What is the purpose of the vent on your cement scale anyway? Remember when you were a kid and you would stick a straw into your drink and put your finger over the top and pull it out and the liquid would stay in the straw until you let your finger off? The same principle is at work with the vent in your cement scale. The reason the liquid didn’t leave the straw is because you were blocking the path for air to replace the liquid. Once you removed your finger, you allowed air to rush in to fill the void left by the liquid leaving the straw.

If a cement scale were not vented, then nothing could weigh into the cement scale because the air inside the scale would have no where to go to allow the cement to replace it. To be very technical, there are larger forces at play than the old finger and straw trick so the air can compress in a cement scale up to a point but in the end, no vent largely means no weigh up.

Now, imagine a partially blocked vent. This can cause two different problems depending on the plant and each one seems to regularly cause confusion for concrete plant operators and owners.

1. Partially blocked cement scale vent with WEAK butterfly fill gate on the silo.
   
   1. In this scenario, a partially (read heavily) blocked cement scale vent creates pressure in the scale because the air cannot leave the scale as fast as the entering cement. Because it’s partially blocked, is creates a temporary pressure inside the scale where the air is being compressed. Because the air is still leaving the scale, just much slower than designed, this creates a vacuum. In this example, the WEAK butterfly gate is actually pulled partially open and you are now pulling cement out of the silo even after the batch computer has closed the gate because of the vacuum created. You will notice the scale weight slowly climbing for a long period of time while the air bleeds off. This is a nightmare on your inventory and mix consistency, not to mention your bottom line.
2. Partially blocked cement scale vent with STRONG butterfly fill gate on the silo.
   1. This scenario is the same as the above scenario except we have a strong butterfly gate on the plant. In this case, the pressure built up in the scale actually causes the air to further compress. In an interesting phenomenon, compressed air actually “weighs” something. More than you would think actually. A lot more than you would think. What you will see in this case is a climbing weight that might look normal but after the concrete batch computer finishes weighing the material, the weight will slowly drop off for a long period of time. This means you didn’t actually have as much cement in the load as you thought because the scale was weighing compressed air and it bled off.

Both of the scenarios are bad for quality and consistency but also cost and inventory reporting. It is vital that you make keeping your cement scale vent a regular maintenance item and believe it or not, you should consider doing it daily, if not every other day.

For other tips or questions, do not hesitate to reach out to our qualified and knowledgeable staff to help you with problems you might be having.