We have a project where we need to weigh product material for selling to customers. This is a project installed in Europe.
This is an industrial product line performing material manufacturing, preparation & packaging. At the final step of the packaging, the product is weight using a load cell scale, information is read by PLC and then the PLC produces a label which is printed out and then applied to the packaged product.
Our general understanding is that: "In all commercial application where product is sold by weight, it is a legal requirement that weighing equipment must be verified as legal-for-trade".
To go through a legal-for-trade process, the weighing system has to be approved by an "approved qualifier".
To make sure to obtain that approval, care must be done to have proper certifiable components for a trade application. Components that detect tilting for instance or that verify standstill, etc.
If a trade approved unit undergoes any alteration or repair, it does require re-verification for the approval to remain valid.
One additionnal concerns is tampering, so that the "approved qualifier" can ensure no tampering happens.
PROBLEM AT HAND
The weighing electronics can be locked-down with a password set by approver or with a seal.
Given how the PLC is involved in the weighing loop (getting data and printing the label), the PLC software cannot be locked-down as it will prevent troubleshooting and adjustment of the entire manufacturing line. How can an "approved qualifier" ensures and certifies legal-for-trade of the system?
What are common strategies employed in such industrial legal-for-trade application?
Would it be required to have dedicated hardware from weighing to printer without involvment of the PLC in the loop? This would require interfacing multiple components from various manufacturers together, the most difficult beeing the interface between the weighing electronics and the label printer; that seems most likely to fail!
Is there something I am missing?