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.


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?

  • 3
    \$\begingroup\$ Discuss with the "approved qualifier" - you are not the first trader with this need and you won't be the last. They will have ideas for solutions that will work. Perhaps the software has "correction factors" that can be "locked" etc etc \$\endgroup\$
    – Solar Mike
    Jun 2, 2019 at 16:26
  • \$\begingroup\$ I agree very much with Solar Mike about talking with those approved qualifiers. Any that have been around for a while in your business area will be a wealth of useful information and worth the time (and money, if required.) But the quality of a system depends a great deal also on other processes that surround a system. For example, if you periodically pull "randomly" from the process line, weigh it with a DIN or NIST traceable scale kept separately, and check the labeled results against its measurements, I think that would add another element of confidence. And that's just one element of many. \$\endgroup\$
    – jonk
    Jun 2, 2019 at 16:42
  • \$\begingroup\$ Since only "reporting the weight correctly" part is legally binding, that can probably be locked down. One can possibly have an option to not print the label unless the weight is acceptable. \$\endgroup\$
    – Indraneel
    Jun 2, 2019 at 16:50

1 Answer 1


I think you're going to need to seal the signal pathway from the weighing platform to the printer such any physical tampering will be evident, this means some sort of cover over the moving parts and the wiring

And also restrict operator customisiation such that they cannot print false labels. If necessary move the printing code out of the PLC that's controlling the line, into a controller that is only responsible for putting marks on paper and can be locked down. (possibly with an auxiliar output only signal path to the controlling PLC so the controller can see the weight, and some sort in input to command it to weigh and print.


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