Measuring devices, calibration

Control of Monitoring and Measuring Devices (Calibrate measuring equipment for valid results)... 

Liquid dosage forms of acetaminophen, intended to be taken in drops by children, shall have one standard dosage form per each milliliter. In this case, the bottle in which the liquid is packaged should be accompanied by a measuring device calibrated to accurately deliver 0.5 ml of the product. 

Volumetric measurements of gas adsorption equilibria reduce, if the mass of the sorbent sample used has been determined, to measurements of pressures and temperatures in gas phases. For this a variety of high precision measuring instruments operating in a fairly wide range and partly also in corrosive environment are available today. Of course these instmments prior to measurement have to be calibrated with meticulous care which may be laborious and even cumbersome. For pressure measuring devices, calibration with pressure maintaining valves of Desgranges Huot has proved to be successful. 

The System of accreditation is a mechanism that ensures official recognition that calibration or testing laboratory has right to perform testing of measuring devices, specific testing or specific types of testing. 

Calibration Cahbration entails the adjustment of a measurement device so that the value from the measurement device agrees with the value from a standard. The International Standards Organization (ISO) has developed a number of standards specifically directed to cahbration of measurement devices. Furthermore, compliance with the ISO 9000 standards requires that the working standard used to cahbrate a measurement device must be traceable to an internationally recognized standard such as those maintained by the National Institute of Standards and Technology (NIST). 

Within most companies, the responsibility for cahbrating measurement devices is delegated to a specific department. Often, this department may also be responsible for maintaining the measurement device. The specific cahbration procedures depend on the type of measurement device. The frequency of calibration is normally predetermined, but earlier action may be dic tated if the values from the measurement device become suspect. 

Cahbration of some measurement devices involves comparing the measured value with the value from the working standard. Pressure and differential pressure transmitters are calibrated in this manner. Calibration of analyzers normally involves using the measurement device to analyze a specially prepared sample whose composition is known. These and similar approaches can he applied to most measurement devices. 

Flow is an important measurement whose calibration presents some challenges. When a flow measurement device is used in applications such as custody transfer, provision is made to pass a known flow through the meter. However, such a provision is costly and is not available for most in-process flowmeters. Without such a provision, a true cahbration of the flow element itself is not possible. For orifice meters, calibration of the flowmeter normally involves cahbration of the differential pressure transmitter, and the orifice plate is usually only inspected for deformation, abrasion, and so on. Similarly, cahbration of a magnetic flowmeter normally involves cahbration of the voltage measurement circuitry, which is analogous to calibration of the differential pressure transmitter for an orifice meter. 

Density and Specific Gravity For binary or pseudobinary mixtures of hquids or gases or a solution of a solid or gas in a solvent, the density is a funcrion of the composition at a given temperature and pressure. Specific gravity is the ratio of the density of a noncompress-ible substance to the density of water at the same physical conditions. For nonideal solutions, empirical calibration will give the relationship between density and composition. Several types of measuring devices are described below. 

Systematic error, as stated above, can be eliminated— not totally, but usually to a sufficient degree. This elimination process is called calibration. Calibration is simply a procedure where the result of measurement recorded by an instrument is compared with the measurement result of a standard. A standard is a measuring device intended to define, to represent physically, to conserve, or to reproduce the unit of measurement in order to transmit it to other measuring instruments by comparison. There are several categories of standards, but, simplifying a little, a standard is an instrument with a very high accuracy and can for that reason be... 

Constriction measurement devices constructed to standards do not necessarily require calibration. One idea of strict standardization is to define the manufacturing, tolerances, and other features in such a way that the instruments made according to these rules require no calibration. The properties are so well known that a certain accuracy can be guaranteed. If the accuracy specified in the standard is inadequate, additional calibration procedures are required. The same applies to Pitot-static tubes made according to standard specifications." ... 

The simplest calibration procedure for a gas flow-measuring device is to connect it in series with a reference meter and allow the same flow to pass th tough both instruments. This requires a reference instrument of better metrological quality than the calibrated instrument. One fact to consider when applying this method is that the mass flow rate in the system containing both instruments is constant (assuming no leakage), but the volume flow rate is not. The volume flow rate depends on the fluid density and the density depends on the pressure and the temperature. The correct way to calibrate is to compare either the measured mass... 

Fast, accurate, and convenient on-site calibration of traditional measurement devices... 

BS ISO TR 7066-1 1997. Assessment of Uncertainty in Calibration and Use of Flow Measurement Devices Linear Calibration Relationships. British Standards Institution, 1997. 

Procedures for the control, calibration, and maintenance of measuring devices will need to cover the various types of devices you employ for measurement purposes, such as ... 

There are two systems used for maintaining the accuracy and integrity of measuring devices a calibration system and a verification system. The calibration system determines the accuracy of measurement and the verification system determines the integrity of the device. If accuracy is important then the device should be included in the calibration system. If accuracy is not an issue but the device s form, properties, or function is important then it should be included in the verification system. You need to decide the system in which your devices are to be placed under control and identify them accordingly. 

These are the measurements required to carry out product verification rather than the measurements to calibrate a measuring device. The measurements to be made should... 

The standard requires the supplier to calibrate and adjust all inspection, measuring, and test equipment including measurement devices at prescribed intervals, or prior to use, against certified equipment having a known valid relationship to nationally recognized standards. 

Regarding the adjustment of measuring devices, adjustment is only possible with devices that have been designed to be adjustable. Mechanical devices are normally adjusted to the null position on calibration. Electronic devices should only be adjusted if found to be outside the limits. If you adjust the device at each calibration you will not be able to observe drift. Adjustments, if made very frequently, may also degrade the instrument. It is best to observe the adage ... 

Prepare procedures for controlling the calibration of measuring devices. 

Provide a calibration laboratory or select an approved laboratory to calibrate your measurement devices. 

Prepare calibration procedures or data sheets for each measuring device. 

Process all measuring devices through your established calibration system. 

Control of monitoring and measuring devices - Maintain and calibrate measuring equipment... 

At this point we introduce the formal notation, which is commonly used in literature, and which is further used throughout this chapter. In the new notation we replace the parameter vector b in the calibration example by a vector x, which is called the state vector. In the multicomponent kinetic system the state vector x contains the concentrations of the compounds in the reaction mixture at a given time. Thus x is the vector which is estimated by the filter. The response of the measurement device, e.g., the absorbance at a given wavelength, is denoted by z. The absorbtivities at a given wavelength which relate the measured absorbance to the concentrations of the compounds in the mixture, or the design matrix in the calibration experiment (x in eq. (41.3)) are denoted by h. ... 

An additional requirement not noted in Table 1 is compliance with GLP7 These practices establish a paper trail for all procedures involved in the determination of residues. With regard to immunoassays, GLPs require calibration of measurement devices such as adjustable pipettors and dedicated spectrophotometers. Computer software output, as noted above, must be verified prior to use. This process can be simplified by limiting the application of specialized software to the operation of an instrument and carrying out the residue calculations in a broadly available spreadsheet such as Excel. On a positive note, in recent years, the software accompanying most microtiter plate readers has become generally easier to use and usually incorporates internal spreadsheets that are compatible with external systems. 

There is much effort in the international community in relation to the topic of UV measurement and the calibration of UV measuring devices. For standards and best practice in absorption spectrometry,... 

It is important that a measurement made in one laboratory by a particular analyst can be repeated by other analysts in the same laboratory or in another laboratory, even where the other laboratory may be in a different country. We aim to ensure that measurements made in different laboratories are comparable. We are all confident that if we measure the length of a piece of wire, mass of a chemical or the time in any laboratory, we will get, very nearly, the same answer, no matter where we are. The reason for this is that there are international standards of length, mass and time. In order to obtain comparable results, the measuring devices need to be calibrated. For instance, balances are calibrated by using a standard mass, which can be traced to the primary mass standard (see also Chapter 5). The primary standard in chemistry is the amount of substance, i.e. the mole. It is not usually possible to trace all of our measurements back to the mole. We generally trace measurements to other SI units, e.g. mass as in 40 mg kg-1 or trace back to reference materials which are themselves traceable to SI units. 

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