0_Einstein

Use 2 Resistors, the one shown in your sketch, and perhaps another R of similar value but much lower wattage rating.

Wire the 2 R's in series.

Apply the AO across the 2 series R's - creating a voltage divider, something like this:

     HTR         R known
o---v^v^v^---o---v^v^v^---o
|            |            |
+-[AI-]    [AI+]          |
|                         |
+-[AO-]-------------------+ 

AO across both R's in series.  AI only across R-HTR 

If R-known ~ R-HTR, AI ~ 5V (if AO = 10V).

You can use Ohm's Law to calculate any value once you accurately measure R-known and verify output voltage and AI input - using a calibration technique to map AO values to actual voltages and actual voltages to AI values.

You will of course need to periodically install another R-Known in place of R-HTR to verify proper operation to account for environmental and other factors causing drift.

There are other ways to accomplish the same task.  That said, you may want to use other methods that improve accuracy and repeatability.  This is to triage whether R-HTR is ~within expected tolerances.

eng_dave

Please see attached pictures of my setup, 

I know the figures for the 0-10v resistance are wrong its just to show what are looking/expecting to see

So we output the 0-10v (when we get 0-10v) we should then get a different voltage return depending on the resistance 9-11.7ohm into the input via the circuit and we can measure the resistance and decide if its a pass or fail 

 

Hi, Thanks for the response

Being totally honest some of that has gone a bit over my head you might need to do dumb it down it slightly for myself haha

If im correct in understanding what your saying would this be a rough example of the circuit?

0_Einstein

Where is the M reference across which the AO is to exert influence?

You need the entire voltage divider to drop across both series resistors and only monitor the voltage across the heater's resistance by a single Analog Input.

 Hi Einstein 

Sorry but I'm still confused, what do you mean about the M reference?

Would you be able to draw a little sketch or something please?

I do not understand what do you trying to do with analog module voltage output? There is no way to reliably measure resistance of 10 ohm when minimum resistance to load voltage output is 1000 ohm! When load resistance is lower there will be significant drop of voltage. Or maybe you have some hi power amplifier connected to the PLC analog module? It must have capability to source at least 1 A of current.

If you want to measure resistance with different voltages with provided schematic - just use some count of external stabilized power supplies switched by relays. BTW supplies must be preloaded with some additional load resistors to increase stability.

If you want to get more precise measurements - use Wheatstone bridge connection, Vout connected to PLC AI module input set to the lowest range.

wheatstones--bridge--bridge