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Mysterious relay in water telemtery panel / SCADA
5

Mysterious relay in water telemtery panel / SCADA

Mysterious relay in water telemtery panel / SCADA

(OP)
Hi folks,

I recently was tasked with replacing some old water system telemetry panels with new SCADA gear.  I did so, but there was a mysterious relay in one of the old panels.  It cycled on/off every two seconds.  Since I was replacing everything, I never found out what this relay did.
Recently I started upgrading a second water system.  I found a relay in this panel too, same 2-second cycle time.
This panel measured only two 4-20ma levels from two water tanks.  I pulled the relay and the loop current fell to zero.  Anyone have an idea what this relay's purpose might be?  Thanks in advance,
-
Robert
 

RE: Mysterious relay in water telemtery panel / SCADA

Did the old system have an external light bulb that blinked in a two-second cycle (on 1s, off 1s) when there was a problem?

The reason I ask is that the underground infrastructure around these parts has the usual cute Yagi antennas pointed off in odd directions, but they also sometimes have an orange light bulb on a pole that blinks when there's trouble.


 

RE: Mysterious relay in water telemtery panel / SCADA

2
Maybe it was multiplexing the two analog signals because the old system only had 1 analog input. I've done that trick when the response time is so slow that reading once every 2 seconds was not a problem.


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RE: Mysterious relay in water telemtery panel / SCADA

That was my thought also, jraef.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Mysterious relay in water telemtery panel / SCADA

(OP)
Thanks, but I have found the same relay in a system with only one analog input.  Maybe it once had two, I've fould all kinds of add-ons / changes that were never documented.

The system that has two 4-20s has two completely different set points, so I think this relay must do something else.
Maybe I'll just have to trace it out.

The other day I was working on one of their sytems that had a phase failure relay in it.  I found that the original installer had wired it out of sequence, so to get it to work, he used the normally closed contacts.  So it would have worked to protect against phase sequence reversal, but not phase loss.  It had been wired that way for 25 years.

RE: Mysterious relay in water telemtery panel / SCADA

Welcome to my world... the world where people don't take the time to truly understand a problem before attempting to fix it, which usually just compounds the problem. By the time I get to it, there are multiple layers of problems.

By the way, Sequim Washington?  


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RE: Mysterious relay in water telemtery panel / SCADA


Some legacy process-control systems sometimes had a 'watchdog' timer or 'health-check' feature.  I wonder if it could be something like that?
 
 

RE: Mysterious relay in water telemtery panel / SCADA

Ironically, I did a small water SCADA system for Sequim back in 1985 or 86. I remember because I had to learn what a "turbitiy meter" was. We were supposed to turn on a well pump system if the intake water from the Dungeness River became too "turbid" and I had no idea what that meant. Nice little town. I was surprised to learn that although it's in Western Washington, Sequim gets less rain per year than San Francisco because it's in the rain shadow of the Olympic Mountains.


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RE: Mysterious relay in water telemtery panel / SCADA

So... You put the clickity clickity relay in there!

Keith Cress
kcress - http://www.flaminsystems.com

RE: Mysterious relay in water telemtery panel / SCADA

No, I don't recall doing it on that project, but I have done it in another system. I had a no-flow shutdown from 2 separate flow transducers, but only one analog input on the micro PLC. So I used a recycle timer to mux the analog signals every 10 minutes because it didn't need to read any more often that that. The pumps could stand no-flow for about 30 minutes (Flygt pumps).


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RE: Mysterious relay in water telemtery panel / SCADA

(OP)
So far I've not worked on a system with a PLC in it.
Yes, the rainfall is only about 17" in Sequim, thanks to the rain shadow.  A search for maximum sunlight on the peninsula for my planned solar trackers is what landed me here.
-Robert

RE: Mysterious relay in water telemtery panel / SCADA

(OP)
I think I'm closer to the answer.
There is a site (a prison) that this system pumps water to.  It is this site that controls this relay coil.  The contacts of this relay go to what I think is a retriggerable one-shot.
If the prison loses power to its telemetry, the one shot times out and disables the pumps.

Eventually I'll get a chance to trace everything.  Tomorrow I get to visit the prison.  Had to get a background clearance.  Busbar:  Can you believe it?  I cleared the background check!  The conviction for blowing up the bathroom 40 years ago has rolled off my record!

RE: Mysterious relay in water telemtery panel / SCADA

 
Time flies when you're having fun..
  
 

RE: Mysterious relay in water telemtery panel / SCADA

Just make sure they let you back out. They can be tricky that way.

"Hey, come in here for a project... Oops, look what we found on your "Permanent Record"! Have a nice stay.".


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RE: Mysterious relay in water telemtery panel / SCADA

 
seq—

It occurred to me that discussions of explosives on an open forum will put us both back on Government Watch Lists.

  

RE: Mysterious relay in water telemtery panel / SCADA

(OP)
Mystery solved!  I went to the prison.  Wow.  Their metal detector wouldn't even let me through with glasses on.  We had to leave all our belongings in a locker.  Shoes x-rayed.

They took us on the complete tour.  They have two 1-megawatt backup generators.  Dentists, doctors on staff.
1000 prisoners, 400 staff.

Anyway, the relay:  It's 4-20mA presented as PWM!  12 second period.  3 seconds on = 4mA.  12 seconds on = 20mA.

Nobody knows whey they chose to run a 108 volt DC pwm.  Distance is 1 mile.  The guy that did it isn't around.

RE: Mysterious relay in water telemtery panel / SCADA

I'd be curious how the conversion from 4-20 to PWM is done. Is it a single relay module that does it?  

RE: Mysterious relay in water telemtery panel / SCADA

Using PWM ratiometric proportional conditioning was/is a way to transmit analog data over long distances. I would imagine they used 108VDC because of the distance involved, and PWM because aside from being relatively insensitive to noise, it also could be configured to make voltage drop over distance irrelevant as well. If you are looking for a Lo signal pulse to be anything under 5V and a Hi to be anything over 10V for example, dropping 80% of your voltage over that mile makes no difference in the outcome.

Now days we would likely do it with hard line (FSK tone transmission, Ethernet, various industrial protocols) fiber optics (Ethernet etc.), or wireless communications, but PWM is still used in some industries and areas where the other methods are not feasible.

This is as close as I could find in a hurry as an aid to understanding the technology behind it.


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RE: Mysterious relay in water telemtery panel / SCADA

Maybe it's morse code for an inmate..

RE: Mysterious relay in water telemtery panel / SCADA

Or an early version of a TENS unit for those lonely nights in the cell?


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RE: Mysterious relay in water telemtery panel / SCADA

2
You have met the BIF (Builders Iron Foundry) 15-second method of transmitting an analog variable over metallic wires. This system came about in the 20's, maybe early 30's and was in common usage until the early 70's in water and wastewater systems. At each end was a rotating device that made one revolution every 15-seconds. At the sending end, usually, was a linear cam working against a moveable switch. The switch was mechanically linked to a float, bellows etc such that the switch was closed when engineering units were at zero for 3-seconds and closed for a longer time up to 100% or 15 seconds. If bellows positioned from an orifice or venture tube the cam would be exponentially shaped. The PWM was always linear to the measured variable. At the receiving end, usually a chart recorder with a 15 second motor with a magnetic clutch mounted to the shaft. The clutch was engaged for the duration of the pulse, and when engaged caused two counter rotating shafts to position the pen arm, one moving it upward if the measured variable had increased and the other moving it downward if the variable had lowered. Point controls were switches mechanically operated by the pen arm. Integration was simply advancing a mechanical counter by a motor driven for the duration of the pulse, ignoring the first two seconds of the pulse. Setting up a two pump, alternating, lead-lag system with high and low alarms meant adjusting the position of five switches along the arm linkage and the inclusion of a mechanical alternating device that was tripped as the pen passed the "On" position.  

AGM (Arizona Gear and Machine) made (makes?) senders and receivers, solid state, that could be used to replace BIF equipment at one end or the other.

Voltages used for transmission were usually DC and two signals could occupy one pair of wire by using two power supplies, different polarities, and diodes at the receiving end.

AC could be used but a relay was required at the receiving end. I used tone and a tone receiver in cases where the TELCO phased out the metallic lines and only offered audio lines.
Steve
 

RE: Mysterious relay in water telemtery panel / SCADA

(OP)
So I finally was able to remove one of the senders.  It was made by AGM and is said to be how they got their start.
It's called a "Pulse Duration Transmitter"  I put it on the bench and found:

1 volt in / 3 seconds out
5 volts in / 12 seconds out

So I programmed the PLC to measure the duration.  Damn thing was accurate over its entire range of better than 1%, which is better than the 4-20mA isolaters that we were going to replace it with.

-Robert

RE: Mysterious relay in water telemtery panel / SCADA

 
Thanks for the followup, Robert.  It's always nice to hear of resolution to sometimes agonizing troubleshooting effort.
    

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