Level Tx to RTU

Yes, you can run 4-20mA 1Km. Steel mills have run 4-20ma 2km and longer routinely.

About 2 years ago, someone asked why his level reading failed whenever the level exceeded about 70% of the max level.

His 24Vdc power supply only had enough voltage to drive about 15mA of current through the combined loop resistance, the wire resistance, the internal resistance of the transmitter and the load, analog input resistance over 2 Km of wire. He fixed it by bumping the voltage about 2V.

The resistance of 2 Km of wire (one Km out, one Km back) is substantial. Wire resistance tables provide the details.

Add up the wire resistance and the analog input's resistance to see what voltage the transmitter needs to drive current through that resistance. Transmitters have a chart of voltage vs resistance or a statement like, "max 600 ohms @ 24V"

A long term issue is splice junctions corroding underground. Corrosion will add resistance. Well sealed wire splices are possible; I've seen telephone installers squirt jel sealant and wrap the splice with really sticky gooey tape that holds up against water intrusion underground.

The alternative is wireless.

How high your reservoir is?

Adding pressure transmitter on pumping side is maybe also possible/alternative. This depens of piping and land shape. (10meters = pressure 1bar on bottom of reservoir).

Height

The reservoir is 3 meter useful volume.

Also, I would to know if a level transmitter with Modbus RTU RS 485 could be used over this distance.

Is radio-telemetry out of the question?

In my 21 years of pumping water into revervoirs, I never had a cable run between pump station and reservoir that did not become problematic. We had everything from lightning strikes inducing damaging voltage spikes, pipe leaks damaging cables (especially if the leak washes out some earth and things shift positions), to troublesome splices, you name it.

IMO, it is money well-spent to use radio telemetry and be done with it. Nothing ever has to be excavated for repair.

I am with Blaster.....wireless.

I built a solar powered Modbus level transmitter station and it has held up well.

http://www.bb-elec.com/Products/Datasheets/ZXTx-IO-xxxx-0712ds-pdf.pdf

Radio

Since, it is only at maximum 1 km apart and with radio comm., licensing is required at the frequency and have to be renewed yearly. That's why I would prefer to connect the level transmitter which may be powered by solar and hardwired either by 4-20 mA or level transmitter Modbus RTU to the RTU at the PS.

Tell me what options you would opt in this circumstances.

Thanks and kind Regards

1000m is near the limit of RS-485, probably at 9600 baud. 4-20mA can easily be driven that distance.

I'd pick a shielded, twisted pair cable that's suitable for underground service. It should work for either RS-485 or 4-20mA.

I'm a Modbus guy, but on a long underground cable run for a single value, I'd opt for 4-20mA because it's current, not voltage and can tolerate some splice and junction box issues, like corrosion, better than voltage signals.

The reason for picking Modbus would be a requirement for multiple values. It's much harder to troubleshoot RS-485 problems than it is to troubleshoot a 4-20mA current loop.