To measure Mn in Precision casting, you can find relevant national standards, such as the determination of manganese content GB / T 5121.14-2008, you can find the detailed answer you need, and there are very detailed steps.
As for what brand of 721 measurement standard is correct, you first refer to which national standard and see at which wavelength the actual measurement is made. If it is within 360-1000, selecting 721 can solve the problem, otherwise UV-visible spectrophotometry is required meter. If the budget is sufficient, you can choose Japan; if the budget is small, you can choose domestic.
Simply put, the Mn content of the standard steel in precision castings is given and known. After processing the standard steel, you prepare a series of solutions with different concentrations c, and then measure the absorbance value A to make a standard curve; Measure the absorbance value with your prepared sample solution, and get the Mn concentration according to the curve, then you can estimate the Mn content of the sample.
The formula is actually A = εbc, A is measured by a spectrophotometer, c of the standard steel is known, and an A-c curve can be made. b- thickness of liquid layer, ε-molar absorption coefficient.