@marceltrapman Optic fibre? Just a passing thought...

@yveaux Perhaps an over-exaggeration

@eddiever I mean no offence by this, but might it not be wiser to describe what you are trying to do so that you might be better informed what to get and why, rather than to make a wild guess ?
For every success on this forum there are a hundred who failed and could readily advise you not to do as they did...

@raptorjr If you can figure out what the filter differential pressure should be when requiring serviced (perhaps from the filter manufacturer or a manual?), it should be possible to find a pressure switch which triggers alarm on the supply side of the filter.
If I recall correctly, some water supply pump pressure switch controllers can be turned down to trigger between 5 and 30 psi, perhaps an old one can be found and re-purposed if it fits the range you are aiming for, however fugly they are...

@raptorjr You are not looking for water flow at all, you simply assumed you were...
Perhaps a sampled differential pressure sensor, or a differential pressure switch might be your solution ?

@mfalkvidd Gland plugs are almost bulletproof, perfect for the selected diameter. The beauty of the free grommets usually accompanying cases is that they may be cut to size, whether that is an antenna or a cable conduit...

@รอเร-อ It was posted earlier, but hopefully you can figure it out from the comments and photos.
The 5v SLAVE pro-mini is facing front, but is connected behind to female sockets glued to the duct cap including the offset I2C lines a4 and a5. The duct main body is a holder for the battery holders, for the SLAVE 4xAA, for the MASTER, 2xAA.
The conduit itself is glued to the base of the case, leaving the cap free to slide but not detach, the pro-mini is free to be detached when pulled from the sockets.
The MASTER has an onboard RTC so is not intended to be removeable. There is a permanently attached USB power cable to enable battery changes without losing the RTC......
Simple re-purposing of preformed plastics for alternative uses...
https://forum.mysensors.org/topic/9424/an-ultrasonic-measurement-saga-finally-over/2

@rozpruwacz Completely agree, if the objective is to change batteries or reprogram, there are alternatives not subject to other than the controlled environment of the case.

As a postscript to my earlier post, one of my 5v Slave (sub) Nodes was assembled using female strips to connect the MCU holding it near the front transparent flexible face of the case.
The female strips are mounted on a slide out tray (actually the cover of a plastic conduit glued to the back of the case), the conduit holds the two AA batteries in a case for easy access and replacement.
The outcome is that the MCU can either be programmed in place using the sliding conduit cover to get the pro-mini clear of the case to use ftdi, or completely removed for external mods. Battery replacement should be easy enough, but haven't needed to do so yet...

@รอเร-อ Avoid external connectors where possible, corrosion or rain penetration are avoidable.

I use hinged transparent window (weatherproof light switch) boxes hacked out on the inside with a Dremel, and can lift the cover to reprogram or change batteries in minutes and close it up on site. A desiccator takes out any air moisture trapped from that opening.
All cables enter the base of the box through a grommet/seal tight around the cable or a 16mm cable duct foam sealed on the end, with internal connectors, and no issues with water entry at all.

@raptorjr A bit vague on flow range, pipe/channel, inside/outside, accuracy required....
If it is a piped flow of clean water, a standard domestic water meter (new or second-hand) with a pulse output (reed or fet) would be my choice. Usually the meter design dictates pulse (1, 10, 100 litres) accuracy, the advantage is that no power is consumed other than to respond to the interrupt as @mfalkvidd outlined, and you have the register to cross-check results.
I have a plastic bodied fet type (not cheap for the sensor) in the house reporting every litre back to Domoticz since last November, two AA cells currently read 3v, they will die at 0.9v sometime in late 2019 or 2020.
I also have a gas meter reed sensor (same principle) reporting over the same period, with the same battery condition).
Zenner and B-meter etc water meters are available down to 10 litres/pulse, the Zenner sensors are silly money but the pocket for them takes a cheap small reed switch which serves the same purpose. There are also cheap chinese apartment style domestic meters also around with reed built in.

IMHO anything else is going to bring more problems over power supplies and accuracy...