# How to make a full Wheatstone bridge with 4 tee-rosette strain gauges?

I am in the process of making a full Wheatstone bridge. I was told to use 4 tee-rosette strain gauges (with axial and radial components in one gauge) to temperature correct the signals. I am interested in measuring amplitudes and phases of stress signals. I understand how to do this for 2 tee-rosette gauges but I am uncertain how to do it for 4 tee-rosette gauges. I guess you need to put them in a series but I am not sure how to wire it. I was also told that it should only take 4 wires out: voltage (V+ and V-) and measuring (m+ and m-).

How do I configure this? I tried something like this but I realized that it will not work because in a full-bridge two resistivities needs to increase and the other two decrease. This alas is not the case for the bridge I drew. Strain gauges are bonded equidistantly around a quasi-square.

@DKNguyen requested further information. The aluminum piece that I am supposed to bond the gauges too looks like this

in which the upper quasi-square is the top view and the bottom rectangle is the side view of the aluminum piece, while the red lines indicate the strain gauge position. It is basically a cylinder with trimmed sides to have flat surfaces to bond the strain gauges to. It is used to measure amplitude and phase of an axially applied sinusoidal wave. A1 and R1 denote axial and radial components of strain gauge number 1. The same notation applies for the remaining strain gauges (2, 3, and 4).

• Oh so it is a tube? Or rod? So you are measuring bend (axial) and torsion (tangential)? Commented Mar 25, 2020 at 14:14
• It is basically an aluminum reference that we use to compare with porous materials. It is kind of like a cylinder with trimmed sides to have flat surfaces for the strain gauges. Commented Mar 25, 2020 at 14:18
• understood. What are you actually trying to measure? Bend? torsion? expansion? It is unclear. How are you actually using this? Are you compressing it axially? I can see 6 wires...two for shared supply and two output for each axis...but 4? You typically have to measure the axis separately or your reading will be nonsensical unless your objective is different. The only case I can see that combines them all is axial compression and measuring the radial expansion strain and axial compression as a single reading which still doesn't seem ideal. Commented Mar 25, 2020 at 14:24
• Sorry, yes. It is compressed axially by a sinusoidal wave. We are interested in amplitudes and phases of the resulting strain signal from the aluminum. Commented Mar 25, 2020 at 14:28
• Understood. See answer Commented Mar 25, 2020 at 14:46

Taken from Sparkfun and added annotations

Note how gauges in the diagram with the same direction of stress are DIAGONAL to each other on the bridge so their effect is additive. Otherwise they would cancel each other out. This might not be the same as being physically diagonal on the mechanical mounting.

Therefore group all elements sharing an axis into two equal groups connected in series within the group. That makes two tangential and two axial groups. Place in bridge appropriately with like diagonal to like, and opposite above and below each other.

I believe best results are obtained when the elements in a single group are mounted far from one another at the expense of messier wiring.

I am unsure if radial expansion strain and axial compression strain are equal and opposite (as it is for tension and compression in bending) but they have to be for what I said to work.

• Thank you, @DKNguyen. I edited my post to clarify some of your comments. A colleague told me it is possible to wire it into one gauge without elaborating on how to do this. Another problem is that there is a limited amount of connectors -- that is why I was told to make it into one 4-pin wire. Commented Mar 25, 2020 at 14:14
• Thank you again, @DKNguyen. You have truly enlightened me. I still wonder however how you would go about wiring them into the said two groups for this to work? I hate to ask but could you provide a schematics? Commented Mar 25, 2020 at 14:46
• @naughty_waves I can't draw now but if you labelled each strain gauge element in your first drawing, I could type it out. Commented Mar 25, 2020 at 14:47
• I updated the image with notations. A1 and R1 denote the axial and radial component of strain gauge number 1. The same applies to the remaining strain gauges (2, 3, and 4). I am truly humbled by your willingness to help. Thank you yet again, @DKNguyen. Commented Mar 25, 2020 at 15:04
• @naughty_waves Your four groups are: Any two Ax connected in series, the remaining two Ax connected in series, Any two Rx connected in series, the remaining two Rx connected in series. For example, A1+A4, A2+A3. R1+R4, R1+R3. In schematic, A1+4 top left, A2+3 bottom right, R1+4 top right, R2+3 bottom left. Commented Mar 25, 2020 at 15:08