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Where to get a PRECISION 1m rule

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LAN_403

Sulaiman

Wed Oct 05 2016, 09:54AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

My plans for a primary reference mercury manometer & barometer are progressing,
I now need a ruler/measure that I can be sure is accurate to 0.1mm in 760mm ....
markings/precision of 1mm or 0.5mm sufficient.

Looking at eBay I could not find a ruler with the guaranteed accuracy required,
any ideas ?

Carbon_Rod

Wed Oct 05 2016, 11:08AM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155

Starrett Vernier 72" Callipers
model 123Z-72

About $2k used...

A local Machinist could also cut a steel reference for you.

Sulaiman

Wed Oct 05 2016, 11:55AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

Carbon_Rod wrote ...

Starrett Vernier 72" Callipers
model 123Z-72

About $2k used...

anything about 100x cheaper ?

I only need measurement precision (scale markings) of 0.5 or 1mm
but I want that precision to be accurate to 0.1mm ... if possible on a budget :)

klugesmith

Wed Oct 05 2016, 12:44PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Looking forward to seeing pictures of your project.

If it's for measuring ambient air pressure, then the marked scale doesn't need to be 760 mm long. You could use a short metal ruler permanently fixed to a long bar, with one mark at the far end. You would need to get it calibrated once.

A 760 mm stainless steel bar elongates 0.1 mm for every 8 Â°C. Maybe you can choose a bar material that will compensate for the thermal expansion (density change) of the mercury column.

Who knows the tempco of common digital readouts on milling machines?

Rich

Sulaiman

Wed Oct 05 2016, 01:12PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

I want absolute pressure measurement over -1 atm to +1 atm using mercury in a U-tube as an absolute reference
as a barometer I will use my vacuum pump for zero pressure reference (<0.01 mm Hg abs.)
(with compensation for scale length and mercury density vs. temperature, standard corrections)

based on a discussion elsewhere, I just measured my 1m steel ruler (KDS Japan) using a cheap Chinese digital caliper,
I set the calipers to 100.00mm and within the limits of my eyesight, the ruler and calipers agree !! :)
good enough for now ... one day I may come across something better

thanks for the thoughts

klugesmith

Wed Oct 05 2016, 01:19PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Oh, and don't forget to look up your local gravity. The same pressure that supports 760 mm Hg at latitude 45Â° will support 762 mm at the equator and 758 mm at the poles. Smaller variations come from local earth composition, elevation, and the tide.

I learned that the torr is defined as 1/760 of 101.325 kPa. Once upon a time, did the exercise of backing out "standard" gravity to see if the standard torr matches the literature density of Hg at 0 Â°C or 20 Â°C.

Sulaiman

Wed Oct 05 2016, 01:42PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

yes, I forgot to mention gravity (but did not actually forget it) ... thanks.
initially I will use lattitude, longtitude and altitude to estimate g
(9.8126 is my current estimate)

I may re-distill the mercury, and measure its density, just to be confident
I bought it via eBay as 'double distilled', but who knows?)

To measure g I may make something like a 'seconds pendulum'

now that I know that my ruler and calipers seem accurate enough (not sure if my metalworking is up to it though )
actually more difficult to achieve effective pendulum length than constructing a barometer :)

Hazmatt_(The Underdog)

Thu Oct 06 2016, 01:13AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

You only need about 25 cm, or 1 foot, because you're really measuring down from the sealed end of the manometer.

Sulaiman

Thu Oct 06 2016, 02:06AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

my manometer/barometer is of the U-tube type
this is to cancel meniscus depression errors
and provide an absolute measurement ... no callibration required.
So I need to measure from one meniscus to the other.

I could measure from the zero line (same pressure each side) to either meniscus then double that
but as a barometer it would need to be 'zeroed' before each measurement ... inconvenient,
plus the zero changes with temperature (glass and mercury thermal expansion)

Single tube barometers (e.g. Fortin type) need to be calibrated against a calibrated barometer.

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