Black and White Cryptometer
Brand: Sheen Instruments – UK
The hiding and spreading powers of pigmented coatings are of fundamental importance and there are many methods by which the opacity of a coating may be examined. These methods do not relate necessarily to painting practice as the change of appearance wanted, the contour and surface characteristics of the substrate together with the rheological properties of the paint will influence the performance of such coatings. The cryptometer was developed to provide a simple and rapid subjective test method for the determination of opacity in terms of hiding and spreading powers; its popularity continues for production and quality control purposes in preference to comprehensive practical testing and the complex measurement of reflectance values.
Their use is called for in some product specifications and national test methods which accept the limitations that the wet film, and not the dry coating is being measured.
The instrument consists of a glass plate, half black half white as the base. Each half of this base has a scale engraved from 0-50mm along one edge starting from the division in the centre. Two glass top plates are included usually with each instrument and these have two metal supports at one end so that these transparent top plates rest at an angle when placed upon the black and white area of the base plate. The top plates differ in the length of their tiny supports so that different angles are formed between the top plates and the base plate. A wedge angle constant (K) is given to each top plate. These range from the smallest angle K= .002 to K= .0035, K= .004, K= .007 and the largest K= .008, the popular constants being K= .004 and K= .008.
METHOD OF USE
The appropriate top plate for the paint to be tested is selected, usually K=.008 for light
coloured paints and K= .004 for dark coloured paints. A blob of paint is placed in the centre of the base plate close to the black / white division. For light colours the top plate e.g. K= .008 is placed over the paint with the supports on the white area of the tile. The top plate is pressed down firmly so that the paint is spread without air bubbles to form a shallow wedge between the plates. This wedge will move with the top plate, the position of which is adjusted until it is observed that the black/white division is just obliterated. The scale reading is then noted from the black scale where the edge of the top plate makes contact with the base plate. When testing a dark coloured paint the top plate e.g. K= .004 is used and the scale reading taken on the white area.
The thickness of paint in millimetres over the black and white division is obtained by
multiplying the scale reading times the wedge constant K of the top plate used. This figure records the minimum film thickness necessary to obliterate over black and white. The coverage or spreading power for this thickness can be obtained direct from the conversion charts below for each of the top plates.
Ref. 302/1 – Black & White Cryptometer with two plates K = 004 and K = 008
Ref. 302/2 – Black & White Cryptometer with two plates K = 002 and K = 007
Ref. 302/K002 – Extra top plate K = 002
Ref. 302/K004 – Extra top plate K = 004 Ref. 302/K0035 – Extra top plate K = 0035
Ref. 302/K008 – Extra top plate K = 008 Ref. 302/K007 – Extra top plate K = 007
DIMENSIONS 17 x 17 x 3 cm WEIGHT 800 grams.