Mysterious relay in water telemtery panel / SCADA 5

[sequimscada]

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

 

[VEBill]

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.

 

[jraef]

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.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[waross]

That was my thought also, jraef.

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

 

[sequimscada]

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.

 

[jraef]

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?


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[busbar]

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

 

[sequimscada]

Hi Jraef,

Yes, I am in Sequim, WA.

http://solar.nikola.com

-Robert

 

[jraef]

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.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[itsmoked]

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

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

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).


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[sequimscada]

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

 

[sequimscada]

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!

 

[busbar]

 
Time flies when you're having fun..

 

[jraef]

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.".


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[busbar]

 
seq—

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

 

[sequimscada]

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.

 

[HSWSac]

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

 

[jraef]

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.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[ozmosis]

Maybe it's morse code for an inmate..