22, 23], The use of peak areas is preferred as compared to the use of peak heights for making the calibration curve [24],... 

Sensitivity The change in the response of a measuring instrument divided by the corresponding change in the stimulus. 

The first factor, especially important with ion-selective microelectrodes, can be eliminated by a suitable modification of the measuring instrument, notably by the use of a coaxial microelectrode (see [167] and section 4.2). If an inter-ferent is present in the solution at a concentration at which it does not affect the ISE potential, factors 4 and 6 are not operative. Penetration of the deter-minand into the membrane, factor 5, is very important for the response times of ISEs with ionophores in their membranes, provided that no hydrophobic anion is present in the membrane solution, as has been theoretically treated by Morf et aL [114]. As shown in section 3.3, the presence of a hydrophobic anion stabilizes the conditions in the membrane, with a marked effect on the shortening of the response time [93]. 

Many different techniques are available for flow measurement and for recording of respiratory functions or flow parameters in particular (e.g. [115,116]). However, not all methods are appropriate for measurement of inhalation flows, either because they have low frequency responses or they influence the shape of the inspiratory flow curve by a large volume or by the inertia of the measuring instrument (e.g. rotameters). They may also interfere with the aerosol cloud from the inhalation device during drug deposition studies. 

The modulus of the gain factor of such a filter (Butterworth) is shown in Fig. 3.7 b (curve 4). The total unit-step response of a measuring instrument equipped with a filter determined by the second-order expression (3.12) is presented in Fig. 3.7a (curve 2). Verification tests of the method have given good agreement between theory and experiment. 

Sensitivity is defined as the change in the response of a measuring instrument divided by the corresponding change in the stimulus . In analysis the stimulus is often the amount of measurand present in the sample under test. Therefore, with reference to Example 2.19, Figure 3, sensitivity is the slope of the calibration curve. Note In some applications, particularly clinical and medical, the term sensitivity is often associated with the lower limit of applicability e.g. limit of detection) of a method. 

When referring to an instrument, the term discriminate is used, i.e. the ability of a measuring instrument to respond to small changes in analyte concentration. This leads to the concept of discrimination threshold, which is defined as the largest change in a stimulus that produces no detectable change in the response of a measuring instrument,.. 

Calibration is at the heart of chemical analysis, and is the process by which the response of an instrument (in metrology called indication of the measuring instrument ) is related to the value of the measurand, in chemistry often the concentration of the analyte. Without proper calibration of instruments measurement results are not traceable, and not even correct. Scales in supermarkets are periodically calibrated to ensure they indicate the correct mass. Petrol pumps and gas and electric meters all must be calibrated and recalibrated at appropriate times. 

To ascertain the suitability of the linear calibration model, plot the residual versus concentration. Remember, the residual is given by (j i — %), where yt is the measured response of the AAS instrument for a given calibrator concentration x and y is the estimated value from the regression equation. The calibration data can be expressed in an Excel spreadsheet, as shown in spreadsheet 5.2. 

A direct comparison of the TDL and extractive probe measurements of CO in the EAF is difficult due to the transient nature of the process and the different transient responses of the two instruments. The DoE report does however show that CO measurements made by both instruments in the EAF match to within approximately 5 to 10% CO over a range of 0 to 30% CO, with a band of noise in the TDL data of approximately 2.5% CO. 

Since a control loop is a dynamic system, its efficiency is governed by the response time of the entire system [4], which includes that of the measuring instrument, the controller, the final operator, and the process itself. Two time-response characteristics of the process are important in selecting the proper control strategy— the dead-time and the resistance-capacitance RC) time-cons font. Dead-time was discussed above an example is the time required to initiate many polymerization reactions. The RC time-constant results from the ability of the process to absorb a change in input without an immediate proportionate change in output. 

The use of long leads between the potential source and the measuring instrument can result in an effective change of the output capacitance of the measuring instrument, thus altering its frequency response. Typically the capacitance of a twin core cable is of the order of 100 pF/m. The effect on the frequency response can be calculated using equations 3 or 4. The remedy to this problem is to keep the cables as short as possible or, where long cable systems are unavoidable, a driver amplifier at the source may be required. 

Calibration The systematic determination of the relationship of the response of the measurement system to the concentration of the analyte of interest. Instrument calibration performed before any samples are analyzed is called the initial calibration. Subsequent checks on the instrument calibration performed through analysis are called continuing calibration. 

An example in which correlation analysis could be useful in an analytical laboratory involves an experimental test of a different instrumental method (e.g., LC with UV detection) as a cheaper alternative to a proven method (e.g., LC/MS). The objective is to provide a measure of the goodness of fit (Section 8.3.3) of the relationship between the two data sets. However, in correlation analysis the nature of the relationship (e.g. linear, quadratic etc.) is not determined in the simple example one would measure the responses of the two instruments for a series of test solutions covering the desired concentration range and compare the two to determine whether there is a... 

The expansion response of the TMA instrument can be checked with a standard reference material of defined thermal expansion. The Polymer Handbook (Brandrup 1999) and the Handbook of Chemistry and Physics (Lide 1998) provide tables of CLTEs versus temperature for numerous materials. Prime (1997) also lists CLTE values for three calibration standards (lead, aluminum, and copper) in the temperature range from -100 C to 180 °C. ASTM E831 describes the standard test method for measuring CLTE of solid materials by TMA. 

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