Utah Integrator at wits end and about to comment ritual suicide with my multimeter probes. Please help a brother out.

Culinary water tank at the end of a one mile shielded 2-pair (untwisted I think) 18 gage wire. One pair goes to hatch intrusion switch, the other tank sensor. The sensor on the tank is Ametek Drexelbrook US-21 ultrasonic. PLC is SCADAPack 350.

Intermittently the loop starts bouncing. Voltage drops from 24 to 10 or so and the current mirrors this. Period of bounce is on the order of a few seconds and may directly correspond to the ultrasonic ranging sensor firing... not sure.

I have had several sensors installed at this location (same model) and each behaves a little differently but I always run into a time of year when they start bouncing. They may go for 6 months without an episode and then it will start. Summer seems to be the worst.

When I power the sensor locally from 3 nine volt alkaline cells it works perfectly.

Any thoughts? Am I at the margins of what this setup can source current-wise and when my ultrasonics fire off and pull a big slug of electrons it droops and then bounces? Could I modify this setup with caps at the sensor or some other approach? My distributor and manufacturer have not much help.

 

Follow up to my own post. The ultrasonic device (also know as Ametek Drexelbrook USonic) sports the following specs:

Input Power
19 to 30 VDC
19 VDC required @ 4 mA
minimum
Output signal
2-wire, 4-20mA, HART (isolated)

 

Hi there,

The best will be to go wireless but if that is not possible, you need to look at this whole installation from scratch. Total loop resistance, capacitance and induction (cable and instrument R/C/L) specs versus supply card/barrier capability.

Have a look at these discussions, it might give you some insight on how and what to look at.

http://forums.mrplc.com/index.php?showtopic=9759

Some other things you can look at is to install loop isolators (4 to 20 in - 4 to 20 out) at various locations since that will also help to boost and keep the signal stable but might not be possible to implement in your case.

Poor/rusty connections at some common point for all the installations might also be a possibility. Earthing is a definite possibility as well as lightning surges will also cause this kind of problems even if it is far off. I have also came across poor gland connections that causes spikes in the signals. Again look for some area where a gland might be common to all instruments installed.

 

Hi again

Also found this info for Ultra sonic level installations.

10. What is the maximum 4-20 mA signal distance?
The sensor can be installed up to 1000 feet away from its point of termination using a shielded, 18-20 gauge twisted pair cable and 24 VDC power supply. The total loop resistance should not exceed 900 ohms

http://www.flowline.com/faqs.lasso

 

Brett,
I think you might be on to something with the bouncing caused by current inrush when the U/S fires. I would certainly try a large capacitor across the terminals, say 2000 microfarad, it won't effect the mA signal.

Have you measured the voltage across the transmitter with full tank?
Does the bouncing occur just at certain levels, I would expect a full tank to be worst case.

Perhaps the U/S is not a good choice at such a distance, another would be one of the submersible level transmitters by Druck or similar. Freeze up wouldn't be a problem just dropped into the tank.

 

Following up again to to my own post.

I asked the same question over at www.plctalk.net and got some good replies. Here is the thread for the interested:

http://www.plctalk.net/qanda/showthread.php?t=58681

It appears that I should have sat down and done the math before I installed the sensor.

Given:

- 10,000 feet of 18g wire = 75 Ohms
- PLC analog input has 250 Ohms resistance to ground
- minimum voltage for the ultrasonic transmitter is 19V
- power supply is 24V

It appears:

- loop resistance is 325 Ohms
- that gives 6.5V of IR drop at 20mA (20mA*325Ohm)
- at 20mA I can only source 17.5V to the transmitter

It is suggested that I turn up the voltage on my power supply and see what happens. Both PLC and transmitter are happy up to 30VDC and the power supply is capable of being turned up to 28.5VDC. I will try that and report back to group.

 

a bit unusual to combine contact sense signals with your analog 4-20 mA, in the same cable, but more importantly, how are you grounding the shields, and are they run separately from any power cabling. Ground loop induction from your process equipment is always a problem with long runs.

 

Thanks for the follow-up, Bret. Follow-up tends to be a rare occurrence on a forum.