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. ?.

 

[IRstuff]

Perhaps fiber optic cable?

TTFN

 

[llrelease]

Can you digitize the analog signal and output it via RS-232 (serial port)? If so, take a look at the dataforth LDM70

http://www.dataforth.com/catalog/pdf/Ldm70.pdf

According to the specs, this modem can be used to transmit data up to 12 miles using a twisted pair cable at up to 57.6K baud rate with hand shaking. I have not used one, but I am interested the application and found this device on the internet. I am sure there are others available as well. Hope this helps.

 

[ICman]

You can go maybe a mile with a 24VDC signal, depending on the wire size and a bunch of other factors.

I would suggest fiber optic or spread-spectrum radio. Maybe a dual-path / dual-radio setup?

I am in the Gas Industry and we have several critical measurements and controls via radio. Historic data is stored locally. PID control is also local, with setpoint coming from a remote location. Local instrument is powered by battery with photocell charger.

Then, there used to be several methods using telephonic communications (the old Bristol Metameter or FSK equipment). Perhaps they still make them? I wouldn't use repeaters on a 24VDC system - much more likely to fail.

Good luck!

 

[xnuke]

I've seen leased line modem-to-modem communication used for long distance communication of signals before, but have never done it myself.

On the process side of things, it seems like there would be a long transport lag. How long do you think it will take for the valve position change to be felt at the pressure sensor?

xnuke

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[BRIS]

Thanks for the response - my field is civil engineering so 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.

The available lag time between the valve change and the change at the pressure sensor can be several hours - (The pipe feeds into a tank with two days storage.

Perhaps a man and a bicycle are my best option?

Thanks again for the advice.

 

[ICman]

Some radio frequencies do not need permits in the U.S., such as those used for cordless telephones. I would think they have cordless phones in Africa, don't they? If not, there may be other frequencies that you can use that do not need permitting. We commonly use bands at 900 MHz and 2.4 GHz.

There are several vendors you might talk to. Try

http://www.scadalink.com

or

http://www.omnexcontrols.com

(both Canadian companies), or search for "spread spectrum" on the Web.

 

[IRstuff]

Fiber optic is probably the cleanest approach to your problem, and the cost does not appear to be that outrageous:

http://www.siservices.com/prod/Fiber_Modems/t1e1_fiber_modems.htm

http://www.rad-direct.com/static_fam_fiber_optic_modem.html

They all seem to support transmission distances well in excess of 9 km.

TTFN

 

[xnuke]

I'd go with radio telemetry. I can only imagine the cost and scope of pulling 9 miles worth of fiber optic cable in Africa...yikes!

xnuke

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[hacksaw]

put a second transmitter near the valve and wire it to your controller normally.