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clay9_24 06-09-2011 04:05 AM

Thermocouple questions
 
Building a controller and am trying to decide on how i need to connect the thermocouples. To quickly explain my potential setup I will have an enclosure with a plc thermocouple card inside, i would like to go through a din rail terminal block before going to a female plug on the front of the panel, and then a connecting cable x feet long to the thermocouple housing mounted on the thing i want to measure.

so that is

plc card > din terminal > panel jack > long cable to thermocouple (maybe a splice cable maybe not)

I will be using type T or K thermocouples and in the past i have wired the thermocouples directly to the plc. Wires ran through a weathertight plug and landed on the plc terminals. If the leads weren't long enough i used thermocouple junction wire but these were not permanent or very clean looking designs.

My big question is:

1. can i have as many connections in the thermocouple wire as i want?
2. would it be better/easier to put a 4-20ma transmitter in the thermocouple head
3. as long as i use the correct metal for type T (copper,constantan) for the panel jackes, splice wire, etc will it work?
4. should i try to minimize the junctions in this wiring?

I have never made a panel with this many thermocouple inputs and am really stumped on if i am going down the right path. Any help, suggestions, or comedic response is greatly appreciate.

marzsit 06-09-2011 09:46 AM

as long as all of the metal is compatable, you can have as many connections as you want as long as the connections are all clean and tight. 4-20ma transmitters are used on very long runs of 20-40 feet or more, you shouldn't have to use them in most cases.

audger 06-09-2011 03:40 PM

for our purposes you dont need to overthink the wiring. if you needed greater then 0.1 degree accuracy, you might want to go the 3 or 4 wire, resistance and temperature compensated thermocouple route, but for brewing, just keeping cable legnth reasonably short and using wires of equal size is going to give you accurate enough results.

i have a few PT100 thermocouples, and when i was testing all the electronics, i tested out a few different configurations, like 1ft vs 10ft cables, cables with different resistances, a 1ft wire to one side and a 10ft wire to the other side of the thermocouple- none of it changed my readings more then .6 degrees at any time, so i just wired it as was most convienient.

marzsit 06-10-2011 10:02 AM

Quote:

Originally Posted by audger (Post 2992852)
for our purposes you dont need to overthink the wiring. if you needed greater then 0.1 degree accuracy, you might want to go the 3 or 4 wire, resistance and temperature compensated thermocouple route, but for brewing, just keeping cable legnth reasonably short and using wires of equal size is going to give you accurate enough results.

i have a few PT100 thermocouples, and when i was testing all the electronics, i tested out a few different configurations, like 1ft vs 10ft cables, cables with different resistances, a 1ft wire to one side and a 10ft wire to the other side of the thermocouple- none of it changed my readings more then .6 degrees at any time, so i just wired it as was most convienient.



those are resistance temperature detectors (RTD) that you're talking about, not really the same thing as thermocouple junctions... they operate on a totally different principle.

MikeinCT 06-10-2011 10:23 AM

Ideally you will want a continous run of wire from your terminal block to the point being measured. How far are we talking? I have used TCs without transmitters for up to 20 feet. The biggest problem with long runs would be extra noise.

Agree with above comment that TCs and RTDs are handled differently. Popular TCs are usually refered to as J, K, S (and others) types.

clay9_24 06-10-2011 05:54 PM

I appreciate the input. Doing some quick sketches i will probably have 20ft or so runs. I bought all type T extension wire, terminal blocks, and plugs. so surely shouldn't be a problem.

If it doesn't look right i'll just buy a transmitter and be done with it. It is a bit cheaper to read 4-20ma signals than it is to read the mV thermocouple signals. At least the plc cards are much cheaper.

EarthBound 06-10-2011 06:43 PM

Quote:

Originally Posted by audger (Post 2992852)
i have a few PT100 thermocouples

Those don't exist. A "Pt100" is a type of RTD. By far the most common devices used in industry have a nominal resistance of 100 ohms at 0 C, and are called "Pt100 sensors" (Pt is the symbol for platinum). The sensitivity of a standard 100 ohm sensor is a nominal 0.385 ohm/C. RTDs with a sensitivity of 0.375 and 0.392 ohm/C as well as a variety of others are also available.

clay9_24 06-10-2011 07:04 PM

Quote:

Originally Posted by EarthBound (Post 2996203)
Those don't exist. A "Pt100" is a type of RTD. By far the most common devices used in industry have a nominal resistance of 100 ohms at 0 C, and are called "Pt100 sensors" (Pt is the symbol for platinum). The sensitivity of a standard 100 ohm sensor is a nominal 0.385 ohm/C. RTDs with a sensitivity of 0.375 and 0.392 ohm/C as well as a variety of others are also available.

why didn't my chem II professor in college explain it like that. I had to listen to an hour worth of stuff of little importance to learn what i would actually use, which is what you said in four sentences.


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