I guess another thing to check is that there is no temperature
gradient at the various cable joins - (T/C to extension cable,
extesion cable to equipment). These connections should be made at an
"isothermal block" and wrapped in thermal insulation to prevent any
temperature difference between the two wire joins.

And you are using the correct type of extension cable for the
thermocouples in use of course?

RP

On 16/01/2008, Spehro Pefhany <speff@interlog.com> wrote:
> Quoting "Chaoua, Rashid" <rashid.chaoua@lmco.com>:
>
> > Hi,
> >
> > I have problem reading accurate temperatures using thermocouples. I have
> > a 40' long refrigerator divided into 4 compartments (3 rooms used for
> > testing and the fourth houses my electronics). In each room I have a
> > thermocouple suspended from the ceiling to mid-height of the room. These
> > thermocouples are connected via thermocouple extension wire to a
> > National Instruments cFP-1804 module and connected to an isothermal
> > connector block. Additionally, each room is kept at different
> > temperatures (ie- room 1: -10C, room 2: -15C, room 3: -20C, room 4:
> > -30C) and the wire is terminated in the coldest room (room 4).
> >
> >
> >
> > My problem is as rooms 1-3 are heated up (via heaters) the temperatures
> > read are incorrect. If I read the temperatures at the thermocouple
> > directly they read the correct room temperature. When I read the
> > temperature at the end of the thermocouple extension wire it reads a
> > much colder temperature (I am using a handheld meter). The same problem
> > occurs when I use my National Instrument device. I need the temperatures
> > read to be accurate (I have a program that uses these temperatures to
> > make a series of decisions) but am unable to find out why I am having
> > such problems. Can anyone out there lend there expertise?
> >
> > Rachid
>
> One of the most common errors, perhaps unbelievably, is reversed extension
> wire which will yield an error of (roughly) double the difference
> in temperature between the two ends of the extension. It's aided by the
> silly US color code for T/C wire.
>
> Best regards,
> Spehro Pefhany
> --
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