How to Select the Right Covering Agent

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.

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

	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.