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Download Model 2182/2182A Nanovoltmeter
Transcript
2-26 Voltage and Temperature Measurements Standard cell comparisons Standard cell comparisons are conducted by measuring the potential difference between a reference and an unknown standard cell. All cell differences are determined in series opposition configuration. The positive terminals of the standard cells (V1 and V2) are connected to the HI and LO inputs of the nanovoltmeter, as shown in Figure 2-13A. The Model 2107 Input Cable (supplied with the Model 2182) should be used to connect the cells to the nanovoltmeter in order to minimize errors caused by thermal EMFs (VEMF). Figure 2-13 Standard cell comparison measurements HI CH 1 Thermal EMFs + DCV1 VEMF V1 LO HI CH 2 V2 LO 2182 + + Standard Cells - Reading #1 = V1 - V2 + VEMF A) Reading #1 HI CH 1 Thermal EMFs + DCV1 VEMF V1 LO HI CH 2 LO 2182 V2 + + Standard Cells - Reading #2 = VEMF + (V2 - V1) B) Reading #2 Once the measurement connections have been made, care must be taken to avoid errors from thermally generated potentials. To minimize the effects of thermal EMFs, a second measurement is taken with the nanovoltmeter leads reversed, as shown in Figure 2-13B. The small voltage difference is calculated by averaging the absolute values of the two readings. Calculation of standard deviation across several redundant readings will help provide this assurance. Once stability has been achieved, the actual voltage difference between the cells is measured. For each comparison, several readings are usually averaged. This process of comparing is then repeated each week, month, or year, depending upon the standards laboratory. The results can then be plotted and compared over time.
