Measuring wire gauge inside cable????

I need to check out cables with 6 wires in them. Checking the soldering connection will be easy, just measure resistance, but making sure the correct gauge was used is tricky. I cannot open the cable and measure the wire physically, must be done with current or something. Any bright ideas for how this is possible? This is mandatory for our check out at work, the wire gauge is critical for the medical equipment to work properly.
The cable length is 3-5 ft, wires gauges are 18 and 26.
Thanks much!

Are you saying you need to distinguish between 18 and 26 AWG wires by external means, or is there more to your request?

Try measuring measuring the resistance of the wire with a large current passing through it as follow...

Connect a power supply directly across one of the wires with a current meter in series. Turn the current limit knob on the power supply all the way down then turn on the power supply. Slowly increase the currrent limit knob until your current meter measures 1A. You are now sourcing 1A through the wires. Take a voltmeter and measure the drop across the wire. The larger wire gauge will have a smaller voltage drop (less resistance) than the smaller wire (more resistance).

Make sure to measure the voltage drop across only the wire of interest. In other words, don't connect the voltmeter leads back at the power supply. Connect them right at the ends of the wire you're testing.

You'll have to experiment to see if you get enough difference to distinguish between the different wire gauges. Its possible you may need to use a larger current.

Thanks for the responses. Yes, I do mean I need to measure the wire gauge by external means.

As far as the current, I thought that would show me which wire is bigger, but not the gauge. Is this correct?

So, which do you want to do: tell which wires are 18AWG and which are 26 AWG, or determine the gauge of every wire in the cable?

You should be able to tell the difference between 18 and 26 by putting an amp thru each and measuring the voltage drop. But you probably can't get the resolution to be sure the 18 is not 20.

Just try it and see what kind of results you get.

Baring taking multiple xrays of each assembly you are left with indirect means to measure gauge. I would argue that a resistance measurement is superior then merely measuring wire diameter as it is the wire resistance that is the pacing requirement, not the wire's physical size.

How much variation does the cable length actually have? Can any cable vary from 3 to 5 feet, or do several models range over these numbers? Lots easier if they are different models.

You can find tables of resistance by AWG per 100 feet so you can easily compute the expected values. Follow that by a reality check using methods such as JMac3108 suggests to establish a minimum and a maximum resistance for each wire and connector.

If you need to do this test frequently make a custom test connector with voltage and current points for each wire in the cable, along with a nice switch box to click in each wire in turn.

Sorry if I did not make myself clear folks, I need to measure the EXACT gauge of each wire to check that they are correct.

ErnieM, each cable will be the exact same length (within a millimeter or so).

I will look into the tables for resistance and see what I can do, let me know if any other ideas come up!

Does the cable have any identification markings on the outside?
If so,that is the best way to find out the conductor size,as it will be included in the specification.
Or,if you can sacrifice on of the cables,cut it,& measure the conductors .

Electronic methods are always going to possess a fairly wide range of uncertainty.