Jose:
As I posted before, I decided not to do anything about installing a sender for my oil gauge. But as you did, I struggled with the solution of installing a short pipe and then a a T-fitting to accommodate both a new temp sender for the gauge and also the original Yanmar temp switch for the idiot light and the alarm on the Yanmar panel. I decided on the option that you are now contemplating to tap into the hose you are showing in the picture. I have the 2QM model, but the configuration is similar.
The reason I went with the hose option is that for the engine T-option, with the sender and switch now way out from the engine, I thought that the temp readings could be very much on the low side. Instead of real time water temp passing over the sensors, the switch and sender would be reading water temperature that also would be affected by the the ambient air temperature of the engine room. Also, the senders/switches are affected by the temp of the engine block which they are normally threaded into. At the end of the T, the metal temperature could be less, particularly since the water is dead-end out there and not flowing normally over the sensors. Sort of analogous (sp?) to taking your body temp by squeezing the bulb or the thermometer between your thumb and forefinger. Could be quite different from the body's core temperature.
With the hose tap alternative, I guess there is also the issue of the ambient air temp affecting the sensor. So is where I mounted the gauge sensor. All I want to see from the gauge is that the water temp is more-or-less normal. Exact measurement is not critical. The hose mounted sensor T-fitting could be wrapped with insulation fabric of some sort to minimize the air temp interference. The original alarm sensor remains in its original spot. This is the thing that will first alert me to a problem.
As kloudie1 cautioned, engine vibration/movement needs to be a concern with the modifications. With a hose mounted arrangement, I observed that the extra weight of the t-fitting and sensor and clamps in the middle of the hose run would cause more flexing and risk of hose failure. In your pic, there are two hoses running parallel... same as mine. I "bonded" the two hoses together at the location of the new mid run fitting using electrical tape then further secured then together with nylon electrical ties. That seems to have done the trick .... the hoses have mutual support. But this location is now on my visual engine inspection list.
Also if you do the hose location, a ground wire to it is needed.
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