Instructions for strain gages
In order to obtain the best possible results from a strain gage installation it is important that care and attention is given to the preparation of the gage, the surface of the specimen, and bonding techniques. Whilst circumstances may call for variations in technique for particular installations, the folowing instructions based on extensive experience, should ensure the complete success of the bonding of Showa foil strain gages. In applications where it is considered there may be special problems, we will be pleased to give any advice and assistance we can.
1. Specimen Surface Preparation.
An area larger than the installation shoud be cleared of all paint,rust etc., and finally smoothed with a fine grade emery paper or fine sand blasting to provide a bonding surface.The area shoud now be degreased with a solvent such as trichiorethylene and finally neutralised with a weak detergent solution. One should use tissue for this operation, wetting the surface and wiping off with clean tissues until the final tissue used is strain free. Care must be taken not to wipe grease from a surrounding area onto the prepared area or to touch the surface with the fingers. This final cleaning should take place immediately prior to installing the strain gages.
2. Strain Gage Preparation.
Normally the gage is ready for applying as soon as it is removed from the packet but experience shows that some engineers prefer to roughen the back of the gage before appllying it. Extreme care should be taken and the area under the tags should be avoided. One method is to sprinkle pumice powder onto a piece of blotting paper and with one finger tip lightly rub the back of the gage over the powder. Remove all products of the abrasion and wipe back of gage with a tissue.
Note: It is advisable not to mix the adhesive until all the gages to be intalled have been prepared to this stage.
3. Strain Gage Installation.
By sticking a short length of sellotape lengthways along the upper face of the gage it may be picked up from a flat clean surface. Holding both ends of the tape,orientate the gage in the desired location and stick the end of the tape furthermost from the tags, to the specimen. Bend the other end of the tape back on itself thereby exposing the back of the gage.
Adhesives.
Three basic types of adhesives are recommended: (1) Epoxy resin,(2) Phenol-Epoxy resin and (3) pressure sensitive (Cyanoacrylate series) adhesive.
The single component pressure sensitive adhesive is recommended where fast bond and thin glue are optimum requirements as this adhesive reacts immediately upon water contained in the atmospheric air.
For an installation where long term stability under adverse atomospheric conditions is the main requirement one should use Epoxy or Phenol-Epoxy system. F3 cement is simple to use and may be cured at ambient room temperature, whilst F1 cement has excellent heat resistance quality. E110 is most suitable for use with MA Series (Polyimide Backing) gages for high temperature application.
1) Epoxy Adhesive F1 and F3.
Coat the exposed back of the gage with adhesive and gently push the gage down into position, at the same time wiping excessive adhesive to the two outside edges of the gage. Stick the whole length of the sellotape to hold the gage in position, cover the area with the piece of polyethylene provided and apply a light weight or clamp as required. Care should be taken that there is an even layer of adhesive and no air bubbles are left under the grid. The installation is now ready for curing. After curing remove the tape as per para. 4.
- F1 10 parts resin : 2 parts hardener 2 hrs. at 100℃
- F3 10 parts resin : 6 parts hardener 24 hrs. at room temp.
Of this two pack adhesive,the base material (A) is inert, and this should be harmless when in contact with human tissue.
The hardener (B) is slightly toxic and can possibly be harmful if allowed in contact with human tissue.
Warning:
- Do not allow the mixed or unmixed materials to contact skin. Protective gloves should be worn.Should skin be inadvertently containated it must be washed off immediately and thoroughly,with soap or detergent and water.
- If heat is applyed to accelerate the cure time of the adhesive then adequate ventilation is necessary to avoid inhalation of resulting fumes.
2) Phenol-Epoxy Adhesive E110
The cement is spread by brush or by spatula on both the specimen surface and strain gages and these must then be left in this condition and dried in a clean atmosphere for 1 to 3 hours in order to allow evaporation of solvents from the cement. If cement is applied by spraygun, the cement should be diluted before it is applied, by methyl-ethyl-keton until its solidity rate reaches to 20%. After drying ,both the strain gages and the specimen surface are contacted face to face and clamped and heated in an oven for 30 minutes at 140℃ to complete bonding.
3) Pressure Sensitive Adhesive 101
Follow strain gage installation instructions as above sticking one end of the tape down to the specimen completely up to the gage. Drop a fillit of adhesive in the 'hinge' formed by the gage and the specimen. Starting at the fixed end with one finger push the gage down at the same time pushing the adhesive along the gage in a single wiping motion until the whole gage is stuck down. Apply pressure with the finger over the whole length of the gage for one minute. Extra attention may need to be given to the tag and lead wire area.
4. Removing the tape.
Remove the tape by slowly and very carefully pulling it back over itself starting at the end furthermost from the tags. Do not pull upwards.
5. Wiring.
Showa strain gages are fitted with short leads and it is standard practice to wire these to small stick-on or self adhesive terminals placed adjacent to the gages. These serve as a bridge-completion point and a change-over point to the heavier wire requred for the run to measuring or recording instruments. The lead out wires from the gage are fragile, and should be handled with care. Preparatory tinning of the ends of the leadout wires, connecting cables and terminals is recommended. be sure to remove all traces of flux or soldering paste with trichlorethlene.
6. Installation protection.
Showa strain gages are encapsulated and therefore are protected from dust and draughts, etc. This encapsulation serves to make any required from of protection all the more efficient. In choosing a protective coating one should study completely the enviroment in which the installation is to function and the length of time the installation will be required to function in such environment. One should also pay special attention to the wiring especially if the installation is required to be immersed in water. There are numerous forms of protection available and we will be pleased to advise you on your particular installation.
