Static Measurement Methods

CLASSIFICATION OF HARDNESS TEST METHODS 1 STATIC MEASUREMENT METHODS... 

Wlien an electrical coimection is made between two metal surfaces, a contact potential difference arises from the transfer of electrons from the metal of lower work function to the second metal until their Femii levels line up. The difference in contact potential between the two metals is just equal to the difference in their respective work fiinctions. In the absence of an applied emf, there is electric field between two parallel metal plates arranged as a capacitor. If a potential is applied, the field can be eliminated and at this point tire potential equals the contact potential difference of tlie two metal plates. If one plate of known work fiinction is used as a reference electrode, the work function of the second plate can be detennined by measuring tliis applied potential between the plates [ ]. One can detemiine the zero-electric-field condition between the two parallel plates by measuring directly the tendency for charge to flow through the external circuit. This is called the static capacitor method [59]. 

Delchar T. Eberhagen A and Tompkins F C 1963 A static capacitor method for the measurement of the surface potential of gases on evaporated metal films J. Sci. Instrum. 40 105-7... 

Closed Vessels. Liquid level can be measured by the static pressure method also at non atmospheric pressures. However, ia such cases the pressure above the Hquid must be subtracted from the total head measurement. Differential pressure measuriag instmments that measure only the difference ia pressure between the pressure tap at the bottom of the tank and the pressure ia the vapor space are used for this purpose. At each tap, the pressure detected equals the Hquid head pressure plus the vapor pressure above the Hquid. Siace the pressure above the Hquid is identical ia both cases, it cancels out. Therefore, the change ia differential pressure measured by the instmment is due only to the change ia head of Hquid ia the vessel. It is iadependent of the pressure within the tank and is an accurate measure of the level. 

A. Stationary (mostly pseudo-stationary) methods -+ static measurement B. Non-stationary methods -> dynamic measurement ... 

Miller, M.E., Stuart, J.D. (2000) Measurement of aqueous Henry s law constants for oxygenates and aromatics found in gasolines by the static headspace method. Anal. Chem. 72, 622-625. 

C, static isoteniscope method, measured range 147.2-287.8°C, Mortimer Murphy 1923)... 

Kakkar et al. (1992) exposed six male Wistar rats to a single nominal concentration of 15,302 ppm for 10 min the animals were sacrificed 24 h later. Earlier studies (not presented) had shown this to be the highest concentration tolerated without any mortality or acute toxicity. Biochemical changes in the brains of these rats suggested impairment of antioxidant defenses. No other signs of toxicity were reported. The exposure was performed under static conditions, and the measurement method was not described. 

Two important, but often under emphasized, aspects of measuring c/w values and isotherms in foods are accuracy and repeatability. Variation in and isotherm values can be due to inherent variation in biological materials, as well as differences in measurement methods, protocols, and equipment employed. Wolf et al. (1985) presented the results of a COST 90 Project on the standardization of saturated salt solution isotherm measurement methodology and Lewicki and Pomaranska-Lazuka (2003) discussed errors in the static desiccator method. 

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