A single covering agent is often not enough for sufficient measuring point protection. Examples for combinations of several agents are given by the AK22 and ABM75 (plastic mass plus aluminum foil). In order to add additional mechanical protection to the metal foil, apply an extra layer of materials such as silicon rubber SG250.
When producing multi-layer covers, ensure that each layer is fully hardened before applying the next layer. In addition, each layer must overlap the underlying layer by several millimeters on all sides.
Frequently, there are several different media acting on a measuring point, such as oil and water. In such cases, for example, oil-soluble ABM75 should be applied directly to the strain gauge, cover with an aluminum foil as a diffusion barrier, followed by an oil-resistant epoxy resin as the final layer.
Multi-layer protection is absolutely essential for indefinable media such as seawater. The top layers that do not come into contact with the strain gauge can be made of other materials from those mentioned here, e.g., asphalt. These materials must, however, not dissolve or chemically change the underlying layers. Apart from that, their electrical insulation resistance is not relevant.
Absolute protection for an unlimited period of time is only possible with a hermetically sealed metallic enclosure. This type of protection can be implemented for standard commercial transducers. However, hermetically enclosing strain gauges for experimental purposes can only be implemented with extremely high overhead or not at all.