Strain Gage – Installation

1.) Prepare the application area. I use a rougher sandpaper to remove the spray paint, then a finer one to ensure there’s a flat, uniform surface. The gage manufacturer recommends “wet abraiding,” or dampening the surface before doing a final fine sanding. Before moving on, it is important to make sure that the surface is very clean and dry. I wipe it with a damp cloth to remove dust then place under bathroom power dryer.

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2.)Place the gage on the surface.  I do this using tweezers or a small pair of needle nose pliers. It is important to only hold the gage by the terminal end (as shown), so as to not damage the precision foil. Be sure the gage is placed in the proper orientation for the loads you wish to read.

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3.) Place a piece of single-sided scotch tape over the gage, with enough on each side to grab and lift (see step 4). This can be the most frustrating step, as static can pull the gage onto the tape before intended, misaligning the gage. In this case, I will either remove the tape, and replace it such that the gage is realigned, or remove the gage from the tape and repeat step 2.

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4.) Lift the tape from one side (as shown) and place a line of glue where the tape meets the part behind the gage (as shown). The Loctite liquid Super Glue is all I have ever used and it has consistently worked very well.

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5.) Place a finger at the base of the tape where it is still attached and (while applying pressure) slide it along the length of the tape. This should “squeeze” the glue out underneath the gage. Hold this down with your finger for 30 seconds to a minute.

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6.) Remove the tape at a very shallow angle (as shown). The gage should remain on the piece. If the gage begins to come up you may need to repeat step 4, or even go back to step 2, to ensure that the gage has not been glued to the tape.

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7.) While the superglue sets, I use this time to apply the pin headers for wiring the gage. This is a tricky process. I use 5-minute plastic epoxy from Loctite. *****An important trick is to mix a small amount  of epoxy and wait a few minutes for it to partially set, while periodically agitating it.***** While this is happening, I’ll place the (set of 2) pin headers as shown. It is important to be sure the longer (connective) ends point outward. I rest the little flange made by the plastic on the headers against the corner of the piece, and orient and hold this as shown. When the putty has reached a putty-like consistency, I will use it to mold a shelf against the side of the piece under the pin header (where the pencil is pointing). See the existing pieces as an example. Once the header stays in place without being held, I let it dry for a few minutes. The shelf may need re-molding throughout this time. I then place a light layer of fresh epoxy over the shelf, and everywhere that the pin header contacts the part (don’t worry about covering the short ends of the header, this is inevitable, but the less-covered the better – see step 8).

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8.) Once everything has had time to set, the header needs to be soldered to the gage. I use individual strands from a multithreaded wire. See other pieces for an example. The small side (terminals) of the pin header are likely buried in epoxy. I use the heated soldering iron to scratch this away and expose the metal. This is best done swiftly, as the heat will be quickly conducted through the header, and which can begin to un-set the epoxy or burn you if you attempt to grab the pin headers. Obviously, be very careful with the soldering iron near the gage. I generally solder the pin-header end first, then use tweezers to bend the wire strands so that they lay across the gage terminals. This simplifies the process of soldering the strands to the gage.

9.) Check the connection. I do this using the ohm-meter setting of a potentiometer, and contacting the two pin headers. They should read approximately 350Ω.