Selection and Overview of Covering Materials for Your Strain Gauge

The quality of a measurement made with a strain gauge essentially depends on the type of installation and design of the measuring point. A thorough preparation of the installation surface, correct connection, and protective covering are important factors for problem-free results.

Immediately after completing the installation of a strain gauge, the measuring point on the surface of the component must be carefully protected from environmental influences by a covering material.

Selecting the Covering Material

In the selection of the covering material, the following factors are to be considered:

  • Environmental conditions during the measurement
  • Duration of the measurement
  • Required service life
  • Measuring accuracy
  • The measuring object must not be stiffened in an impermissible manner; and

  • The material that comes in contact with the measuring point, including the connection cable, must also have a very high insulation resistance and not be capable of triggering any chemical reactions or corrosion.

Condition of Measuring Point

  • The measuring point must be in a perfect condition before being covered—free of dirt, moisture, perspiration from hands, and solder residue.
  • The measuring point must be covered directly after installation.
  • If the strain gauge installation is unavoidable under humid conditions (deadlines, poor weather, humid rooms), the measurement object should be baked dry in an oven, where possible or if this is not possible, the measuring point should be dried using a hair dryer or other devices serving the same purpose.
  • The covering agent must be fully bonded with the area surrounding the measuring point. Defects and capillaries are access points through which aggressive media can enter. The covering agent bond with the surrounding area must remain unchanged during the entire service life of the measuring point. The surrounding area must, therefore, be cleaned as thoroughly as the adhesive point and should reach approx. 1 to 2 cm over the outer adhesive edges. Hand perspiration (fingertips) can cause rust to creep under the covering agent, rendering it ineffective, despite the initial perfect condition.
  • The cable entries must be very carefully sealed. The covering agent must surround the wire ends on all sides, including from underneath, to ensure that there are no channels or capillaries through which moisture can penetrate the cover. Embed cable wires individually in the covering agent, in the case of multi-core cables, and cover a part of the cable sheath as well with the covering agent. In critical ambient conditions, roughen the cable insulation first and degrease it with a chemically pure solvent.
  • In addition, only the covering materials recommended by strain gauge manufacturers should be used to protect the SG and adhesive layer.

Measuring point with protective cover

 

Commonly used covering materials include the following:

Covering material
Temperature range of resistance in air
Application method
Curing conditions
Components

AK 22

Viscous putty

-30°C … +75°C

[-22°F … +167°F]

Kneading on by hand Viscous, kneadable, sticky putty

ABM 75

Aluminum foil with kneading compound

-196°C … +75°C

[-321°F … +167°F]

Pressing on by hand 0.05 mm thick aluminum foil, coated with 3 mm thick kneading compound

NG 150(1)

Nitrile rubber

-269°C … +150°C

[-452°F … +302°F]

Brush on with brush Air-drying at room temperature Transparent, solvent free single-component nitrile rubber

SG 250

Transparent silicon rubber

-70°C … +200°C

[-94°F … +392°F]

Application from tube Room temperature Transparent, solvent free single-component silicone rubber

PU 140(1)

Polyurethane paint

-40°C … +140°C

[-40°F … +284°F]

Brush on with brush Room temperature … +80°C (… +176°C) Solvent containing single-component polyurethane paint

SL 450

Transparent Silicone resin

-50°C … +450°C

[-58°F … +842°F]

Brush on with brush In temperature stages from +95°C to +315°C (+203°F … 599°F) Transparent, solvent containing silicone resin

(1) PU 140 and NG 150 cannot be combined

Combined Agents

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.


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