Do I use definite/indefinite articles properly in this example?

Question

I have so many problems to find the proper usage of defnite vs indefinite articles. Could you please see the example below and tell me if I made a mistake, and what is the proper general rule/ why the used article is wrong? This is just an example where both sentences are general rules, and it makes me really confused about the second sentence especially.

"A manager (one of many) mentioned they - according to guidelines - don't have to calibrate equipment every time they replace a water filter in a coffee dispenser. But, usually, as they replace the filter in the coffee machine the equipment calibration changes."

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Edit:

Ok, no problem I'll try to describe my doubts. I just don't 'feel' like they are all correct. Well, this is just a feeling and it's not very easy to explain.

1. I don't feel when 'an object/or a noun(?)' is really a definite one. Is this 'a particular object' which is specified/definied or 'a word' which we'have already mentioned in the previous sentence?
2. In the 1st sentence doesn't make it definite when we put 'coffee' before the word 'dispenser'?
3. or, doesn't make it definite that we generally spoke about coffee machines before eg. 2 days before?
4. 'the equipment calibration' as a noun mentioned for the first time in the second sentence. Why not to use 'an equipment calibration'? Also, this just sound strange to me 'an equipment calibration.
5. Should we use 'the' in the second sentence because we just used both words 'filter' and 'coffee machines' or should we mean 'the same particular object(s)' in both sentences?
6. Should we use undefinite or definite articles in the second sentence. This is only a very general statement, so why not to use indefinite articles (in both)?

Answer 1

"A¹ manager (one of many) mentioned they - according to guidelines - don't have to calibrate equipment every time they replace a² water filter in a² coffee dispenser. But, usually, as they replace the³ filter in the⁴ coffee machine the⁵ equipment calibration changes."

¹ This is the subject of the sentence, as in who is speaking of their experiences with the coffee filter. Since there is only one manager being called into question, 'a' is an appropriate article to use here.

² This is the appropriate article to use here since it is preceded by "every time they replace." It hints that they do it often, but it isn't an action of much consequence outside the given discussion about coffee filters currently being held. If you used 'the' here, it would indicate more importance placed on the object of the coffee/water filter, when it doesn't need it - not all coffee dispensers may have a filter, for example. It's only talking about this specific case, and this specific coffee dispenser.

³ This 'the' is used to place familiarity on the now familiar coffee dispenser. You know which the speaker is talking about because he mentioned it the last sentence. If he hadn't done that, then you wouldn't know, and therefore wouldn't be able to use the article 'the.' It appears as if it only has one filter, too, so using 'a' would be confusing because it would suggest there's more than one.

⁴ Again, since you know he's talking about a coffee dispenser, and not something else like a potted plant, 'the' is okay. It is the topic of conversation.

⁵ This one's tricky as it relies on a meaning of the word 'as' that's a little obscure. It means, roughly, "because they do this, this happens as a result, always." The meaning here differs from the first sentence in that the managers technically don't have [aren't required by the rules stated] to calibrate the coffee dispenser, but they should, as evidence by the second sentence, which states that their act of replacing it alone re-calibrates it automatically, and because of that, would require the manual re-calibration suggested of the managers in sentence one.