Vertical Float level indicator with reed switches is normally use inside the hydraulic oil tank to indicate the oil level of the hydraulic system. Normally, it has a diameter of 25mm to 35mm diameter. Some have 2 level and some have 3 level.

You have to be careful with the installation. It may not work as you has expected. The reel switch operated by magnetism located at the floating actuator. When you install the reel float switch inside your hermitically seal transformer and when your transformer is energized, the magnetic flux that generated by your transformer may cause faults signal to your reel switch and thus, it will cause tripping to your transformer.

You still need to drill a hole at the top of your transformer steel cover, as I have explain early, it is also dangerous as there is HV and LV voltage circulating in the transformer oil.

Before you proceed to install the reed switch, you need to have a deep study about the advantage and disadvantage of this reed switch modification.