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Immersion sensor spec.
2

Immersion sensor spec.

Immersion sensor spec.

(OP)
Hi folks,

I'm debating with a client on an immersion sensor spec.  He's insisting on eliminating the usual thermowell in the pipe, in order to improve response time.  It's really nonsense, but he is the client.  I can argue all the points about time constants, accuracy, serviceability, and the comparitively long time constant of the water mass, no problem.  

What I don't know how to do is calculate the forces that will be exerted on the bare probe (1/4" stainless x 5", see attached for a sketch) by the moving water.  I am mainly electrical / controls focused, with a good background in HVAC but little in hydraulics.  I thought I'd ask if anyone can point me to a starting point toward calculating this myself and following through.  What's a good reference book, textbook, or web resource?

I have no problem hiring a consultant, but I'd like to learn myself as well.  I have never questioned the strength of the usual brass or stainless thermowell, since their manufacturers make them for that purpose.  This time, I'm the manufacturer.  Our probe is designed to go in a thermowell.  Fun.

Thanks tons!

Good on ya,

Goober Dave

Haven't see the forum policies?  Do so now: Forum Policies
 

RE: Immersion sensor spec.

(OP)
And sorry about the thread title. I left it blank and the system grabbed part of my first line. It's late at night, forgive me.

Good on ya,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

RE: Immersion sensor spec.

Couple drops of oil between the well and the probe should improve the response.

Customer is aware that he has to depressurize the process to change a failed probe, right?
... and that the probe is going to fail sooner than it otherwise would because it will be in an environment for which it's not designed?
... and that it may fail in ways that will screw up the process, unpredictably, since it's outside of its application envelope, right?
... and that it may fail by just developing an offset or a gain change that he may not notice right away, since it's outside of its application envelope, right?

As for how it interacts with the water, that depends on the water velocity, the temperature, the concentration of nasties like chlorides, and the amount and nature of fine particles in the water, and probably a dozen other things I haven't thought of.



Mike Halloran
Pembroke Pines, FL, USA

RE: Immersion sensor spec.

Is this temperature going to be monitored by a human? If so, faster response = more noise in the reading, and humans typically see more noise as a "bad thing". Thats what I've seen from chemical plant operators. They desire to see a single straight line versus a wide band painted on the chart.

The probe vendor might become an ally in this foolishness. Maybe together you can carry the day!

Good luck,
Latexman

RE: Immersion sensor spec.

Go to this link and scroll down till you see the heading DRAG NOTES

http://www.roymech.co.uk/Related/Fluids/Fluids_Dra...

Then further down you will see DRAG FACTOR FOR VARIOUS LONG OBJECTS then if your Reynolds number Re is less than a 1000 the Cd for a long cylinder is given as 1.2.
Transpose the formula for DRAG COEFFICIENT to obtain drag force.

RE: Immersion sensor spec.

Breaking News: ASME Thermowell Standard


We rarely email SwiftyCalc members, but here are four pieces of news that we
think you may want to know: (A lot better than getting spammed 4 times....)

The upcoming edition of ASME B31.3 will mandate ASME PTC 19.3TW where applicable. http://www.jms-se.com/news.php

Training on the Impact and Application of the ASME Thermowell Code http://files.asme.org/Events/B31CodeWeek/32676.pdf by 19.3TW committee members Allan Gilson, PE, I&C Section head at Black & Veatch Energy and Mitch Johnson, JD, President of JMS Southeast, Inc. will be featured at the ASME B31 Code Week Workshop in Norfolk, VA on Sept.17 http://events.asme.org/b31codeweek/educational_wor... . SwiftyCalc members who attend will receive a door prize.

Practical Thermocouple Thermometry, 2nd Editionby Thomas W. Kerlin, PhD and Mitch Johnson is now available featuring updated troubleshooting tips and case studies. Published by the ISA.

Do Your Thermowells Meet The ASME Standard? http://www.flowcontrol-digital.com/201208/201208#&... by Mitch Johnson & Allan Gilson, PE is featured as the cover story in this month's Flow Control Magazine.

JMS has been recognized as the world's best manufacturer of temperature sensors and we are always looking for ways to improve. If you have a suggestion as to how we can improve or if we can help you with your next temperature measurement application involving a thermocouple, RTD, thermowell or transmitter, please let us know!

Recent improvements to SwiftyCalc include Matching Sensor capabilities, Data Sheets and, coming soon: project interface where large quantities of calculations are needed and SPI/InTools modules.

Best regards,

Mitch

800.873.1835
cell 704.929.0315

RE: Immersion sensor spec.

Hello DRweig,

I do have a certain experience with thermowell calculations. There has been a recent change in ASME PTC 19.3 standard, which has complicated things a lot IMHO in comparison to the older one, but the basis is that the thermowell must withstand:
- external static pressure
- fluid impingement
- vibrations (resonance and fatigue must be avoided as much as possible)
I'm afraid I don't have a strong background in thermocouple/RTD accuracy calcs, but I suppose that, provided that the probe alone can withstand pressure and impingement, induced vibration might be an issue since the hot junction is exposed and much more sensitive.

Hope it helps.

Stefano

RE: Immersion sensor spec.

(OP)
Thank you all,

Desertfox, that link is exactly what I needed. I can find anything else I need from the terms.

For the rest of you, I have already made all of those arguments to our client (accuracy, unknown resonance/vibration, small change in response time, process unknowns, need to drain system for replacement, and others).

For this application, our tests show step-change time constant = about 22 seconds for bare probe and about 50 seconds for probe in well with thermal grease at water speed of 10 ft/sec. Final settling to stable reading is exactly the same for well and no-well cases, about 300 seconds. The time constant for changes in water temperature in a large chilled-water HVAC system is very large compared to even the 300-second settling time, and step changes just plain don't happen. The small advantage in time constant is WAY insignificant in comparison.

Unfortunately, I seem to be arguing with not only my client, but a new government standard that prohibits the use of thermowells in these applications as part of their building energy code. I have essentially lost already, and thus we need to do the necessary investigation to completely characterize the response and mechanical reliability of this installation. Yuck. There is no manufacturer to go to for help, as we are the manufacturer.

Mitch, JMS has been very helpful to us in the past. Our company does not really compete against them since we are in the building automation world where very inexpensive and not-so-accurate thermistors are the predominant choice. In fact, I was already aware of the new ASME code due to a JMS e-mail alert. Thank you!

Man, eng-tips is the greatest.

Good on ya,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

RE: Immersion sensor spec.

not uncommon practice in water services, but there are risks,

for example if the element fill material is ignored, you risk exceeding the maximum allowable stress of the element sleeve above 3 m/s, depending on the degree of insertion and the local fluid velocity. You'll know when this happens when you fold the sensor over.

At lesser fluid velocities, depending on the density of the element fill, cyclical flow induced stresses may cause fatigue failure of the sleeve.

By the way you can decrease the element response time constant to 1-2 seconds, by placing it in better thermal contact with the S.S. sleeve,

where bare sensors are used to monitor pump temperature differential, flow induced vibration can alter the sensor calibration because of the cyclical G's. Typically a problem with Pt elements.

you do have to be careful about use of the current thermowell standard 19.3, as it is strictly limity to conventional thermowells dimensions.

RE: Immersion sensor spec.

Hi DRWeig

Your welcome

RE: Immersion sensor spec.

"new government standard that prohibits the use of thermowells in these applications as part of their building energy code"

Ugh. What standard is that? We recently installed our R&D product into a building in Atlanta, showing fairly dramatic gains in efficiency, which is typical for us. What was different was that _we_ instrumented the water piping ourselves. A lot of discussion and debate went into the bare probe vs. thermowell question; but we went with thermowells in the end, for maintenance reasons; and because we could prove to ourselves that no significant loss of accuracy would occur. It was a good decision, as we had several probes get damaged during installation, and require replacement.

RE: Immersion sensor spec.

(OP)
Thanks btrueblood. I'm an Atlanta-area guy, by the way.

I hoped nobody would ask which standard, because my contract has an onerous confidentiality clause in it (yuck). Our client needs my analysis to help procure a particularly huge job. They don't want anyone who is not already in negotiation for the project to find out until they have a contract in hand.

There are a couple of manufacturers who make bare probes that have been certified and tested at HVAC-system water velocities. No bending, no resonance, no vibration damage is what they guarantee. We'll see. On top of all the maintenenace and tuning issues mentioned above, some manufacturers are bent on saying that their existing products will be OK -- without providing any documentation to support it.

Ah well...

When the project is finally under contract, I'll ask permission to upload our report to eng-tips in this thread. Wish me well. I'll be sure to let y'all know which government and which standard to watch out for. That's public record, but I can't mention it until I get permission.

Good on ya,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

RE: Immersion sensor spec.

instrumentation that doesn't fail, that is a major first...

be sure to carry plenty of insurance, or get out of town before start up, cause they'll be looking for resolution

RE: Immersion sensor spec.

(OP)
Agreed. My report has some strongly worded "don't-do-it" lines, I even had them highlighted in bright red.

Good on ya,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

RE: Immersion sensor spec.

Dave,

Why not make your probe with the same o.d. and shape as a thermowell? At least then you'd have a stiff product (i.e. unlikely to vibrate) with a lot of wear/erosion margin. The maintenance issue is "not yours" - i.e. if they have to drain the system to replace it, it's not your problem.

Understand the hush hush part, I'm under similar restrictions.

RE: Immersion sensor spec.

by the way some of the manufacturers that produce direct immersion sensors as you describe, claim astronomical velocity ratings, even your 5" probe will fail above 12-15 ft/s in water, depending on use of the maximum allowable stress under B31 or the yield stress.

RE: Immersion sensor spec.

(OP)
Thanks both. That's a good idea, btrueblood! I'll pursue it.

Good on ya,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

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