Understanding OP AMP output

I'm using a LM318N in the below configuration with a Force Sensing Resistor


My output voltages with various RMs are as follows,

5K 1.3 to 3.1
10K 1.3 to 3.5
47K 1.3 to 3.6
100K 1.3 to 3.4

I do not understand why I get a minimum of 1.3V to start with all RMs
Secondly the range and top pressure readings are all about the same not varying as per the FSR data sheet
Is this a function of the op amp and will I get a different result with say an LM358 or a LM324 ?

hi,
I assume you are using a single voltage supply for the LM318.??

If Yes, then the output will not swing to 0V, you require a dual supply.

The LM324 will swing close to 0V output with a single supply, when loaded with a 10K t0 0V.