RTD or transmittter??
c131frdave
Posts: 38
Hello. I have a project about to start, and I need some advice.
I need to control some pumps. These pumps have an electronic control that do most of the hard work for me. There are two contacts that are normally jumpered, but can be used to turn the system on and off by a relay. I want to control that relay with a basic stamp.
The pumps pump water to a heat exchanger. The pumps first pump the water to a boiler and then to the heat exchanger. The water needs to maintain a temperature of between 80 and 120 degrees. I don't need the pumps to be on very often or for very long because the boiler water is 180 degrees and holds about 1/2 of the whole system's capacity. Right now, every 2 hours a guy goes out, looks at the thermometer of the water in the heat exchanger, and if it is below 80 degrees, walks over to the pumps and turns them on for 10 minutes. He watches the thermometer until the water gets to about 120 degrees and then turns off the pump.
I want to remove the thermometer, plug in an RTD, and wire that to the basic stamp. The basic stamp could close and open the relay to the pumps as needed.
My only issue is how to do it. I can get an RTD with a 4-20 ma transmitter, or I could just simply use the RTD by itself and to the capacitor discharge speed thing to figure out the temperature. The wires to and from the stamp will be fairly long- about 30 feet, so I think I'd rather use a RTD with a transmitter since the wire distance doesn't matter for an amperage signal, where with using a straight RTD with resistance monitoring will require some difficult calibrations (my wire will have resistance of it's own, and it is exposed to outside elements that might have influence as well).
So, if I want to use a transmitter, how do I get the stamp to look at a 4-20ma signal and give it a digital value I can use in a program? Is there anything I am not thinking about that might mess with my idea? Thanks!
I need to control some pumps. These pumps have an electronic control that do most of the hard work for me. There are two contacts that are normally jumpered, but can be used to turn the system on and off by a relay. I want to control that relay with a basic stamp.
The pumps pump water to a heat exchanger. The pumps first pump the water to a boiler and then to the heat exchanger. The water needs to maintain a temperature of between 80 and 120 degrees. I don't need the pumps to be on very often or for very long because the boiler water is 180 degrees and holds about 1/2 of the whole system's capacity. Right now, every 2 hours a guy goes out, looks at the thermometer of the water in the heat exchanger, and if it is below 80 degrees, walks over to the pumps and turns them on for 10 minutes. He watches the thermometer until the water gets to about 120 degrees and then turns off the pump.
I want to remove the thermometer, plug in an RTD, and wire that to the basic stamp. The basic stamp could close and open the relay to the pumps as needed.
My only issue is how to do it. I can get an RTD with a 4-20 ma transmitter, or I could just simply use the RTD by itself and to the capacitor discharge speed thing to figure out the temperature. The wires to and from the stamp will be fairly long- about 30 feet, so I think I'd rather use a RTD with a transmitter since the wire distance doesn't matter for an amperage signal, where with using a straight RTD with resistance monitoring will require some difficult calibrations (my wire will have resistance of it's own, and it is exposed to outside elements that might have influence as well).
So, if I want to use a transmitter, how do I get the stamp to look at a 4-20ma signal and give it a digital value I can use in a program? Is there anything I am not thinking about that might mess with my idea? Thanks!
Comments
Industrial Stamp
It says it will accept 4-20 ma signals. Anybody know exactly how to write code to make this work?
Alternatively, there are all sorts of excellent digital thermometers and thermostats that work over a -40C to 125C temperature range. Check out Dallas/Maxim's website for these (www.maxim-ic.com/products/sensors/thermal-temp-sensors.cfm?). There are even temperature switches where the trigger threshold is set by an external resistor and their output is a simple open drain switch.
Many of these thermometers and thermostats use the I2C two wire bus protocol. This can be handled by a BS2, but there are built-in statements for I2C control in the BS2p/pe/px Stamp models that make things easier. If you're going to use a BS2, look at the Nuts and Volts Column #85 (www.parallax.com/Resources/NutsVoltsColumns/NutsVoltsVolume3/tabid/445/Default.aspx) for I2C interfacing examples with a BS2.
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· -- Carl, nn5i
and mount the Stamp + thermocouple local to the heat exchanger , then use 30 feet of 3 conductor wire with your supply voltage and a control wire for the relay.
Jeff T.
RTD is a resistive thermal device, resistance varies proportional to temperature.
A thermocouple is two metals that form a very small voltage across their junction that varies with temperature
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Chris Savage
Parallax Engineering
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· -- Carl, nn5i
I would stay w/ the RTD, And I would use A 3 wire 100ohm Platnuim "The Industry Standard" w/ A 'Wheatstone Bridge" Also the Industry Standard...The " Wheatstone Bridge", Cancel's out the Resistents in A long "RUN" of wire.,,So what is read in resistents is the Act. value of the RTD.,,All of the "LOOP" Resis. is removed!!!
I don't recomend A T/C for this App. since A T/C is based on the Diff. between the "HOT" and "COLD" Junction,, And @ 80 to 120 degrees F',,, The Seebeck voltage will be so small.,,It will be difficult at best to have any Resol.
W/O Transmitter:
The RTD w/ the W/s Bridge can be easily read w/ the "RCTIME" command and a added cap.
with Transmitter:
The $stamp can also read A 4-20 mA loop w/ a 250ohm resister in the loop and A A/D converter....If You read the voltage across the 250ohm resister @ 4mA will read 1volt and @ 20mA will read 5volts.
This 4 to 20mA loop is from a Temperature Transmitter out put w/ a 24 volt supply,,,And not the RTD loop,"input to transmitter"...The Transmitter has the Wheatstone·Bridge ciruit built in...
The reason for the 4 to 20mA and the 1 to 5 ; Is to make sure the loop is intact, If it were 0 to 20mA or 0 to 5 volts, And the loop was open, It would read "0";This would Gen. an error w/ 4-20mA or 1 to 5, Since "0" is'nt A valid Value
__________Roll W/ RTD________________________________$WMc%__________
Post Edited ($WMc%) : 11/22/2008 1:13:13 AM GMT
The PLC has, or can have (I think they're optional; mine has'em) A/D converters that can accept as input a current of 4 to 20 ma and convert it into a digital value that the Stamp inside can read.· To me this seems quite straightforward, exactly what you need.· You can use 22 AWG phone wire, and over thirty feet the effect of the wire resistance will be negligible.· 22 AWG wire·has about 16.4 ohms resistance per thousand feet.· You will use a pair of wires, so 32.8 ohms per thousand feet, thus the total wire resistance in your 30-foot current loop will be much less than one ohm.· That's negligible.· You don't need Wheatstone bridges, or any other kind of bridges, or any of that stuff.· Telephone wire is twisted pairs, and we're talking about quite low impedances, so noise shouldn't be a problem.· If it is, use shielded twisted pair (about four bucks' worth of shielded CAT5 network cable, for instance).
Use a thermistor.· Apply to it a constant voltage that will cause it to flow, say, 12 ma at the middle of your temperature range (you'll need to know its resistance, and the resistance of other parts of your current loop,·at that temperature).· Using the resistance at each end of your range, calculate what current will flow at the temperature extremes with the voltage you'e applying.· If they're between 4 and 20 ma, you're home free.· If not, the resistance is varying too much for the 4-20 ma range of the ADC.· In that case, hang a resistor (of resistance equal to that of the thermistor at midrange)·in parallel with the thermistor, and halve the voltage; recalculate.· This is really easy stuff.
When you think you've got it all figured out, get a bucket, and some Saran wrap to protect the thermistor, and an old-fashioned thermometer,·and run some experiments at the kitchen sink.
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· -- Carl, nn5i
Take a second look at My app. w/ the Wheatstone bridge....From My frist reply...Its used for Measuing small values of resistance.. and cancealing long runs of wire to the RTD.
·"Wheatstone bridge"
·"3,4, wire RTD"
·"T/C"
__________________$WMc%________
Post Edited ($WMc%) : 11/22/2008 2:28:22 AM GMT
What I am looking at right now is one of these. The Stamp PLC looks perfect, but it does need the AD converter. As I have very limited knowledge of how to get data from sensors, even though it might be easy and straightforward to you guys, it's a very steep mountain for me to climb to figure out how to program this.
Anyway, I like the RTD transmitter in the link because it still gives a visual temperature reading, which we need for visual inspections.
Again, thanks for your help and advice. I wish I know 1/100th of what you guys know...
This application, by the way,·does not involve any long runs of wire.· Thirty miles would be a moderately long run, but thirty feet is not.· Thirty feet of wire (sixty feet, actually, because it makes a round trip) will add·much less than 1% to the resistance of a thermistor.· That's negligible.· You should·be able to calculate the need, or lack of need, for what you recommend.· Not only be able to calculate it, but actually do so.· The calculation shows that one can design this system as if the thermistor were only an inch away.
Engineering is not the practice of mindlessly making simple problems complex, nor is it the practice of using, say, a Wheatstone bridge because it's one of the things one happens to have heard of.· Engineering is a quantitative practice -- an engineer first figures out the simplest way to do a thing, then calculates its expected performance to see if that's good enough, before adding needless complexity.· If the calculation is particularly burdensome or uncertain, he builds a prototype and measures it.·
An old boss of mine, decades ago, used to say to his engineers, You're suppposed to be an engineer -- give me a number!· He never had occasion to say it to me.
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· -- Carl, nn5i
I didn't mean any "sneer" or anything negative to Your postreply...I could have written My last postreply A lil. better, For that I apologize."I abbreviate way to much,This makes My postreplys seem "negatvie" sometimes,,,I'll try to improve on this!"........
You asked what an RTD was in A prev.postreply, So I threw a lil. info out about them, And the Most common setup to use w/ an RTD.This isn't something that I came up with, Its an Industrial Standard, and something Engineer'ed a long time ago...I failed to mention that the 4 to 20 mA loop was from the output of A Temperature Transmitter and not from the RTD,,And the 250ohm resister is in series with this 24 Volt loop...Its in this loop that 1 to 5 volts can be read across the resister,"output from transmitter"....I brought up the Bridge circuit to directly read the RTD w/ A $stamp and A A/D converter..."No Transmitter"
__________I hope I've cleared somethings up____________________$WMc%____________
Here's a suggestion to KILL all your abbreviations IMMEDIATELY! Try using a spelling checker as most of us do.
Just trying to be helpful.
Regards,
Bruce Bates
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When all else fails, try inserting a new battery.
Can You supply some info on how to apply a spell checker to this forum?
I'm happy to oblige. Here is the one I use with Internet Explorer, but Opera has add-ons one of which is a spelling checker. I suspect most of the popular browsers you find today also either have it built-in (you may have to go digging for it), or offer it as an add-on as Opera does. Here is the link for IESPELL which I learned about here on this very forum!!
http://www.iespell.com/ .
If I can be of any further help, just ask.
Regards,
Bruce Bates
P.S. I was typing too quickly on a keyboard I'm not used to and misspelled my own NAME! Opera's spell checker caught that as well. What more can I say?
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When all else fails, try inserting a new battery.
Thank's...........I'll check out the link http://www.iespell.com/
___________________________$WMc%_________
You might also stop putting all of those extra periods in at random places. To be honest, between the periods, the typos, and the abbreviations, I find your posts unreadable. I know you're putting some real effort into the ideas, and it's a shame that the writing gets in the way of your contributions.
I appreciate all input, misspelled or mispunctuated or not. However, being that I have no formal electronics training or knowledge, I do appreciate plain speak over technical jargon. But all help is welcome, and I thank you for it.
Er, no, no hate. Just trying to help people be better contributors. Sorry if my tone implied otherwise.
As you said a couple of posts up, there are people here who have an amazing amount of knowledge about this stuff, as well as people like you who ask these good questions that wind up inspiring those folks to share that knowledge. I've talked with Bruce off forum, and (not to put words in his mouth, but...) that's how he feels too. I haven't spoken with Carl elsewhere, but look at this little gem in his post:
For us non-engineers, this is good "big picture" stuff to go with the technical details. IMHO, c131, you started a really good thread here. That's why I was reading it closely enough to respond.
(1) did all the calculation/prediction/checking I could manage,
(2) ordered PC boards and parts, then
(3) built a prototype and got it working, then
(4) decided I didn't like it much because it would be too easy to hook it up wrong and burn stuff up,
(5) did a major redesign with different connectors,
(6) ordered a new set of PC boards, and more parts from Mouser,
(6) built a new prototype,
(7) discovered the Real Time Clock didn't work any more because I left a trace off the PC board layout,
(8) said a few things that even $WMc% could spell,
(9) soldered in a piece of wire to fix that, and
(10) it's working again.
Sigh.
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· -- Carl, nn5i
Post Edited (Carl Hayes) : 11/22/2008 4:47:14 PM GMT
I'm going to download that spell checker. " http//www.iespell.com "; Is that " checker " or " che'ker " ?
________No hate here __ just poor spelling on My part______________$WMc%___________
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· -- Carl, nn5i@arrl.net
Good man. Tackle the issue head on - that's the way to do it.