Maximum control cable length

[BRIS]

I need to regulate a flow control valve from a pressure transducer located 9 miles downstream. For safety reasons the client requires the communication is hard wired. I am advised that the length is too long to send a signal from the pressure transducer to the control valve via cable (hard wire).

What is the maximum length that we can send a signal by cable ? Can I provide an amplifier to boost the signal? Cost is important - how much am I looking at to send a hard wired analogue signal from a pressure transducer to a flow control valve over a length of 9 miles. ?.

 

[KRSServices]

Have you considered utilizing the phone lines and/or telemetry systems. This technology is always improving and for the most part is pretty reliable. Wireless is good too, although I have found it has a few more hiccups.

KRS Services

http://www.krs-services.com

 

[ivanlocke]

Depending on location and such you may be able to make a deal with a communications company where they will install fiber, a line for you and lines for them, in exchange for easement rights.

 

[tjmurf]

What is the output of the transducer, voltage or current?
If its current and loop powered you can transmit for miles!

If the positioner has a 4-20 mA input, this means the input resistance is 250 ohms. Add the positioner resistance to the overall signal cable resistance. Then, using ohms law calculate the require voltage for the maximum current based on the total resistance.

Assuming the 250 ohms for the positioner, at maximum current the positioner will drop 5 volts.

The resistance of 18 gauge wire is 0.00651 ohms per foot * 2 because it is in series with the positioner = .01302 ohms per ft. of cable, * 5280ft/mile = 68.75 ohms per mile, * 9 miles = 620 ohms.
Positioner resistance 250 ohm + Cable resistance 620 ohms = 870 ohms.

Ohms Law I * R = E
20 mA * 870 ohms = 17.4 volts are required, “minimum.”

If the positioner is the only load on the loop, a standard 24 VDC instrument power supply should work. Any additional 250 ohm loads for chart recorders etc. will require an additional 5 VDC for each. It should work.
You could also use a larger wire gauge which will lower the resistance per/ft.
Make sure the cable is a shielded twisted cable.

 

[BRIS]

Thanks for the response - my field is civil engineering and wires and signals are a dark science to me.

I should have added that we are in Africa and technology is basic. Radio is an option but will require permits which are difficult to obtain.

I assume that the transducer can be current and loop powered but I am not sure what that means?.

Thanks again for the advice.

Brian

 

[tjmurf]

You stated, "For safety reasons the client requires the communication is hard wired. "
Why not post the names and specs of the equipment / instrumentation you propose to use. Maybe I can help; however, if you can use wireless, that would be the way to go. I just wanted to state the distance is not too great and it is possible.

 

[BRIS]

At present I need a feasible concept and your statement that the distance is not to great and it is possible really answers my question.


Yes - the client has specified hard wire for safety. Dual radio has been proposed as an alternative but the client is not convinced it will provide adequate safety.Also there is a bureaucratic problem with permits.

Thanks again

Brian

 

[shobi]

I think with a regular loop powered transmitter it should not be a problem. But how are you going to control the flow with a pressure transducer? Do you mean a differential pressure transmitter?

 

[wri]

One significant problem with a long run of continuous copper is the extreme likelyhood of surges being induced by near lightning. This condition is present everywhere and should not be overlooked. If the owner needs point to point control then a fiber optic link should be considered.

If a line of sight can be established between the ends of the control loop then radio can be successfully used, even spread spectrum which is license free in the US.

Radio could be much cheaper all around and when installed correctly with good grounding is very immune to lightning.

As far as redundant radios, I argue this point regurarly that your standby radio should be on the shelf. In the unlikely event lightning should take out a radio station, every online piece of equipment is equally vulnerable. Radio telemetry systems can be built that are extremely dependable and robust and can last 20 years or more running continuously.