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

 

[jraef]

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


"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

 

[SteveWag]

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

 

[sequimscada]

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

 

[busbar]

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