Home temperature reading with a thermocouple

I am working on a home project that I am logging temperatures indoors and outdoors. I am using type J thermocouple going to a 1746-NT4.
I have a direct run for the outside and basement but am using an extension to the main floor and upstairs. I am seeing a -5 F reading when the room is 60 F and seems to be more acurate when the room temp reaches around 70F (acuracy seem better on the straight runs).
I have read that type J's may have up to a 6 degree error and just want to make sure this is probably the reason and not because of the (non)T/C extension wire I am using.
I am not scaling in the program but do have a adjustment on each channel. Seeing that an adjustment would also effect the reading when the room is around 70F, this is not ideal.
Has anybody ran into this and if so, what was a work around for this issue.

(on a budget, so upgrading wire or any other hardware is not a option right now)

Explain "extension"? TC wire and TC extension wire is iron and constantan. If you introduce copper wire, then you have created an addition "two dissimilar wires" junction. Your control system will see the average reading of your real junction and your error junction. This will be incorrect from what you are trying to do. [FONT=Verdana, Arial, Helvetica, sans-serif]
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Not a proper answer, but information you might be interested in.
I have been logging the temperature inside and outside my house for the last 5 years.
I use www.drdaq.com connected to my PC

If you introduce an extension wire that is made of any material other than the TC itself then the junctions at both ends of the extension must be at exactly the same temperature. If they are not then you will introduce a significant error into your reading. Compensating for different junction temperature in an intermediate material isn't cheap and is somewhat complex as it requires that you add precision measuring devices at each junction and develop a compensation table, thats why most of the time either extension grade wire or a TC transmitter is used.

Even though you are on a budget, upgrading to TC extension wire is probably the most economical option.

There is quite a bit of information about it at http://www.omega.com/techref/

Have you thought sbout using something other than a thermocouple, perhaps the sensors from a cheap indoor/outdoor thermometer. They will be much easier to interface than type J as others have pointed out.
Roy

Thank you for your replies. From the looks of everybody's comments, my two choices would be either purchasing TC extension cable OR purchasing a sensor (unsure where I'd find this...but I will search).

The cable I am using IS copper and both wires are the same. But as far as the errors go, I do see that I believe. At times I see reading that are right one. Other times I adjust the value up to 4 degrees be accurate. Then a day later...I'm back to 0 adjustment.

And yes, money is an issue and being that I am not doing much with this right now besides learning and keeping my skills up.

Mouser electronics has good temperature sensors from $0.3 ...

I see this item on the Mouser site

http://www.mouser.com/ProductDetail...=sGAEpiMZZMtffRBI/27p9NkdTwuk7VtiC4k0e5pH958=

It's a thermister so is that compatible with the 1746-NT4 or would I need to use an analog card. I do also have a analog card in the rack if needed.

Or a pt100 that is very precise and easy on a PLC.
The TC setup you are using will not work and you will see the tempdifferences in the rooms not the actual ones.
TC is max 10 meters. a thermistor will also be good and cheap.
when using a resistor and some experiments you still can use the TC input.

I've been away for a bit but I've been logging the temperatures from all four channels and noticed a pattern that I thought was interesting.
sensor locations:
0 outside straight termocouple cable
1 basemeent " "
2 main floor Extented cable
3 upstairs " "
3b internal temp (CJC)

When I see a jump in the basement temp, thats when the main floor and upstairs readings jump up way to high. The basement temp now is usually around 58 but when we come home, the furnace is on longer to make 70 degrees in house. Today I also was doing laundry ( have a point...really..haha)and I have the exiting exhaust bypassed into the basement for additional heat and humidity. The basement went up to 68 degrees and was pretty humid. And when I looked at the two upper temps, they were way off (actual 70...reading 79).
Is there some way the CJC is getting messed up with this humidity and/or heat? I know my extension wire is less than ideal but I really do see actuarate readings 95% of the time. It's aalmost like if I do laundry, things go nuts.
btw...the plc is in the basement and in a direct patch of the exiting dryer air (but about 15 feet from the actual vent).

Sorry for being long in my words but it seems I found something that may be a cause.

I have taken notes from everybody on ideas for a better sensor. I'll be trying those when time/money permits.