A PLC controls the rate of water flow to a continuous mixer. The operators are reporting that batches have recently started coming out far too thin (too much water). Preliminary troubleshooting has established the input side of the water control loop is problem-free. That is, simulated inputs to the PLC always produce the correct response in the PLC. The water valve has been stroked, and it works perfectly.

Where do you need to look for the problem?

Just because there's too much water in the final mix doesn't mean too much water is being added. There may be problems with the other inputs to the mixer in addition to, or instead of, problems with the water.

If you were mixing batches, you could weigh a batch. If it's too heavy, you have too much water; if it's too light, then there is too little of something else. Unfortunately, this is a continuous process.

An easy way to eliminate cause possibilities is to use the PLC to simulate the outputs for water and each of the other ingredients. Force the outputs at 10%, 25%, 50%, 75%, and 90%, and use a DMM to measure the signal at the corresponding analog output module terminals. If you run through this calibration several times and results are consistent, you can assume that any problems are between the output module(s) and the final control elements.

Use the recording setting of your DMM to look for induced spikes on the output wiring. Check the input wiring this way also, as it may not have been tested for spikes.