Getting proper information from water and admix meters
Does the batching computer for your concrete plant always give exactly the right amount of admix and water? It probably doesn't, but you'll never know it. The big admix companies use a practice that makes their equipment look better than it really is; as a result, you could be using (and paying for) more admix than you think
Admixture companies usually pre-wire their ontrols so that the power for the meter comes from the filling valve solenoid output on the computer. As a result, at completion of the filling , when the valve turns off, the meter stops counting. This does not necessarily stop the flow of admix, however. If the flow was fast, it probably continues for a fraction of a second before the valve closes.
This can translate into a few ounces or many milliliters in every batch. Even worse, consider the valve that sticks in the OPEN condition. Admix will continue filling, possibly at a much reduced rate, during bottle discharge and all the time up to the start of the next batch. The control system will not record it and only a very alert operator will spot it.
The solution is to re-wire the admix controller, which we do on almost every installation. Power the meter from a solid power source (see diagram). As long as the batching computer monitors the meter after shutting off the valve, it will catch the errors above.
Wire water valves and meters in the same manner.
P.S. – Master Builders assures us that they wire their admix systems; the above change is not necessary! Thanks for the feedback.
Improve jogging performance with flow control valves
Big feed gates with large operating cylinders can open very quickly. When jogging at the end of the feed period, the objective is to release small bursts of material with each jog
Concrete plant batching controllers usually control the “open time” of the gate in steps of 1/10 to 1/100 second. Since it takes 1/10 to 2/10 second to start the gate moving, and it can be full open in 3/10 second, a setting of 2/10 may drop no material while a setting of 3/10 can drop 50lb/20kg. This is too much for accurate batching. The computer’s automatic tuning can try to adjust the jog time but it could jump from 2/10 to 3/10 second and back again in successive batches. This is futile and will result in lost time and tolerance errors.
The solution is to add a flow control valve in the opening direction only. When adjusted to a speed of 1 second (approximately) from closed to full open, the open time plus the size of the opening can be adjusted in small steps. Since the volume of material dropped in each jog is roughly proportional to the area under the curve (see diagram), you can control the amount of each drop by the same 1/10 second steps. This allows the automatic tuning (assuming that your system has it) to easily adjust to the best setting. Update, 2016: Most PLCs now have timers with 1/100 second increments, so a faster closing rate is possible – but the principle still applies!
Checking your scale calibration
We had a situation several years ago where a concrete plant had been making sub-standard product. We found that the cement scale read 20% too high and it weighed out too little cement on each batch
Understandably, our customer was very upset. The fact is, however, that the customer is responsible for checking his equipment, and that includes scales. This was an extreme case, but it shows very clearly what can happen when we blindly believe our instruments. Even if the customer checks the scales every 6 months, things like this can happen. We recommend that the customer checks his batching scales every week by having someone apply his body weight to each scale, while someone else reads the display. This is rough and ready, but will find gross errors quickly before much damage is done. A better way is to use our SHUNT CALIBRATOR unit, which connects to each scale in turn. It simulates a known weight, checking the complete load cell circuit as well as the indicator. This avoids the need for test weights during weekly tests, but you should do a six monthly calibration test to meet quality plan requirements.