FED-STD-791D
6. PROCEDURE
6.1 Calibrate the spectrophotometer in accordance with the manufacturer's instructions.
Prepare a series of at least five standards consisting of mixtures of fatty oil meeting the
requirements of A-A-59197 and mineral oil meeting the requirements of 0.0 15%. If the
sample being analyzed contains more than 15% fatty oil, dilute it with sufficient mineral oil
which meets the requirements of fatty oil in 0.0 15% mineral oil, to reduce the fatty oil content
below 15% so that the standards prepared above can be utilized. Determine the absorbance
values of the standards in accordance with 5.2 and plot the percent fatty oil against the
absorbance values at 1743 cm-1.
6.2 Thoroughly mix the sample but avoid producing air bubbles. Fill the cavity in the
sealed cell, avoiding any air inclusion. Samples suspected of containing any moisture should not
be used as the moisture will fog the sodium chloride or potassium bromide windows or change
the effective path length of the cell. Place the filled cell in the beam of the infrared
spectrophotometer and scan between 2000 and 1500 cm-1. Determine absorbance of the band at
1743 cm-1. Determine absorbance of the band at 1743 cm-1 using the Base Line Method. This
method consists of drawing the base line as nearly parallel as possible to the radiant energy
background as shown in Figure 1. The absorbance is then calculated from the equation in
Section 7.1. Determine the percent fatty oil in the sample from the plot prepared from the
standards (see 5.2). It is suggested that standards bracketing the expected fatty oil concentration
be run concurrently with the sample undergoing test to check the previously prepared plot
prepared from the standards and to compensate for any variability in equipment operation. If the
absorbance of these standards do not agree with the previously prepared plot, prepare a new
working curve prior to analyzing the sample.
7. CALCULATIONS
7.1 Calculation of absorbance of 1743 cm-1 band:
A = log10 T B
TP
where:
A = absorbance
TB = the distance from the zero line to the base line
TP = the distance from the zero line to the selected absorption peak.
8. REPORTING
8.1 Report the percent of fatty oil found in the cutting fluid.
9. PRECISION
9.1 Use the following criteria as a basis for judging the acceptability of results (95%
confidence):
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