I am well acquainted with all three types: Vernier, dial, and digital. I have all three types in my shop.
As for absolute accuracy, a good Vernier will always be more trustworthy than either a dial or a digital. There is almost nothing that can go wrong with a Vernier if it is not mechanically damaged. But small particles of dust or chips can easily throw a dial caliper off and I spent an entire career fixing electronic devices that went south and that should be enough said about that.
But the question was which is easier to read, Vernier or dial. That question completely leaves out the digitals which are the hands down winner in this respect of readability.
I would have to say that the dial calipers are a lot easier to read than the Verniers. I find that I can easily read a dial to 1/4 of a division 0.00025". Now, that is not to say that a dial caliper is accurate to that level, just that I can easily read the dial to it. One thing that this means is that, if you are comparing parts and need one that is perhaps a half thousandth larger, the dial caliper may be the best one to use as it would allow comparative readings to that level. The Vernier would not and a half thousandth is usually the least count on a digital caliper and there is that pesky +/-0.0005" spec in every reading made with it so finding a part that is just 0.0005" different would not be an accurate procedure because you would be within the basic error margin of the instrument. See my example below.
I was introduced to Vernier calipers in my college days by an old physics professor who carried a 10X magnifier with him at all times just so his ageing eyes could see the lines on those Vernier calipers better. Even in my 20s I found that a magnifier does make it a lot easier to find the pair of lines that are aligned. And, with a high quality Vernier caliper, not a cheap import, you can often see that two adjacent pairs of lines are both off in opposite directions. This kind of reading relies on having lines of extremely consistent width and very straight edges and that is why I completely eliminate the cheap imports from this. Also high contrast between the lines and the background is very desirable. And you MUST be aware of possible parallax effects and move your head side-to-side to be sure they are not fooling you. So, at the very best, you MAY be able to read a Vernier caliper down to the 0.0005" level. BUT that is just a reading, not something that is supported by the caliper's specs for even the very best of the Vernier calipers. And it is far more nebulous than the differences on a dial's readings so the dial would still take the championship in readability as compared to the Vernier.
EXAMPLE of close digital readings: Most digital calipers read to 0.0005". That is absolutely all the digital display can show. There is no dimming or other change of that 0.0005" figure to let you guess to a finer reading: it is either a "5" or a "0", there are no inbetweens. So, let us say we have two parts that are 1.0002" and 1.0003". Our digital micrometer confirms those measurements. A digital caliper that is in perfect working order and perfectly calibrated will read 1.0000" for the first because it rounds that tenth place down and 1.0005" for the second one due to rounding up. In comparing these two parts with a digital caliper we are shown a 0.0005" difference when, in reality, that difference is only 0.0001". So if we choose the second one becaues we wanted a somewhat larger part, it may be very disappointing. A vernier would not be able to show any real difference in those parts and we could only conclude that they are within 0.001" of each other. It would be virtually useless for this comparison. However, a dial caliper that is also in perfect working order and perfectly calibrated, would show the needle at about the same position, approximately 1/4 of the way between 1.0000" and 1.0001" and this tells us that these two parts are within two or three tenths of the same size and perhaps do not show enough difference to be of any practical use.
Hence, the dial caliper is actually
a lot better when making
comparative measurements. And this is due to the fact that dial readings are easier to see.
All that being said, with all three types equally available in my shop, I use an 8 inch digital caliper for 99% of my own measurements. And my most common reason for going to another instrument is because I need the increased accuracy of a micrometer where I also use a digital most of the time. My digital micrometer reads down to 50 millionths so I can be somewhat confident of a reading in tenths with it.
I worked with a pig.. oops, guy I mean
that was the biggest slob and he swore by verniers I think because you could leave them in filth and not screw up the rack like on dials. He'd grab them, slap them on the part, then declare "It checks 1.251 and a half"
That seems to me the exact opposite. Reading a vernier to within a thou or two would be damn near impossible for me now with my eyesight, while dials are just as easy to read as when I first started this trade.
