Can I have multiple scales on a single weight controller channel?

It is possible to have multiple and independent scales attached to the same channel, and in closely monitored situations it is possible to share like scales on a single controller.

You would not need any switches if the scales were the same capacities. If the scales are different capacities you would need to re-calibrate each time you switched scales. C2® calibration would be easy, but a procedural error could cause you weighing problems.

If you had two ANY WEIGH scales of the same capacity this could act as a single scale, only with independent action.

Think of a platform with two load cells. If you put the weight on either side of the scale the total weight is read. The two load cells are summed (parallel) and outputs only the amount of force applied. When two scales are attached there is a small amount of weight shared with the other load cell. If you have isolated load cells, the force on one load cell is not seen by the other. This will give you two independent scales. The scales must both be the same capacity and you are limited to a total number of load cells as specified in your controller's operations and installation guide.  Some model Hardy controllers can accept four load cells and others can accept eight load cells.  Additional load cells will require an external power supply.  There is a separate Webtech answer that explains using external power supplies for additional load cells. See related answers below.

We have seen applications using three identical single load cell platform scales with attached tanks. The PLC would zero the scale (gross weight) and then select a tank to fill. The process would fill the one tank to 75 lbs and stop. The scale would tare the weight (net weight to zero) on the scale and fill the next tank to 45 lb (net weight), tare the weight (net weight to zero) and fill the last scale to 30 lb (net weight). The gross weight of the scale was 150 lb, and the PLC would track the weight in each tank using net and tare. The PLC would then discharge all three tanks to zero. In this application these chemicals could not be mixed directly with each other and were dropped into the mixer from separate sides, all other ingredients were already added, and the mixer in operation. 

An example would be adding water to acid being a problem, but acid to water is not a problem.

You could partially discharge or fill each tank separately and track the changes in the PLC, but that is tricky. Tracking the gross weight and reducing the total of one tank, being filled or dispensed, by the loss or gain in weight would be a simple tracking program in the PLC.  Similar to an inventory program. 

Tank number one has 75 lb and that tank is partially discharged. The overall weight drops by 10 lb and now there is only 65 lb in the tank. Tank 2 and 3 weight has not changed. This process could be repeated for each tank and scale, tracking the change and totals of each tank in the PLC.

Using C2® calibration would make calibrating your weigh system much easier. Alternatives to C2® would be hard calibration and placing identical test weights on each scale and calling the total test weight applied the span value.