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.

Commonly used covering materials

 

AK 22

Viscous putty

Ord. no.:
1-Ak22

ABM 75

Aluminum foil with kneading cmpd.     

Ord. no.:
1-ABM75

NG 150(1)

Nitrile rubber

Ord. no.:
1-NG150

SG 250

Transparent Silicone rubber

Ord. no.:
1-SG250

PU 140(1)

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]

-269°C ... +150°C

[-452°F … +302°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

3 bottles
each with 25 cm3

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

35 SG

20 SG

250 SG

90 SG

Application method

Kneading by hand

Pressing on by hand

Brush application

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

1 year

6 months

9 months

6 months

Ingredients

Viscous, kneadable, sticky putty

0.05 mm thick aluminum foil coated with 3 mm thick kneading cmpd.

Solvent-containing one-component nitrile 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 (400 bar)
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

cndtl.

cndtl.

yes

yes

cndtl.

cndtl.

no

cndtl.

cndtl.

yes

cndtl.

cndtl.

no

no

yes

yes

Ketones:
Acetone
Methyl ethyl ketone (MEK)

cndtl.

cndtl.

no

cndtl.

cndtl.

no

no

no

no

no

no

no

no

no

no

cndtl.

yes

cndtl.

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

cndtl.(2)

cndtl.

cndtl.

yes

no 

no

yes

condtl.

yes

yes

yes

yes

yes

no

no

no

no

no

yes

yes

yes

yes

cndtl.

yes

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

condtl.

condtl.

condtl.(2)

condtl.

no

no

condtl.

no

no

condtl.

no

yes

yes

yes

Liquefied gases (excluding oxygen)

yes

UV resistance

yes

yes

yes

yes

yes

(1) PU 140 and NG 150 cannot be combined      (2) 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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