Troubleshooting Tips
When troubleshooting look for the following:
  • A shorted mixer probe could be caused by dirt and/or condensation at the probe or mixer terminals.

  • The center conductor of the probe may be broken or excessively worn. The probe's ceramic insulator may be cracked due to a whack from a loose mixer blade or hammer blow.

  • Chemical impurities or contaminations like clay, silt, clinkers, chunks of ice or soil in the aggregate can make pronounced changes.

  • Using two probes per mixer as shown on this sketch, generally results in more consistent uniformity.

  • The probe must be in contact with the fresh mix at all times to secure correct readings. Never permit the concrete to harden or pack over the probes and do not use a hammer to clean them.

  • The wiring between probe and instrument may be broken or the connector may be loose due to vibrations. It is recommended that standard automotive spark plug wire is used, preferably encased within rigid conduit. Oils, chemicals and heat will break down ordinary insulation. A snap on connector at the probe is not only convenient but also will prevent some problems mentioned above.

  • The moisture meter should be mounted on a wall or location not subjected to vibrations.

  • Don't overlook the need for steady water pressure. A gauge easily observed by the operator is cheap insurance against this occurrence. Fluctuations in water pressure may be due to a faulty regulator or strainer.

  • A leaky water solenoid valve may be the culprit when the mix becomes too wet.

  • While checking the scales for proper weights, inspect the weigh hoppers also for hardening or hung up materials.

  • On initial installations, the meter dial may have to calibrated. The meter manufacturer's instructions should always be followed.
Additional Information

Although the moisture meter may be functioning properly, a host of external conditions may arise which could cause erratic readings, unless the following variables are taken into account.

Conductivity factors are influenced mainly by composition and concentration of the ions. The temperature in turn affects their mobility. Applying an electrical current to the mix, to measure its conductivity and relative moisture content, will establish the "character". This character of the individual aggregates is influenced by particle shape, degree of crushing, specific gravities, finess modulus, separation, moisture absorption and the degree of mineralization. Water used for concrete should be clean enough for human consumption. When in solution, all these ingredients collectively form the character of the concrete mix.

It is quite likely, that even though the water content of a mix formula remains constant, the meter will show it to be wetter or drier depending upon changes in temperature or other characteristics of the aggregates. When aggregate gradation changes form coarse to fine, or vice versa, a corresponding change in the current flow through the mix will result. This is due also to the change in total surface area of the particles contacting the probe.

When the aggregate is coarse the particles may contact the probe in relatively fewer places, whereas fine particles will contact the probe in more places. Thus a given current will flow through the fine mix having less water and less resistance than that of a coarse mix.

A drier mix will result if the batch is finer than the sample to which the control was adjusted. Conversely, a coarser batch will be wetter than desired. Increased temperature of the mix will increase the mobility of the ions and the changed conductivity would give similar results.

Miscellaneous Factors to Keep In Mind

Mixing, shovelling, changes in storage methods, rainstorms etc. are all elements that will cause an aggregate to alter its character. Outside storage piles of materials may be affected by rain, (particularly acid rains) and wind, snow and ice. Materials can be blown or washed away, leaving an undesirable gradation. Watering down the stockpiles can prevent the roll down of large particles in addition to wind erosion. It may pay to cover the stockpiles with tarps. Depending upon the circumstances, variable factors can be self compensating or cumulative. Proportionally use materials from different areas and levels of the piles and keep storage bins as full as possible to minimize segregation.

The sun will raise the temperature of cement in silos, water in storage tanks and aggregate in stockpiles. Cement may be delivered at warm temperature.

Cements of different types or from producers could have been mixed in the same storage bin, introducing yet another factor.

Inconsistent use of plasticizers such as fly ash or other admixtures could upset the balance. Calcium chloride additions are usually made after the final water has been added for the same reason.

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