ORP Signal Conditioning Problems

I am trying to buffer an ORP sensor with a gain of 2. See the circuit below. The ORP sensor in water measures about 160 mv. When I connect it to my circuit, the output goes to about 1.4 V. It should be 320 mv. D8 is a dual Schottky diode for op amp protection. C26 is a 16 V ceramic cap & C18 is a 25 V electrolytic cap. When I remove C26 there is no change. What's going on?

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Why is the purpose of C18 being parallel with the feedback resistor?
What output voltage do you measure when you ground the input?

Try removing C18.

Also why do you need D8?
The LVM324 has built in protection for the input circuits.
Try removing D8.

Apart from just using the ORP sensor, have you tried to test the amplifier by injecting a signal from an adjustable DC source and measuring the output of the amp to see if it is working correctly on it own without the ORP sensor?

JimB