RTD Transmitter

[jeebusmn]

Is there ever really a good reason to use a 3 or 4 wire RTD card instead of a 4-20 mA temp transmitter? From what I read, the transmitter isn't expensive ($80-120 U.S), provides a signal that is more immune to noise, can be recalibrated easily for another RTD sensor (as opposed to matching the card to the RTD), and no special cabling is required. Were RTD cards used just because transmitters at the time were too expensive?

 

[itsmoked]

Transmitters often require another power source.
They introduce more span and zero errors. This can add to calibration hassles.
They can increase system space requirements.
They're yet another "thing".
They reduce reliability.
They cost more money.

Otherwise if none of these bother you they might be preferred.

A lot of systems I build for need 20 or 30 RTDs.. Wanna buy 20 or 30 transmitters? Not I.

I probably wouldn't use them unless I only needed a few and the measurements were being made over, say, 200 feet away.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[JLSeagull]

We use RTD's as the input to adjacent Rosemount temperature transmitters that cost WAY LOTS MORE than $100. Our distances are often closer to a kilometer. They are normally used for process control of refinery or chemical plant applications that require high reliability and noise immunity.

We also minimize the types of signal types to eliminate a wide variety of home-run cables and associated junction boxes. Among the reasons that exist, some of the older and more primative DCS systems only accommodated low-level signals such as thermocouple and RTD's for open loops. The controller I/O required high-level 4-20 mA signals. Other DCS companies that were associated with large "batch" processes as well as most PLC companies had more variety for controller I/O than that popular primative DCS. Consider the direct RTD inputs where you have many temperature signals and few flow, pressure or other 4-20 mA signals.

 

[Ashereng]

itsmoked and JL Seagull have stated my experiences as well.

I typically use RTD inputs direct when I have a lot of them in a single area. One example is in a pump bay (where you have say, 8 pumps side by each in close proximity) where you have temperature reading for winding, bearing, this and that.

I typically buy Rosemount transmitters. For temperature, they usually run about $1500.

Can you provide the make and model of the $120 US temperature transmitter?

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[itsmoked]

Hi Ashereng.
There are few good reasons to pay that kind of money these days...

I googled for "temperature transmitter".
First one I found:
Scroll to very bottom. $235

http://www.laurels.com/transmitter-4-20-temperature.htm

Second: Under $100

http://www.jumoplus.com/store/home.php?cat=337

Here's a page full from a 'not' inexpensive supplier.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Ashereng]

Hi Keith,

Thank you for the links. Yes, these temperature transmitters are down in the hundreds of dollars.

The ones I was thinking of, are:
- field mount
- -40 - 40 °C ambient on all components
- Class 1 Div 1 Gr CD
- local indication
- NEMA 4x (IP 66/IP67)
- 3 and 4 wire RTD inputs

Different application I guess.

I guess I probably should have asked for a bit more detail.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[jeebusmn]

Asherang,

Honeywell makes a transmitter for $115 us.

http://www.lesman.com/downloads/s04_tempsensxmtrs.pdf

 

[Ashereng]

jeebusmn,

Yes they do.

As you add in the ex-proof and NEMA 4X housing, local LCD, HART or Fieldbus, mounting, documentation etc, the price comes up from the $115 closer to $1000 +/-.

Thanks for the link though.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[jeebusmn]

How much does an RTD card cost and how many points do you typically get with a card?

 

[Ashereng]

It has been a while since I bought a RTD input card. If memory serves, I believe they come in 8 channel, 16 channel and (this is stretching it) 32 channel (although that may be a DI input card).

Price varies, and I have been told, they have come down significantly since I last bought them. But to take a guess, I would say around $200 - $300 a card?

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[danw2]

Like my Daddy says, you pay your money, you take your choice.

RTD inputs cards are priced somewhere between $75-$150/point, depending on the class of PLC or process controller.

The four point SLC 500 RTD/resistance input card, cat # 1746-NR4, lists for somewhere around $900, but I think we've paid about $600 for it, at $150/point (several years ago)

A recent Honeywell HC-900 hybrid controller 8 channel universal card that takes eight, 3 wire RTD inputs was priced at $650, or $80/point.

Automation Direct's four channel RTD card, cat # F2-04RTD, (I've never used one) is priced at $300, at $75/point.

Dan

 

[ScottyUK]

Glancing over a UK supplier's price list for the SLC-500 I/O the RTD and thermocouple I/O is about double the cost of a 4-20mA I/O card of equivalent channel count. I wouldn't trade the reliability and simple installation of using the purpose-made RTD card for a small saving on capital cost at the expense of more things to fail and more spares to hold, plus field-mounted converter heads have a tough life in my industry - hot, vibration, wet and sometimes chemical-rich environment.


----------------------------------

Sometimes I wake up Grumpy.
Other times I just let her sleep!

 

[jmw]

I just bought a 4 wire PRT ATEX approved for insertion via a swagelock for £175. (say $150).
But then I'm allowed to. I told myself I could.
However, for large comapnies you can bet your VP Purchasing has been out to dinner on on the glof course with the VP sales and Marketing of a Large multinational supplier who can do "single source" "One stop shop" etc. and they have signed a "Strategic Alliance".
You cost your job out with all sorts of good cheap products (i.e. cheaper and better than the Major companies offering) and try and get it past Purchasing.
Then you'll get it in the neck because your projected costs come in way under final costs when Purchasing (possibly without telling you) orders the equivalent from the single source supplier.
Now you just spend I hour looking for cheaper and better and you may find it. If this is a one off, you probably used up the cost savings through your time. Then again, they may not be an !approved vendor".



JMW

http://www.ViscoAnalyser.com