I can;t figure out what your trying to say either.
Generally, you have a sensor and a meter or you can have something like
https://proteusind.com/wp-content/uploads/2016/06/8000TRM.pdf
and a readout.
You can see some sensor wiring here:
https://assets.omega.com/manuals/te...flow/paddlewheel-flow-meters/M3814.pdf#page-4
A sensor might just be +5, gnd and an open collector output transistor.
If you requre an open collector sensor and the sensor is not, you can add a Shockley diodse or sometimes a regular diode.
Look at the output of your sensor, unpowered with the diode function of your DVM. Also check for resistance to the + power supply.
Check the readout too, to see if the input is pulled up to some level.
You can load the readout with a potentiometer until he voltage is 1/2. Measure the pot and this is the input Z of the readout.
OPTOMOS relays, basically an optocoupler have really low input current requirements, on the order of 1 mA, but you;ll have to add a resistor.
There are lots of optocouplers, but the OPTOMOS relay, the schmidt trigger, FET and bi-polar transistor might be worthwhile to look at.
A diode might be all that's required.