The white standard WS DIN 5033 is a barium sulphate tablet produced with the powder press. This standard is mainly used as a primary standard for the calibration of working standards for reflectance photometers but it can also be used as a working standard itself.
The white standard must have a very even, flat, clean and mat surface; the values of the spectral reflection factor must be known. The even surface is achieved by pressing the barium sulphate powder in the press against a matted glass which is later removed.
Within one batch, the spectral reflection factor's reproducibility is ± 0.1% and therefore better than the ± 0.2% required by DIN 5033, Teil 9.
White standards with a nominal diameter of 40 mm can be produced using the powder press 40 described here. The nominal diameter of the white standard should always be larger than the measuring port of the instrument to prevent false measuring results caused by the metal mount.
The barium sulphate powder is pressed into a metal ring which is closed with a plate at the back after pressing. In this way, the tablet is entirely surrounded by a metal mount. The tablet surface is flush with the front surface of the holder; this permits exactly defined positioning of the white standard against the measuring port of the photometer. If produced correctly, the tablet is so compact that it will not lose any powder even when positioned vertically or with its surface pointing downwards.

The sample cup shown permits fibers and non-compressible powder to be reproducibly presented to the measuring instrument. The material to be tested is filled into this sample cup and pressed against the inside of the cover glass by a plate. Two plates with different diameters are supplied for the sample cup and can be mounted on a spring-loaded stamp. The slightly larger plate is used if a powder-like substance is to be filled into the cup. The small radial play of the plate in the cylinder prevents even the finest powder from falling through between the plate and the cylinder wall. The diameter of the other plate is 0.6 mm smaller and should be used for fiber-like material. The greater radial play of this plate prevents jamming of the plate caused by fibers getting between the plate and the cylinder wall.

Use

• Unlock knurled bottom plate by turning it counterclockwise and remove.
• Put material into the sample cup and spread.
• Insert the stamp of the bottom plate into the cup so that the two dots are opposite each other.
• Press the bottom plate into the cup and lock by turning clockwise.

For practical work, a white standard made of opal glass is available which must be calibrated against the white standard.