Analog input to T100MD-888+ ??

[cdenk]

I have a Tri-Plc T100MD-888+ PLC. It has Analog 0 - 5 volt input, and 5 volt reference voltage available. I have a fuel level sensor that is 0 - 90 ohm range to interface, only need around 50 or so steps, and can handle calibration/non-linearity with lookup table or Basic programing.  Presently there is 30 feet of Cat-5 cable in metallic conduit  to the remote location, I would prefer signal conditioning at the PLC end of the cable.

Also what would be the easiest way to get temperature readings, say -30F to +130F, nearest degree resolution would be more than enough.

Spent all the budget on the PLC, buying a $250 signal conditioner is not an option.

[support]

Do you mean that the fuel level sensor has a variable resistance in response to the fuel level change?  The lowest cost means is to build a voltage divider using a resistor in series with the  sensor and apply a 5V to the combination and tap the center of the two resistor to measure the voltage developed across the sensor. To reduce self heating, you may use a series resistor such that the voltage drop across the sensor will fall between 0 to 1V.

You may be able to use a thermistor to measure temperature using a circuit as described above. Thermistor transfer function is non-linear so you will have to build a look up table in DM and use the look up table to interpolate for intermediate values.

[cdenk]

Yes the fuel sensor is a basically a variable resistance with 90 ohm empty, and zero ohm full.  It is only 2 terminals, not 3 like a pot. I was trying to limit the 5 volt reference current to less than 4ma (less than half the 10ma limit without changing RA3) to allow some capacity for other sensors. Then resistance to ground then needs to be around 1K ohms to ground, so the total resistance is 1090 ohm at empty, and the voltage measured zero to .41 volts.  The .41 is 8% of the ADC 5 volt full scale and 4096 * .08 / 4 = 84 steps, which is acceptable. I would prefer to have more current of the 5 volt reference available for other sensors, but think I could live with that.  If my thinking is off, don't hesitate to comment.

Someone has suggested using an OP AMP, which I am learning with a little help. At this point my expertice is a little more than E = IR, just enough to be dangerous, but I can see far enough to build up a breadboard and see what happens.  

[support]

You could get more current if you connect the sensor and series resistor to 12V or 24V. Compute  your resistor size such that at maximum voltage develop across the sensor would be less than 5V. The only issue here is self heating. If R=90 while V = 5V, then the power dissipated through the sensor = V^2/R= 0.28W and the series resistor will likely to dissipate more heat. If the flow sensor can handle this power dissipate without losing accuracy or burn out then you can get much higher resolution since you will be able to use the full range of the ADC for flow measurement.