British Standard Model – C3
Single daylight    (2 plate)  C3/1
Double daylight  (3 plate)  C3/2
Triple daylight     (4 plate)   C3/3

British Standard Model – C4
Single daylight    (2 plate)  C4/1
Double daylight  (3 plate)  C4/2
Triple daylight     (4 plate)   C4/3    

Definition

The set is the residual strain in a rubber test piece after it has been subjected to stress for a given time and then allowed to recover for a given time, the temperature being substantially constant during the test. ‘Compression Set’ is the residual compression strain after compression, (a) to a given compression strain, or (b) a given compression stress.

Purpose

The apparatus, under reference C.3, C.4, applies to the constant strain method of test.

Apparatus

Two or more circular steel plates 12.5 mm thick and 126 mm diameter are clamped together with three bolts. Spacers of the appropriate thickness in the form of rings around each bolt are placed between the plates to control the thickness of the test pieces whilst compressed. Loose stainless steel discs are interleaved between the main plates to provide a highly polished surface in contact with the test pieces. These discs may easily be replaced should they become scratched, pitted or damaged. The apparatus is available with single daylight (2 plates) double (3 plates) and triple daylight (4 plates). It can also be supplied with spacers of the required thickness to meet the BS (Model C.3) or the ASTM (Model C.4) specifications. When ordering please state whether BS or ASTM model is required, and also the number of daylights (or the number of plates).

Procedure

Cylindrical test specimens are cut or moulded and then conditioned and their thickness measured as specified, using a Wallace Thickness Gauge, ref. S.4.

Test pieces of either 13 mm dia. or 29 mm dia. may be used. The two types of test pieces do not necessarily give the same values of compression set and comparison of values from the two types should be avoided. The test pieces are placed between the plates of the apparatus and the bolts tightened uniformly until the plates are in contact with the spacers. After a predetermined period the load is removed and the test pieces allowed to recover. Their thickness is again measured. Compression set at constant strain is the difference between the original thickness of the test piece and that after recovery expressed as a percentage of the initially applied compression:

Preparation of Test Peices

Special rotary cutters for the preparation of test pieces are available as under: