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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. 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 covered directly after installation. Only the covering materials recommended by strain gauge manufacturers should be used to protect the strain gauge and adhesive layer.

  • The measuring point must be in a perfect condition before being covered—free of dirt, moisture, perspiration from hands, and solder residue. 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.
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  • 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.

Choose the Correct Measuring Point Protection

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AK 22

Viscous putty
Ord. no.: 1-AK22

ABM 75

Aluminum foil with kneading cmpd
Ord. no.: 1-ABM75

NG 120

Nitrile rubber
Ord. no.: 1-NG120

SG 250

Transparent Silicone rubber
Ord. no.: 1-SG250

PU 140

Polyurethane paint
Ord. no.: 1-PU140

SL 450

Transparent silicone resin
Ord. no.: 1-SL450
Temperature range of resistance in air in °C -30°C...+75°C
[-22°F ... +167°F]
-196°C … +75°C
[-321°F … +167°F]
-40°C ... +120°C
[-40°F … +248°F]
-70°C … +200°C
[-94°F … +392°F]
-40°C … +140°C
[-40°F … +284°F]
-50°C … +450°C
[-58°F … +842°F]
Package contents 1 kg 11 pcs.
205 mm x 100 mm
Tube
with 150 g
Tube
with 85 g
3 bottles
each with 30 ml
3 bottles
with 25 g each
One Package sufficient for approx. 30 SG 200 SG 200 SG 20 SG 250 SG 90 SG
Application method Kneading by hand Pressing on by hand Application from tube Application from tube Brush application Brush application
Curing conditions Air drying at room temperature Room temperature Room temperature
… + 80°C
In temperature levels from 95°C - 315°C
Storage life at room temperature 2 years 2 years 15 months 6 months 9 months 6 months
Ingredients Viscous, kneadable, sticky putty 0.05 mm thick aluminum foil coated with 3 mm thick kneading cmpd. Brown, solvent one-component silicone rubber Transparent, solvent free one-component silicone rubber Solvent-containing one component polyurethane paint Transparent, solvent-containing silicone resin
Weather yes yes yes yes yes yes
Water:
Pressurized water
Condensation
Tropical climate
Water vapor
yes
-
-
-
no
yes
-
-
-
condtl.
yes
-
-
-
no
yes
-
-
-
no
yes
-
yes
yes
no
yes
-
-
-
no
Oil:
Engine oil (RT/70°C)
Mineral oil (RT/70°C)
Hydraulic oil (RT/70°C)
no
-
-
-
no
-
-
-
yes
yes
yes
yes
yes
-
-
-
yes
-
-
-
-
-
-
-
Greases - - - - yes -
Solvents in general no condtl. condtl. no - condtl.
Fuels:
Gasoline
Kerosene
no
no
no
no
yes
yes
yes
no
no




Aromatic / Aliphatic mixtures condtl.
Aromatics:
Benzene
Toluene
Xylene
no

no
no
no

no
no
condtl.
no
condtl.
condtl.
no

no
no
no


no
no

no
no
Chlorinated solvents:
Dichloromethane
Carbon tetrachloride
Perchloroethylene
1.2-Dichloroethane
o-Dichlorobenzene
no
no



no
no



no
no
no
no
no
no
no
no



no
no



no
no



Alcohols:
Ethyl alcohol
Methyl glycol
Butyl alcohol
Isopropyl alcohol
Ethylene glycol
condtl.
condtl.



yes
yes



condtl.
condtl.
no
condtl.
condtl.
yes
condtl.
condtl.



no
no



yes
yes



Ketones:
Acetone
Methyl ethyl ketone
condtl.
condtl.
no
condtl.
condtl.
no
no
no
no
no
no
no
no
no
no
condtl.
yes
condtl.
Terpenes:
Dipentenes
Turpentine




condtl.
condtl.
yes






Acids:
Hydrochloric acid conc.
Sulfuric acid 50 %
Acetic acid 50 %
Nitric acid 50 %
Oleic acid conc.
Lactic acid conc.
Air containing acids
no
no
no
no
no


condtl.(1)






condtl.
condtl.

no
no

condtl.
yes
yes
yes
yes
yes


no
no
no
no
no


yes
yes
yes
yes
condtl.
yes


Alkalis:
Sodium hydroxide 10 %
Potassium hydroxide 10 %
Ammonia 28 %
Air containing alkalis
condtl.
condtl.



condtl.(1)




condtl.
no
no

condtl.
no
no



condtl.
no



yes
yes
yes



Liquefied gases (excluding oxygen) - - yes - - -
UV resistance yes yes yes yes yes -

(1) Up to 5 % (destruction of aluminum foil!).

condtl. = Conditionally resistant (at least 10 days at RT)

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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