The right tool for the job or just a right tool.

[MikeSharp01]

I wish I had paid more attention in Metrology lectures way back in the 70s! So which of these is a class 1 tape then?

Actually 2 of them are class one yet they do not agree over 8m at any rate. The third is a cheap plastic tape and it clearly under reads the other two - the essential question is which one is correct?

 

The yellow one is my everyday tape but only 8m long as you can see. The thin white one is 20m but still class 1. The other cost me £7 in wicks a fews years back. 

 

So as a rule they are pretty useless when measuring is shared across different tapes. Conclusion - using 1 tape for everything so that all the errors will be consistent at least.

 

 

[ToughButterCup]

6 minutes ago, MikeSharp01 said:

I wish I had paid more attention in Metrology lectures way back in the 70s! So which of these is a class 1 tape then?

[...]

Conclusion - using 1 tape for everything so that all the errors will be consistent at least.

 

I measure with a tape and a laser (where possible), and halve the difference ( 1 or 2 mm sometimes).

But I always trust a steel rule - especially over a laser.

[Mr Punter]

I just looked up what these mean.  The top 2 are about 1mm out and class 1 allows +-0.9mm over 8m, so they may be OK.  The third one is way out but may be kept for [other reasons]

[MikeSharp01]

17 minutes ago, Mr Punter said:

The third one is way out but may be kept for [other reasons]

Yes its normally used for setting out the croquet lawm at millstone manor. I have it here because its 50m amd allows me to do quick and, as you can see, dirty measurements over the full depth of the site.

[jack]

43 minutes ago, MikeSharp01 said:

So as a rule they are pretty useless when measuring is shared across different tapes. Conclusion - using 1 tape for everything so that all the errors will be consistent at least.

 

I'm sure there was someone, ages ago, who'd set up an onsite test length where visiting trades could test out their tape measures before relying on them for critical measurements.

[SteamyTea]

Our old mate Joiner used to have a metre rule on the wall, he told everyone to check their own rules and tape against it.

 

"I have 12 inches, but don't use it as a rule"

 

[MikeSharp01]

1 hour ago, SteamyTea said:

Our old mate Joiner used to have a metre rule on the wall, he told everyone to check their own rules and tape against it.

Trouble is 1m is not really enough @jack has the right idea with the outside length - but how did they know their length was correct, However the issue is not an absolute one as unless you have access to lab conditions you are unlikely to be able to get good enough conditions to reference measure within 0.5 mm over distances greater than several meters.

Edited September 19, 2019 by MikeSharp01

[Conor]

3 hours ago, MikeSharp01 said:

I wish I had paid more attention in Metrology lectures way back in the 70s! So which of these is a class 1 tape then?

Actually 2 of them are class one yet they do not agree over 8m at any rate. The third is a cheap plastic tape and it clearly under reads the other two - the essential question is which one is correct?

 

The yellow one is my everyday tape but only 8m long as you can see. The thin white one is 20m but still class 1. The other cost me £7 in wicks a fews years back. 

 

So as a rule they are pretty useless when measuring is shared across different tapes. Conclusion - using 1 tape for everything so that all the errors will be consistent at least.

 

 

Temperature can be a significant factor when it comes to metal tapes. Do that again in 3 months time when it's -5c and you'll find the metal tape will be shorter.

[jack]

19 minutes ago, MikeSharp01 said:

Trouble is 1m is not really enough @jack has the right idea with the outside length - but how did they know their length was correct

 

They didn't. I suspect it was more to make people aware of the potential issue than to act as a calibrating standard.

 

The main issue with typical metal tapes for me is that bit on the end that slides slightly to account for whether the end of the tape is being pushed into, or pulled away from, whatever's being measured. I just always assume there's a couple of mm error and try to make sure that an error of that magnitude doesn't impact what I'm doing.

 

One example is where the absolute measurement isn't as critical as making sure it's replicated properly (ie, precision is more important than accuracy). An example would be making legs for a workbench. Best to cut all legs at once, or at least use the same physical template for each of them. Doesn't matter if they're all 5mm shorter or longer than the design (which is slightly arbitrary), as long as they're the same.

There's something called a "story stick" in woodworking that's a nice development of this idea.

[MikeSharp01]

10 minutes ago, Conor said:

Temperature can be a significant factor when it comes to metal tapes. Do that again in 3 months time when it's -5c and you'll find the metal tape will be shorter.

Agreed but both were at the same temperature here and they should expand and shrink almost the same as they are the same material and from the same manufacturer! I will of course have the slight challenge of what to check them against that is not itself affected by the heat.

 

[Jeremy Harris]

Shouldn't be too hard to make an accurate light/laser distance measuring system as a reference.  Many years ago I made a system for measuring distance when cave surveying (before laser distance measures were affordable) that used a tiny red diode, intended for us in fibre optics, collimated through a lens to a near-parallel beam about 20mm in diameter, that was sent to a distant retro reflector (a big bike reflector) and then bounced back to a 50mm lens (taken from an old camera) that focussed the reflected light on to a fast photodiode.

 

I solved the problem of how to try and measure very short times of flight, buy just modulating the red LED with a 100 Hz sine wave.  Part of the light from this LED was sent via a small hole to another photodiode receiver.  Both photodiode signals were amplified through identical tuned amplifiers, centred on 100 Hz.  The outputs from both amplifiers went to a dual comparator, that was gated to produce a 100 Hz pulse train with a pulse width that was proportional to the phase shift between the transmitted and received light.  It was dead easy, even in the 1980s when I built this, to accurately measure the width of a pulse at this sort of frequency, with a high degree of precision.  The pulse width was directly proportional to distance, and pretty accurate out to the limit imposed by the power of the red LED.  I tried an IR LED, which gave a much greater range, but being invisible it made the instrument much harder to use in a cave.  The red LED provided a very visible indication when the beam was aligned with the retro reflector, lighting it up very clearly, and as both the transmitter lens and receiver lens were mounted in the same machined block of aluminium, wherever the transmitter pointed the receiver was also pointing.

 

Pity that red laser diodes weren't available then, as they would have massively improved performance.  As it was it made cave surveying a lot quicker, as I included an electronic inclinometer in the box (and old Mk46 torpedo attitude sensor), together with an ex-sonobuoy flux gate compass, and a data logging capability, so pressing one butting recorded bearing,  distance and elevation angle, which was all that needed.  We'd often record each survey leg twice, taking both a forward and backward set of measurements, and always survey in a closed loop if possible, so we could use the least-squares error correction method on the 3D data set to evenly spread all the errors and get the loop to close.  Heck of a lot easier than surveying with a handheld compass, inclinometer and fibron tape (which used to get really muddy).

[Bitpipe]

12 minutes ago, JSHarris said:

Shouldn't be too hard to make an accurate light/laser distance measuring system as a reference.  Many years ago I made a system for measuring distance when cave surveying (before laser distance measures were affordable) that used a tiny red diode, intended for us in fibre optics, collimated through a lens to a near-parallel beam about 20mm in diameter, that was sent to a distant retro reflector (a big bike reflector) and then bounced back to a 50mm lens (taken from an old camera) that focussed the reflected light on to a fast photodiode.

 

I solved the problem of how to try and measure very short times of flight, buy just modulating the red LED with a 100 Hz sine wave.  Part of the light from this LED was sent via a small hole to another photodiode receiver.  Both photodiode signals were amplified through identical tuned amplifiers, centred on 100 Hz.  The outputs from both amplifiers went to a dual comparator, that was gated to produce a 100 Hz pulse train with a pulse width that was proportional to the phase shift between the transmitted and received light.  It was dead easy, even in the 1980s when I built this, to accurately measure the width of a pulse at this sort of frequency, with a high degree of precision.  The pulse width was directly proportional to distance, and pretty accurate out to the limit imposed by the power of the red LED.  I tried an IR LED, which gave a much greater range, but being invisible it made the instrument much harder to use in a cave.  The red LED provided a very visible indication when the beam was aligned with the retro reflector, lighting it up very clearly, and as both the transmitter lens and receiver lens were mounted in the same machined block of aluminium, wherever the transmitter pointed the receiver was also pointing.

 

Pity that red laser diodes weren't available then, as they would have massively improved performance.  As it was it made cave surveying a lot quicker, as I included an electronic inclinometer in the box (and old Mk46 torpedo attitude sensor), together with an ex-sonobuoy flux gate compass, and a data logging capability, so pressing one butting recorded bearing,  distance and elevation angle, which was all that needed.  We'd often record each survey leg twice, taking both a forward and backward set of measurements, and always survey in a closed loop if possible, so we could use the least-squares error correction method on the 3D data set to evenly spread all the errors and get the loop to close.  Heck of a lot easier than surveying with a handheld compass, inclinometer and fibron tape (which used to get really muddy).

 

I just KNOW you have one of these in the garage... 

 

[Oz07]

I set out some joinery in a workshop once with a pattern making rule. It was only on something small as the rule was only around 300mm. What a cock up that was. Old boy was a pattern maker in his day, why he bought that rule into a bespoke joinery workshop ill never know.

[Declan52]

1 minute ago, Bitpipe said:

 

I just KNOW you have one of these in the garage... 

 

Guy down the road from me has one. Still smile every time I see it.

[Bitpipe]

Just now, Declan52 said:

Guy down the road from me has one. Still smile every time I see it.

 

But does it have the flux capacitor? 

 

And when will Dundonald get a DeLorean museum to rival the Titanic one in Belfast?

Edited September 19, 2019 by Bitpipe

[ToughButterCup]

2 minutes ago, Bitpipe said:

I just KNOW you have one of these in the garage... 

 

Right on that one mate. 

Plus  J was cross because a budgie pooed on his measuring tape once too often.  Reeeeeally cross.

[Declan52]

53 minutes ago, Bitpipe said:

 

But does it have the flux capacitor? 

 

And when will Dundonald get a DeLorean museum to rival the Titanic one in Belfast?

Nope the peelers took it out!!!

 

The film driven about John delorean is worth a watch.

[dpmiller]

3 hours ago, Bitpipe said:

 

But does it have the flux capacitor? 

 

And when will Dundonald get a DeLorean museum to rival the Titanic one in Belfast?

 

Why Dundonald? The factory was/is in Dunmurry... Now used by Montupet, producing automotive castings

[Simplysimon]

4 hours ago, jack said:

There's something called a "story stick" in woodworking that's a nice development of this idea.

when making multiples of an item, or one which isn't square or is curved, a workshop rod is drawn, 'a story stick'. this may be one which shows sections to position joints or full sized if a shaped item for the placing on and marking of joint positions.

[SteamyTea]

4 hours ago, Oz07 said:

I set out some joinery in a workshop once with a pattern making rule. It was only on something small as the rule was only around 300mm. What a cock up that was. Old boy was a pattern maker in his day

I worked at a model and pattern makers once (well twice).

Model makers would work to 5 thousands of an inch, pattern makers to the nearest quarter of an inch.

Cabinet makers never did any work, they just sharpened their own chisels all day, and moaned.

[Oz07]

8 hours ago, SteamyTea said:

I worked at a model and pattern makers once (well twice).

Model makers would work to 5 thousands of an inch, pattern makers to the nearest quarter of an inch.

Cabinet makers never did any work, they just sharpened their own chisels all day, and moaned.

It's the rulers which allowed for metal expansion/contraction which my beef is with!