How to transmit 4-20mA at 2000 meters away? 1

[Emanuel F Lemos]

Hello guys,
I need a technology suggestion to tackle the following problem:
Send a 4-20 mA signal from a PIT (Pressure Indicator Transmitter) to 2000m depth of sea level.
Most instruments give me the option of Modbus RS485, but this one goes only up to 1200m in range. Others offer me 4-20mA + Hart, however in this case it is impracticable to send this signal, of that amplitude at 2km distance (from what I researched).
Do you know of any new technology or other options to make this transmission?
Thank you very much in advance.

 

[Skogsgurra]

Back to "basic" 4-20 mA (no Hart) allows for total loop resistance around 1000 ohms if your supply voltage is 24 V DC.
1000/4000 (two conductors in series) is 250 mohm/meter. So you need a Cu conductor with area better than around 17.5/250=0.07 mm2. I guess that most cables for deep sea use have more area than that.

In difficult cases, there are systems that have a higher compliance voltage. But I don't think you need that.

Question: Is the basic 4-20 mA to "crude"? Do you need better precision? Or do you need high band-width. That could be a problem with a current loop and high cable capacitance. The first Atlantic telegraph cables had that problem. But length was somewhat more than 2000 m

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[JG2828]

Can you send it over fiber?
We use RLH products to convert many different types of signals to fiber in electrical substations.

https://www.fiberopticlink.com/product/4-channel-420ma-010vdc/

 

[danw2]

2,000m distance is doable with a two wire, loop powered 4-20mA pressure transmitter over copper wire of a sufficient gauge/diameter to have a resistance that in combination with the receiver's input resistance is low enough that the DC loop power supply voltage can pump 23.0 mA through the total loop resistance (wire resistance plus analog input resistance).

Here's a typical load line for a 2 wire loop powered pressure transmitter

The total loop resistance is the wire/cable resistance and the analog input resistance. Although not universal, 250 ohms is a widely used, common input resistance (any industrial analog input shows the input resistance on its spec sheet).

The transmitter will operate (fire-up) at 10.8 volts, but will not drive any loop resistance.

A 250 ohm loop resistance (an on-the-bench situation: the transmitter, a 16.48Vdc power supply and a 250 ohm resistor in the loop to provide enough resistance for HART to operate) will let a tech configure the transmitter with HART handheld.

A 24vdc power supply will drive almost 650 ohms, or 400 ohms of wire/cable length in addition to the assumed 250 ohms of an analog input. The load line graph shows higher voltages can be used for loops with higher total loop resistances.

Wire resistance is out-and-back, so 2000m run is 4000m of wire resistance.

18AWG is roughly 0.75mm2, which an internet table says is 24.13 ohm/km. Four km of 0.75mm cable (`18g) is roughly a 100ohms resistance, well below the 400 ohms that a 24Vdc power supply can handle.

An off-the-shelf 24Vdc power supply will drive that loop.

I don't have a clue about the mechanics of supporting the weight of a copper cable that length nor whether the requirements for immersion in seawater, but electrically/signal-wise, copper cable will work.

 

[Emanuel F Lemos]

But how can I use this converter if I don't have power supply up to sea deep? And the panel for this (above the water, 2000m) doesn't exist.

Thanks friends.

 

[Skogsgurra]

No panel? No PSU? Kidding?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[JG2828]

But how can I use this converter if I don't have power supply up to sea deep? And the panel for this (above the water, 2000m) doesn't exist.

You would need to install the RLH fiber converter in a box 2000M below the ocean (if this is possible), then run 24vdc power wiring and fiber from this box back to the surface. You will need another box at the surface with another RLH converter and the 24v power supply to power both converters.

However after a little researching there could be issues at that depth with various electronic components rupturing due to the pressure. See this link.

https://www.uutech.com/ptepaper/

 

[Compositepro]

It would make better sense to use a higher voltage to send power to a power supply than to use 24 volts to supply the power.

 

[JG2828]

It would make better sense to use a higher voltage to send power to a power supply than to use 24 volts to supply the power.

According to the spec sheet, that RLH converter will run on 48VDC if that makes a difference.

 

[itsmoked]

danw2's probably gnashing his teeth about now... He's already nailed it. Just use 18AWG and a 24V supply and be done with it. You can't have wire smaller than that hanging in the ocean and not expect it to be broken by the first fish that runs into it. You need some mechanical strength.

And, where is the advantage of fiber if you have to send copper down to power it?! A crazy complication at 2000 meters (nearly 3,000 psi).

Keith Cress
kcress -

http://www.flaminsystems.com