Level sensor with relay outputs

I sugest you do it by yourself.

take a plastic plate float on water surface and a simple on/off circuit enough.

if you need measure level high value. you can select a ut sensor. it somewhat expensive.

it uses a

Field Adjustable Set Point - high resolution/high accuracy field adjustment mechanism, Adjustable set points from 0.1" H₂O to 30 psi, Gold inlay contacts, Variety of options for ports and mounting, Small size and lightweight, PCB Terminals Available

http://www.designflexswitches.com/designflex_psf102.htm

With a cost of 20 Euro you can buy a level sensor circuit that has three electrodes which give the level of the water and an out put that can powers even a 220 V power relay

If I understand what you are asking, you want to interface a relay, perhaps for the purpose of alarm signaling, to the 4 to 20 mA output of an existing level transmitter. That is going to require more than just a simple relay. Relays are boolean devices. You need to recognize the mA signal and actuate a circuit at a specific output. Sometimes this function in included in the level transmitter. Sometimes it is a function of the DCS, or panel indicator. If you can't find the functionality in the manual for the field devices, you need to consult your vendor. He'll hook you up.

If I understand your situation, you already have a level transmitter that puts out a 4-20ma signal for remote indication. What you want is called a limit alarm. It can be as simple as a large relay looking device that plugs into an 8 or 11 pin socket, to an 1/8 DIN panel meter with two output relays. The small relay type device has two pots that are accessed through a hole in the case. I have not used one of these in a while. Wilkerson, Action, and several others used to offer these ~$200. Most of the panel meters with limit alarm option are microprocessor based and you set the points you want the relays to trigger and the histerysis through parameters in a menu. All the people who do temp and process controllers should have an offering, ~$200-$400 the Red Lion PAXP might work. Another option would be one of the new pico PLC/relay replacers. Some of these are less than $200, have one or more inputs that can be set for 0-10 analog. You can configure a program through a device mounted keypad and don't have to use software. In this option, I would recommend using a 250 ohm burden resistor across the input to change your 4-20 into a 1-5 volt signal. If you prefer to use the PC, software will run you another $100-$150. Allen Bradley or Klockner Moeller

It is really very simple.

Just connect the output of your 4/20 ma output device to inverting & non-inverting op-amp connected as comparators.

You can also look up the GE SCR manual to connect transistors to act as comparators with built in hysteresis to prevent relay chattering.

Cost would be actually be negligible. A few dollars at the most.