Each module includes three strain sensor and each ones give 120 Ohm.
I would like to read the sensors by using Arduino: should I just wire them by considering them like resistors?
How can I test them? I tried to manually apply a force but the resistance value is always almost the same. Unfortunately, I do not have a datasheet since I purchased them on alibaba and they came with no documentation.
What can be a typical resistance range for them?
Buying sensors that has no datasheets sounds "not good" especially as Your knowledge is not "on top".
I suggest You use Google on strain gauge sensors and check how they work.
Bending using the fingers sound a little bit unsafe to me. Don't overload them!
You tell they show 120 ohm while resting. I guess they can change both positively as well as negatively. If You want use the entire range build a voltage divider using a 120 ohm resistor, nd the sensor. Vcc / 2 would be the analogue value read for sensors in neutral.
The change in resistance is going to be very small, and difficult to measure.
Information found by entering the model number into a search engine suggest that the gauge factor for those strain gauges is around 2.
Basically in simple terms this means that the percentage change in resistance is about 2 times the percentage change in length of the gauge - and that is minute.
It is normal to arrange 4 strain gauges into a wheatstone bridge and then use an instumentation amplifier to get a voltage that can be fed into an analogue input of the Arduino.
Except that the OP has purchased a rosette (for unknown reasons), consisting of three independent strain gauges, each of which is normally completed with its own quarter bridge Wheatstone arrangement.
Look at the HX711 board. You can find them on eBay.
You should know that bonding the strain gauges to some substrate is not a simple glue and all done. As stated above the strain in the gauge results in very small changes in resistance. If you bonding is not up to par movement at the bond line can completely invalidate any measured results.
Perhaps Omega engineering website can give you some hints.
All of the strain gauge applications I’ve seen involve a high complexity of mechanical design as well as sensitive electronics.
You may be better off dissecting an existing electronic scale, than trying to use bare gauges…
