# Serial cable : Resistance too high? / Thermocouple loop resistance

Hi everyone,

In a datalogging system I'm setting up, I wanted to standardise my connections and use a standard SUB-D 9 M/F cable.

So... I bought one ! But much to my surprise, the resistance of each conductor is around 3ohms for 6ft. (2 meters actually).

Is that a normal value for a regular quality computer cable? Is it there for some reason?
Strangely, the shielding cable (which seems to be just another conductor like any other) gives me around 1ohms... much lower...

And now the subsidiary question : I want to use this cable to wire 8 thermocouples (commong ground) to an ocean controls TK249 thermocouple shield, is this resistance gonna be detrimentary to the accuracy of the readings?

Thanks,

Marc

That is high. You should see close to 0 for 6 feet.
Any resistance will lead to high readings, current x resistance = voltage that may see as an offset from actual readings.

That's weird...
They all have the same value : 2,9-3ohms.

There goes another useless buy!! Damn!!

Sure it's not your meter/probes? What do you see when touch the probe tips together?

marc426:
Hi everyone,

In a datalogging system I’m setting up, I wanted to standardise my connections and use a standard SUB-D 9 M/F cable.

So… I bought one ! But much to my surprise, the resistance of each conductor is around 3ohms for 6ft. (2 meters actually).

Is that a normal value for a regular quality computer cable? Is it there for some reason?
Strangely, the shielding cable (which seems to be just another conductor like any other) gives me around 1ohms… much lower…

And now the subsidiary question : I want to use this cable to wire 8 thermocouples (commong ground) to an ocean controls TK249 thermocouple shield, is this resistance gonna be detrimentary to the accuracy of the readings?

Thanks,

Marc

Added resistance won’t be a problem, but moving the thermocouple wire to copper wire transition point further from the shield can/will effect the cold reference junction compensation that one uses with TC sensors. unless the temperature at the shield and at the other end of your extension cable (that is using copper wire) there will be an error in the measurement. The bigger the difference the bigger the error.

marc426:
And now the subsidiary question : I want to use this cable to wire 8 thermocouples (common ground) to an ocean controls TK249 thermocouple shield, is this resistance gonna be detrimental to the accuracy of the readings?

You evidently do not know what a thermocouple is, do you?

To connect a thermocouple, you have to use thermocouple wire (which is why it is sold as such), not just "any old cable"!

Arguably, if you are measuring hundreds of °C and your extension cable is entirely at room temperature and accuracy is not that important, it will not matter much.

CrossRoads beat me to it - what reading do you get on the meter with the probes shorted?

I evidently don't know
Even though I do... But yes it slipped out of my mind that the cable should be TK cable as well...

Back to our problems, no, the problem is not from the multimeter, it gives 0,1-0,2ohms when shorting the probes...

But what I'm hearing right now is : won't make much of a difference if the cable is @ ambiant temperature (which it will be) and it won't be more than 6ft. long anyway...
So great!

Marc