I you look at "scales", you see just a whole piece (see the above picture), it's not like a pair of scissors.
Probably, in the past, the scales has 2 pieces like these
But nowadays, in modern world, we don't use that "2 piece scales" any more.
Is it correct to say "use a pair of scales to weigh the products" or "use a scales to weigh the products"?
"A pair of scales" is a very old-fashioned device, referring to the two-sided scales that requires a set of weights to be manually added to one side while the item to be weighed sits on the other - your second picture. These are also sometimes called a "set of scales", likely because they come as a set with the required weights.
Some people may still say "a pair/set of scales" to refer to modern scales, but the kind shown in your first image - kitchen scales - as well as other kinds of modern weighing scales such as bathroom scales, are usually just referred to as 'scales'. An article is not required as it is a plural noun, so you wouldn't say "a scales". When referring to these kind of scales, most would simply say:
Weigh the products using scales.
It is also common to specify kitchen scales. Another commonly used term is weighing scales. Again, these would not need an article because they are a compound plural noun.
I have always heard it used in the singular, "a scale". Like, "Weigh the vegetables on a scale" or "I checked my weight using a bathroom scale."
I don't recall ever hearing anyway say, "I weighed it using scales."
I just checked thefreedictionary.com, https://www.thefreedictionary.com/scale, and they have, definition 3, "scale", "an instrument or machine for weighing". But then they say, "often 'scales'". So apparently both the singular and the plural are correct. Perhaps this is a regionalism, that in the UK they say "scales" while in the US we say "scale" or whatever the boundaries are.
amazon.com refers to such devices as "a scale", singular. See, for example, https://smile.amazon.com/s?k=scale&ref=nb_sb_noss_2, where they list "kitchen scale", "bathroom scale", etc. And of course if Amazon says it that way, it must be right.
Traditionally, things like scales, glasses, trousers have been seen as coming in two's and therefore are always used in the plural form.
Nowadays, "scales" can be made from a single part. Actually, most often they are a single part, like in your first picture. But grammatically, we don't care about that. They are plural scales. Just checked the amazon website, and all their weighing equipment is sold as "scales". You wouldn't say "a pair of scales" for one-part scales, but they are scales.
So I would say that "Use a pair of scales" and "Use a scales" or "Use a scale" would all be wrong. "Use scales" would be right - unless the person has an actual pair of scales like your second picture.
(Things get funny if you have more than one scales. For example, I bought two each of the items on your two pictures, four items in total, and you ask me what I bought... I'd answer "two scales and two pairs of scales" but have no idea if that would be correct. Just avoid doing this :-)
The specifics have already been answered, but generally speaking, a scales sounds wrong immediately because you're matching a singular article with a plural noun. You either need to pair it with a determiner that takes a plural ("the scales", "some scales", "your scales") or use a singular group noun ("a pair of scales", "a truckload of scales").
As an English native speaker with an A grade in English Grammar O-level ...
I was taught to say 'Use some scales to measure the amount of flour needed' for example.
You could say 'Use a ruler to measure that distance' but 'scales' implies a plural although it's not one. 'a set of scales' is equally fine though. As in ..
'Use a set of scales to measure the sugar for the cake'.
'some' gets round the awkward plural sounding form. 'use a scale to measure the currants' is technically correct but the measuring device isn't called 'a scale', they are called 'scales'.
It's just one of those English curiosities.
