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Temperature

06/26/2010 2:51 PM

Why we use temperature Transmitter after Using of RTD ?

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#1

Re: Temrature

06/26/2010 3:13 PM

One reason might be that the controller does not accept a resistance input, so there is a transmitter (or signal conditioner) to convert to 4-20ma or other common controller input signal.

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#22
In reply to #1

Re: Temrature

01/31/2025 4:41 PM

This is a old post, the question that needs to be asked is the transmitter's sourcing or sinking.

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#2

Re: Temrature

06/26/2010 4:15 PM

Your boss doesn't like you. No matter what suggestion you make, she (he?) will glare at you and tell you to do it a different way. She already thinks that you constantly make lame suggestions just to be heard in the crowd. You've yet to show her that you understand all of the reasons why something was done one way and what real answer there maybe for configuring a system another way all together. Until you demonstrate to your boss that you have a complete understanding of what it takes to do a job and why a different configuration will improve reliability, reduce cost, improve resolution or any of the other reasons a minor redesign will be worth the effort, your stature in the company will not improve.

Study the circuit configurations you already have at work and learn from them. When you think that configuration A is not as good as B ask a trusted coworker or this blog for help in understanding. But don't just as why A and not B. It shows the person you're bothering that you are so lazy that you won't even explain what A does in the system and why B may work much better. In a feudal society, pestering your lord with this kind of a question can cause body parts to be swiftly and painfully removed, if not worse.

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#3
In reply to #2

Re: Temperature

06/26/2010 6:36 PM

Aw come on moderators.

My answer was carefully crafted to be no more obnoxious than the pathetic initial question. It went directly to the point of why I felt that it was pathetic question. The OP made absolutely no clarification why they felt an RTD was adequate, what problems were encountered using the RTD or even what temperature range was being measured. The OP literally only asked why B is being used instead of A. He didn't even identify what type or model of a temperature transmitter was used. This kind of laziness should not go without some response. The CR4 FAQ on rules for posting states:

  • Provide as much information as possible. The most frustrating questions on this site are like this: "I need a sensor to test water flow - which is the best one." What are you testing for, what are you trying to determine? Is it in a stream, a pipe, an aquifer? Are you looking to measure velocity or mass? Are you testing for particulates, mineral concentration or pollution? Is it wastewater, potable water, slurry, etc.? The more you let us know, the better the chance that someone on the site will be able to help you find an answer (thanks Silas Marner).

Now this "guest" violates this guideline and my correction is deemed OFF TOPIC with no explanation. Oh, one last thing. If you're going to correct the OP's poor spelling in a title, when you review the few initial answers, please correct this spelling too.

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#6
In reply to #2

Re: Temrature

06/27/2010 4:58 AM

redfred

i want to vote GA on this reply, but wasnt able because you have so far 6 votes off topic.

consider my vote for GA.

I would answer in the same tone

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#4

Re: Temperature

06/27/2010 12:10 AM

Sir,

I suspect it relates to cost and complexity. Consider the materials needed to make the cable for an RTD. They are quite costly. Therefore, you want to keep the sensor cable short. However, the location where you want to display the measured temperature may be a significantly greater distance away. Therefore you need a reliable and proven method for transmitting this measurement to the location it is needed. If you have the time and can get pricing, I suggest that you compare the material and installation costs for an RTD with a long probe length and an RTD connected to a local transmitter to the remote meter site. I expect you to find that the more complex approach will be much lower in cost.

--JMM

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#5

Re: Temperature

06/27/2010 12:28 AM

You can get Local readings at Grade level making things easy during commissioning and maintenance

Regards

Jose

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#7

Re: Temperature

06/27/2010 11:02 AM

Consider resistance drop as the cable length increases and another point 4-20 transmitters are usually with inbuilt feature of signal isolation providing signal transmitting line safe from other cable line interference.

Hope you got the answer......Luv Jay

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#8
In reply to #7

Re: Temperature

06/27/2010 7:05 PM

Jaya's got the right idea. Principally, it has to do with long leads introducing resistance outside the Wheatstone Bridge resistances that make up the RTD.

The other reason: RTDs are voltage-driven (rather than current-driven) devices, and long leads are great voltage receivers/antennas. Using a temperature transmitters as a voltage-to-current transducer (that acts as a high-resistance current source) eliminates this problem.

Cheers!
DZ

P.S. As concerns the comments re. 'simplistic language', or whatever. Not everyone masters the English completely, chummies; so be understanding and don't shoot down seekers of knowledge. Unless they're looking for answers to homework, that is. :P

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#9
In reply to #8

Re: Temperature

06/27/2010 8:24 PM

As I understand it, with a 3-wire RTD, the resistance from lead length can be subtracted out in the bridge arrangement.

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#11
In reply to #9

Re: Temperature

06/28/2010 3:43 AM

Correct. That is why 3-wire is used.

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#10

Re: Temperature

06/28/2010 1:41 AM

The temperature variation of the surrounding in which the connecting cable (between the RTD and the secondary instrument) will definately affect the measurement accuracy. This will be more if the distace between the RTD and the secondary instrument is more than 200mtrs. This is applicable for even for three wire RTDs. Hence where we are requiring better accuracy, it is better to have transmitter. Incidentally we can have local temperature also in the transmitter.

R.Thiyagarajan.

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#12

Re: Temperature

06/28/2010 8:29 AM

On short runs it usually not necessary. Every manufacturer of temperature indicating controllers, such as the Red Lion T16 and Precision Digital PD 540, PD560, PD570 series have an RTD input as well as a 4-20ma input. Most processors (PLC) have RTD input cards available. However, these are expensive enough that you had better have at least four or so RTD's to make elimination of the transmitter cost effective. On a Foundation Fieldbus system, the Rosemount T848 eight RTD input transmitter saves cost as soon as you get to two RTD's. However, as mentioned, this is a short run situation. We use them at this plant for process skids where the distances are 25 feet or less from RTD to the panel with the RTD input device. Beyond that, you might consider using a transmitter immediately at the RTD to eliminate noise concerns. That is a mighty small signal coming from the RTD.

Truth be told, I spec just RTD's for runs on our process skids as mentioned above because of the cabling, not the transmitter costs. Most customer specifications are extremely particular about the make and style of cabling from a 4-20ma transmitter, but will accept any RTD cable put on by the RTD manufacturer, and they are ALWAYS about 1/3 the diameter of a good shielded 4-20 ma capable cable as required by the specificiations. Much easier to handle than the bulky double insulated braided shield cables used when a tranmitter is installed.

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#13

Re: Temperature

06/28/2010 4:41 PM

Phys,

Thank you for your very interesting comment on supplying RTD's-direct over transmitters because of wiring/cable sizes.

In 30 years of doing instrumentation, I have a list of 23 reasons for selecting a transmitter rather than use RTD-direct, a few of which have been cited here. But my list of reasons why someone might use RTD-direct rather than a transmitter is much shorter, 5 reasons, most involving "can't deal with complexity".

It's rare that I pick up useable information on a forum, but this is one such event. GA to you, dude!

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#14

Re: Temperature

06/29/2010 10:40 AM

RTD's are simpler to install in the field, and long length is really not a factor.The 3 wire RTD simply puts the 3rd leg into the wheatstone bridge in series with the measuring element.If more compensation is needed, a 4th lead can be added to the opposite leg in series to further compensate for lead length.There are, however still some noise issues and isolation that must be dealt with.Transmitters are becoming cheaper and cheaper, with smart transmitters now dominating the market.A smart transmitter accepts a setpoint, and tolerances, and does not transmit a signal unless the parameters are exceeded.

This saves bandwidth on networks, and saves processor time on controllers.

Therefore, transmitters are becoming the instrument of choice.

Hope this helps.

HTRN

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#15

Re: Temperature

06/29/2010 9:29 PM

RTD is a temperature element.

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#16
In reply to #15

Re: Temperature

06/29/2010 11:14 PM

No its not. Its the Research into Testimonial Dinners.

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#19
In reply to #15

Re: Temperature

07/08/2010 10:25 AM

Please carefully reread # 14.

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#17

Re: Temperature

06/30/2010 6:01 AM

Function of transmitter is to converts measurements from a sensor into a signal, conditions and to display it on some device.

So transmitter is use to display the output as temperature (4-20 mA)

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#18
In reply to #17

Re: Temperature

07/01/2010 9:37 AM

Correct.

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#20
In reply to #17

Re: Temperature

07/08/2010 10:27 AM

Please carefully reread #14

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#21

Re: Temperature

01/23/2025 7:17 AM

And the reason that someone else in the <...we...> cannot answer this question is what, please?

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