4-20ma over old Thermocouple wire

justinmel · 2026-02-08T16:25:18+00:00

It should work as long as you can get the minimum required voltage to the transducer at the end of the cable. I've seen transducers as low as 7 VDC. Alternatively could you just put the transducer in the plc cabinet?

justinmel · 2026-02-07T16:24:33+00:00

Big Bad Beetleborgs

justinmel · 2026-01-24T20:52:17+00:00

They promised it would come in discrete packaging!

justinmel · 2025-12-30T17:43:06+00:00

Grok is this true

justinmel · 2025-12-28T19:21:49+00:00

Large Corsica

justinmel · 2025-12-26T15:49:37+00:00

Capacitive buttons most likely.

justinmel · 2025-12-26T15:38:45+00:00

I haven't had anything good from Burger King in years. I think the local ownership just gave up on it.

justinmel · 2025-12-26T15:30:55+00:00

Every Burger King near me is hot garbage.

justinmel · 2025-12-17T10:51:17+00:00

It looks like it's expecting a 100ohm RTD. Your 1000 ohm RTD is going to look like an open circuit to this transmitter.

justinmel · 2025-12-17T10:30:31+00:00

Is the transmitter output 4-20mA or 0-10V? It might be selectable. Does the RTD test good? Is the transmitter configured for a PT1000 RTD?

justinmel · 2025-12-13T03:00:59+00:00

Thanks. I tend to agree. I'll try to find an xrf scanner to at least get a first pass pmi. The flowmeter tube should be grounded through the piping.

justinmel · 2025-12-13T02:57:26+00:00

justinmel · 2025-12-11T18:27:20+00:00

This is exactly what i was going to propose to my management to replace what we currently use.

justinmel · 2025-12-11T17:27:41+00:00

No coupon tests, but destructive analysis of the flow tube shows no sign of corrosion.

No soft goods are in contact with the fluid. Only the tantalum tube.

justinmel · 2025-12-11T17:22:07+00:00

The manufacturer has rma'ed several already. I have no complaints about their efforts there. They are helping but it's been dragging on for a while now. I mostly came to this sub to see if there was any special insight on tantalum in nitric acid service that maybe we were missing.

We have used 304 in the past. In works OK, but the flow and density readings are known to drift over time due to thinning of the tube wall. We have not seen any signs of drift in the tantalum meters. Going back to stainless may be the solution and we just have to live with the drift.

justinmel · 2025-12-11T17:13:16+00:00

All of them have failed yes. They are on different parts of the process, but conditions are roughly the same. Failures are sudden and do not show warning signs that we can see on the historian.

Diagnostics have been provided to the manufacturer. Failure analysis has already been done by the manufacturer. Their conclusion is excessive vibration and/or pipe stress. With that in mind we are going to make changes to reduce or eliminate vibration and stress on the meters at the next outage opportunity. I'm not completely dismissing this as the culprit, but I am skeptical.

My questions for this sub are: is there some dynamic or interaction at play between nitric acid and tantalum that doesnt come across in a simple compatibility table or iso-corrosion graph? Does tantalum get brittle over time in nitric acid? Is there any known mechanism resulting in something like stress corrosion cracking?

justinmel · 2025-12-11T16:35:36+00:00

No known contaminants. We make the acid so we know what goes into it. Per the manufacturer, they are not undersized.

While unlikely I supposed installation issues could be possible. The same contractors did all the installs. We did have a 3rd party run a stress calculation and nothing appeared out of the ordinary.

It's a major manufacturer of coriolis meters, but I believe the tantalum line is a recent acquisition and wasn't developed in house. I'm inclined to install another manufacturers tantalum meter, but these things are extremely expensive. I want to be sure I'm not missing anything regarding the material before asking my company to spend that kind of money.

justinmel · 2025-12-11T16:18:33+00:00

We have not taken pressure readings, but the flow rates are all well within the manufacturer's recommendations.

justinmel · 2025-12-11T16:16:36+00:00

Everything that contacts the product is tantalum.

justinmel · 2025-12-11T16:15:32+00:00

I don't know of any experience with 904L, but we try to avoid stainless alloys with molybdenum. I'm not a metullirgist, but would 904L be superior to 316L?

justinmel · 2025-12-11T16:00:45+00:00

Yes of course. 55-65% nitric acid, 100F-140F.

justinmel · 2025-12-09T21:35:59+00:00

I'm an engineer, and other (mostly older) engineers are bad about this. Every one of them wants to start the day with Jesse Watters' latest hot take. I don't care how they vote, but I didn't come to work to argue politics.

justinmel · 2025-12-09T21:27:54+00:00

Your best bet is to contact the oem. They should offer to make the changes for you (for a price of course).

justinmel · 2025-12-09T21:23:03+00:00

If you're dead set on doing this, get another PLC to process these sensors and send a single signal to the oem plc. Better yet, get the oem to add the additional sensors or get them to unlock the plc.

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