Previous in Forum: Frustum, Step-Box and Chimney?   Next in Forum: Transistors and Lamps
Close
Close
Close
22 comments
Participant

Join Date: Jun 2013
Posts: 4

RTD's

05/18/2015 9:43 AM

I need to know how to hook up a 3 wire rtd (100 ohm platinum). can it be connected to a dvm (agilent 3458a) to measure current, voltage or ohms? And how to correlate that reading to actual temperature. I have an rtd with a readout for my standard and a temperature bath.

Login to Reply
Interested in this discussion?
You can "subscribe" to this discussion to be notified of new comments.
Click on the Subscribe menu at the top of the page.

Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
3
Guru

Join Date: Nov 2007
Location: Eastern Arizona mountains on Route 666 about a mile from God's country
Posts: 1676
Good Answers: 122
#1

Re: R T D's

05/18/2015 10:00 AM

www.omega.com

Here is an excellent source that contains a wealth of information to answer your questions.

__________________
They said; "Brain size?" I heard; "Train size?" so I said: "I'll take a small one, thank you."
Login to Reply Good Answer (Score 3)
4
Power-User

Join Date: Jul 2010
Posts: 134
Good Answers: 18
#2

Re: R T D's

05/18/2015 2:28 PM

>can it be connected to a dvm (agilent 3458a) to measure current, voltage or ohms?

>current

No, an RTD needs a constant excitation current to provide a reading

>Voltage

No, that's what the meter/indicator reads internally when its excitation current passes through an RTD

>ohms

Yes, sort of, the meter measures resistance, whose measuring units is ohms.

>3 wire rtd

It is rare that I recommend a 4 wire RTD but when your intended use is "RTD with a readout for my standard", that's the case for a 4 wire RTD, along with a meter/indicator capable of true 4 wire RTD measurement.

One pays dearly for increased accuracy in the world of calibration. The relative prices of meter/indicators is one reflection of their ability to handle true 4 wire RTDs.

Another indication is 4 separate terminals, one for each RTD wire.

An RTD used as a 'standard' should be at least a class A RTD (if not AA). Many industrial RTDs are class B. The manufacturer part number decodes which is which, or the cal cert should say.

There are tables of resistance (in ohms) vs temperature for various RTDs, the most common of which is the Pt100 with a 0.00385 Alpha curve.

The DVM manual should have a spec page with its accuracy spec.

There are RTD indicators that read out directly in temperature that can be used for more or less permanent installations if the using the DVM proves awkward.

Login to Reply Good Answer (Score 4)
Guru
Hobbies - Fishing - Old Salt Hobbies - CNC - New Member United States - US - Statue of Liberty - New Member

Join Date: Mar 2007
Location: Rosedale, Maryland USA
Posts: 5198
Good Answers: 266
#3

Re: R T D's

05/18/2015 4:37 PM

It's a resistance device which is linear with the temperature change. So the temperature can be computed. Problem is it need to be calibrated first. Which you would have to know the resistance for three temperatures. Usually 0°C, 100°C and any other.

__________________
Life is not a journey to the grave with the intention of arriving in a pretty, pristine body but rather to come sliding in sideways, all used up and exclaiming, "Wow, what a ride!"
Login to Reply
Guru
Technical Fields - Technical Writing - New Member Engineering Fields - Piping Design Engineering - New Member

Join Date: May 2009
Location: Richland, WA, USA
Posts: 20872
Good Answers: 774
#6
In reply to #3

Re: R T D's

05/19/2015 1:57 AM

If truly linear, only two calibration points are needed.

__________________
In vino veritas; in cervisia carmen; in aqua E. coli.
Login to Reply
Power-User

Join Date: Nov 2011
Location: New Hampshire
Posts: 406
Good Answers: 3
#17
In reply to #3

Re: R T D's

05/22/2015 3:41 PM

Some come with a calibrating resister and the proper procedure for calibration.

Login to Reply
Guru

Join Date: Mar 2007
Location: at the beach in Florida
Posts: 29931
Good Answers: 1668
#4

Re: RTD's

05/18/2015 8:22 PM
__________________
Break a sweat everyday doing something you enjoy
Login to Reply Score 1 for Off Topic
3
Guru
Technical Fields - Technical Writing - New Member Engineering Fields - Piping Design Engineering - New Member

Join Date: May 2009
Location: Richland, WA, USA
Posts: 20872
Good Answers: 774
#5
In reply to #4

Re: RTD's

05/19/2015 1:49 AM

That graph is wrong.

__________________
In vino veritas; in cervisia carmen; in aqua E. coli.
Login to Reply Good Answer (Score 3)
Guru
Engineering Fields - Instrumentation Engineering - EE from the the Wilds of Pa.

Join Date: Feb 2006
Location: middle of nowhere, Pennsylvania
Posts: 2603
Good Answers: 63
#7
In reply to #4

Re: RTD's

05/19/2015 8:24 AM

Hardly "off topic". There is the answer to the OP. They are resistance devices with published resistance/temperature curves. What more do you need?

__________________
Remember when reading my post: (-1)^½ m (2)^½
Login to Reply
Guru

Join Date: Mar 2007
Location: at the beach in Florida
Posts: 29931
Good Answers: 1668
#12
In reply to #4

Re: RTD's

05/20/2015 12:22 AM

That chart does seem to be in error....alternate site for info...

http://www.emory.edu/NMR/mysite06/My%20Stuff/My%20Notes/pt100_table.pdf

__________________
Break a sweat everyday doing something you enjoy
Login to Reply
Guru
Safety - Hazmat - New Member United States - US - Statue of Liberty - New Member Engineering Fields - Chemical Engineering - Old Hand

Join Date: Mar 2011
Location: Lubbock, Texas
Posts: 14331
Good Answers: 161
#14
In reply to #12

Re: RTD's

05/20/2015 10:01 AM

Note that the link shows it to be a Pt100 table, although the table itself is not well labeled, other than EU alpha, and a designator code.

__________________
If it ain't broke, don't fix it. Just build a better one.
Login to Reply
Guru

Join Date: Jul 2010
Posts: 664
Good Answers: 175
#8

Re: RTD's

05/19/2015 8:58 AM

Solar Eagle's graph, from the Sarco site, is in error.

A Pt100 is 100 ohms at zero Deg C, not 0 ohms at zero Deg C, as the abbreviated table below shows.

Login to Reply Score 1 for Good Answer
Guru
Safety - Hazmat - New Member United States - US - Statue of Liberty - New Member Engineering Fields - Chemical Engineering - Old Hand

Join Date: Mar 2011
Location: Lubbock, Texas
Posts: 14331
Good Answers: 161
#9
In reply to #8

Re: RTD's

05/19/2015 9:36 AM

I wonder what goober working at Spirax-Sarco put up that chart. Obviously, that would make Pt, Cu, and Ni the cheapest high temperature superconductors in the world. 0 Ohms, really? A blind squirrel can find a nut, but this time the hog ran through the whole acorn patch without even stepping on one! Wow!

Solar Eagle - kudos for good intentions by showing the relative slopes of the resistivity (if that part of the chart is more or less correct.) Was the resistance axis supposed to have been labeled, "change in resistance", or R-R0 ?

__________________
If it ain't broke, don't fix it. Just build a better one.
Login to Reply
Guru
Hobbies - Musician - Engineering Fields - Chemical Engineering - New Member Engineering Fields - Control Engineering - New Member Engineering Fields - Instrumentation Engineering - New Member

Join Date: Jan 2007
Location: Moses Lake, WA, USA, Thulcandra - The Silent Planet (C.S. Lewis)
Posts: 4216
Good Answers: 194
#10
In reply to #9

Re: RTD's

05/19/2015 6:55 PM

Or maybe the temp scale is actually K

__________________
"Reason is not automatic. Those who deny it cannot be conquered by it. Do not count on them. Leave them alone." - Ayn Rand
Login to Reply
Guru
Technical Fields - Technical Writing - New Member Engineering Fields - Piping Design Engineering - New Member

Join Date: May 2009
Location: Richland, WA, USA
Posts: 20872
Good Answers: 774
#11
In reply to #10

Re: RTD's

05/19/2015 10:55 PM

But even then, the graph would still be wrong.

__________________
In vino veritas; in cervisia carmen; in aqua E. coli.
Login to Reply
Guru
Hobbies - Musician - Engineering Fields - Chemical Engineering - New Member Engineering Fields - Control Engineering - New Member Engineering Fields - Instrumentation Engineering - New Member

Join Date: Jan 2007
Location: Moses Lake, WA, USA, Thulcandra - The Silent Planet (C.S. Lewis)
Posts: 4216
Good Answers: 194
#15
In reply to #11

Re: RTD's

05/20/2015 5:05 PM

But of course

__________________
"Reason is not automatic. Those who deny it cannot be conquered by it. Do not count on them. Leave them alone." - Ayn Rand
Login to Reply
Guru
Philippines - Member - New Member Engineering Fields - Instrumentation Engineering - New Member Engineering Fields - Control Engineering - Who am I?

Join Date: Oct 2006
Location: Philippines
Posts: 2055
Good Answers: 49
#13

Re: RTD's

05/20/2015 9:46 AM

If the distance is not too long (<10 ft), you can just use two wires (the test probes of the DVM). You measure the resistance is then use the RTD-Temperature conversion table (like what SolarEagle provided in #16) to obtain the temperature.

Note, however, that most transmitters apply only 1 mA of current to the RTD when measuring its resistance. Higher than that, you might get into a "heating effect" where the current heats up the RTD and give you an incorrect resistance (hence, wrong temperature) reading. I don't know how much current your DVM delivers when measuring resistance.

Why not just get a RTD temperature display that is designed to read and display RTD temperatures? The display's manual should have instructions on how to wire it.

regards,

Vulcan

__________________
Miscommunication: when what people heard you say differs from what you said. Make yourself understood.
Login to Reply Score 1 for Good Answer
Power-User

Join Date: Jul 2010
Posts: 134
Good Answers: 18
#16

Re: RTD's

05/20/2015 9:17 PM

For any reasonable use of the words 'calibration' or 'standard', a 4 wire RTD is mandated by the Agilent's resistance measurement methods:

The Agilent 34461A offers two methods for measuring resistance: 2-wire and 4-wire ohms. For both methods, the test current flows from the input HI terminal and then through the resistor being measured. For 2-wire ohms, the voltage drop across the resistor being measured is sensed internal to the multimeter. Therefore, test lead resistance is also measured. For 4-wire ohms, separate "sense" connections are required. Since no current flows in the sense leads, the resistance in these leads does not give a measurement error.

page 2 http://literature.cdn.keysight.com/litweb/pdf/5988-5512EN.pdf?id=1000002205:epsg:apn

A 2 wire connection has an inherent offset due to the very accurately measured resistance of the lead wire, which needs compensation for use as a cal standard.

Login to Reply
Participant

Join Date: Jun 2013
Posts: 4
#18
In reply to #16

Re: RTD's

06/03/2015 1:01 PM

then I can measure the resistance directly with my agilent with 2 wire? if so do I need to correct for the resistance in the leads?

Login to Reply
Guru
Safety - Hazmat - New Member United States - US - Statue of Liberty - New Member Engineering Fields - Chemical Engineering - Old Hand

Join Date: Mar 2011
Location: Lubbock, Texas
Posts: 14331
Good Answers: 161
#19
In reply to #18

Re: RTD's

06/03/2015 1:58 PM

You can do two wire, but as you asked, yes, you must correct for lead (and connection) resistance for highest accuracy. Really, in most instances a 4-wire set up is much preferred as compensation is done somewhat automatically in that case.

__________________
If it ain't broke, don't fix it. Just build a better one.
Login to Reply
Participant

Join Date: Jun 2013
Posts: 4
#20
In reply to #19

Re: RTD's

06/03/2015 2:35 PM

I'm beginning to see a little more about rtd's than I did before. I was given a 3 wire to work with. the difference in ohm values at 0 deg. C from the chart (100.00) and my dvm (100.79) should be due to the resistance the leads and connections.

Login to Reply
Guru
Safety - Hazmat - New Member United States - US - Statue of Liberty - New Member Engineering Fields - Chemical Engineering - Old Hand

Join Date: Mar 2011
Location: Lubbock, Texas
Posts: 14331
Good Answers: 161
#21
In reply to #20

Re: RTD's

06/04/2015 4:06 PM

That sounds about right. Now you just need to remember that offset, or figure a way to compensate it out. I think that takes an active summing junction that injects just enough current to compensate, but I am just a lumberjack.

__________________
If it ain't broke, don't fix it. Just build a better one.
Login to Reply
Participant

Join Date: Jun 2013
Posts: 4
#22
In reply to #21

Re: RTD's

06/05/2015 9:08 AM

thanks for the help. I was missing the total resistance. I had the resistance of my leads compensated for but not the resistance of the wires on the element its self. again thanks for the help, I needed it.

Login to Reply
Login to Reply 22 comments
Interested in this discussion?
You can "subscribe" to this discussion to be notified of new comments.
Click on the Subscribe menu at the top of the page.

Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Copy to Clipboard

Users who posted comments:

Bud (2); calbtech (3); Iris (1); James Stewart (4); Mikerho (2); Munster (1); ozzb (1); Phys (1); SHOCKHISCAN (1); SolarEagle (2); Tornado (3); Vulcan (1)

Previous in Forum: Frustum, Step-Box and Chimney?   Next in Forum: Transistors and Lamps

Advertisement