Looks do-able to me.

        Reading proximity sensors, reading load cells and displaying values
on request while regularly logging them is simple PIC stuff.

        Measuring liquid heights can be done several ways, but as you are
already reading proximity switches why not use that method for this task
too?  Use a captive rod in a guide above a float to trip proximity switches
above your tanks (KISS principle).  Also gives a visual indication of the
amount in the tank as the rod rises above the rim.

           ______  _
             ||  <||
             ||x <|| sensors
             ||  <||
           _ || _ ||
        ___| || |_||_________
        |  | || |           |
        |  | || |           |  Use courier font to see diagram
        |    ||             |
        | ___||___          |
        |_|float |__________|
        | |______|          |
        |                   |
        |       liquid      |
        |                   |
        |___________________|

        This should also work for your special additive liquids as long as
you use compatible materials for the float and rod.  For increased accuracy
use tall, thin tanks (large, vertical pipes?).

        Humidity sensors are available in 1-wire devices.  [I assume you
need to know the humidity in several feeds to guess the amount of water
contained in the raw materials (sand and gravel) to calculate the real
amount of material being added and the water to be added]

        Good luck.

        <original message>

        To automate a plant of Concrete.

        The idea is to use a PC for the general control of the plant and to
arm a
        network (Possibly RS485) with terminals that will be slaves working
with
        PICs, to complete different tasks, as the following ones:

        1) To control the pneumatic servos and read proximity sensores and
other.

        2) To Sense the weight of the materials with cells of loads,
indicate the
        value measured in a  7 segments display and to send the measures
througt the
        network when they are requested or periodically. This partly would
be
        resolved, but I accept ideas and suggestions in this respect.

        3) To Sense the level of water in the main dosificador tank.
        <snip>