Understanding and using bubble level readings.

Demon69 · Feb 1, 2016

I dont know about you guys but sometimes I get those brain lock times interpreting a bubble level readings.
The main question is how do you interpret a string of point to point readings? As in how do record them and what do they mean to you? Do you work in angles or the rise and fall the level in each position. Am I the only one who ends up staring at numbers and just goes blank? lol

If we take 250mm 0.05mm in 1 meter as the example. Ive got that as a 10 seconds ish.

Pictures welcome, sometimes struggle with just numbers.

Demon69 · Feb 1, 2016

If you were looking at these readings. How would you determine the dimension h? Does what ive done look about right or is there an easier way?

Demon69 · Feb 1, 2016

How would you about recording these point to point readings, and what might they you about the surface being measured?

Heavey Metal · Feb 1, 2016

A level is a go no go Gage not a measuring device.

dcsipo · Feb 1, 2016

Demon73 said:
How would you about recording these point to point readings, and what might they you about the surface being measured?

just hope Pete wont jump on this one

...I am not certain what I am looking at. The problem is that the only thing you know is that between the two ends of the level you have about 10 arc sec of tilt change for every division move. It is unknown if the left went up or the right went down, the only thing you can do is to record the angle of the 15" line relative to level. Lets say you start at the edge of the surface and you are 0 degrees, i.e. level. so then you move over to the right 6" and now you are at -10 arcsec then move another 6" and at +5 (if you trust your eyes) then another 6" back to 0 and then -10 and so on. what i mean is the you cannot rely on the elevation number you know at the end of the level, you have to back calculate the intersections from the points of the center of the level with the angle of the tilt.

If you look at the diagram above also notice the in some instances two level positions may not even be at the same height

.

dee
;-D

dcsipo · Feb 1, 2016

Heavey Metal said:
A level is a go no go Gage not a measuring device.

it is a measuring device,it measures angles relative to level.

US Federal Spec A-A-50685:

1. COMMERCIAL ITEM DESCRIPTION LEVEL, PRECISION MASTER, SINGLE FACE
The General Services Administration has authorized the use of this commercial item description as a replacement for Type IX, class 1, single face, precision master level in superseded specification GGG-L-211C.
1.1 This commercial item description covers a single face, precision master level used to calibrate other levels.
2. REQUIREMENTS.
2.1 The item shall be made of steel or close grain cast iron of a design to preclude warping or deflection due to temperature change. It shall also consist of a single faced stock, one adjustable level vial, one f~ed cross test vial, a vial holder, a cover and a case. The vial shall have a cover or top plate of heavy nonferrous metal or plastic to shield vials from heat of operator's hand. It shall be 15 inches long and have vial accuracy of 0.0005 inches per foot of single faced stock. Concavity and convexity of single faced stock shall not exceed 0.0002 inches per foot. The vial shall be a glass tube with a vial
sensitivity of 10 seconds, maximum. The vial shall have no less than three
auxiliary lines, permanently marked, spaced l/10 inch apart on both sides of both bubble defining lines. The angular value of these lines in either degrees or inches per foot shall be permanently marked on the level.
2.2 The level, including all parts and accessories, shall be free from burrs, rust, external sharp or rough edges, imperfections, and any other defects which may impair serviceability, durability, appearance, or operational performance.
3. QUALITY ASSURANCE.
3.1 ~ The contractor shall certify that the product offered meets the salient characteristics of this description and that the product conforms to the producer's own drawings, specifications, standards and quality assurance practices, and is the same product offered for sale in the commercial marketplace. The government reserves the right to require proof of
such conformance prior to first delivery and thereafter as maybe otherwise provided for under the provisions of the contract.

It lives in this pile of GSA specs....

Document Number Document Title FSC P reparing Activity Date Superseded By Document Status
GGG-B-461A Blocks, Step, Machinists' 5220 AR 17-Aug-1966 Active
GGG-G-61 Gages, Plug and Ring, Plain and Thread (Inactive) 5220 AR 23-Aug-1932 Active
GGG-P-61A(2) Parallels; Steel and Cast-Iron, Precision-Ground 5220 84 6-Jul-1966 Active
GGG-P-441A Plates, Angle, Adjustable and Solid 5220 PA 9-Apr-1965 Active
GGG-P-453A(1) Plate, Surface, Cast Iron 5220 99 8-Apr-1965 Active
GGG-P-463C(1) Plate, Surface, (Granite) (Inch and Metric) 5220 NBS 10-Sep-1973 Active
GGG-B-121B Bar, Sine; Plate Sine (and Fixtures) 5220 AR 4-Jun-1964 Active
A-A-58089 Comparator, Bench Type, Electronically Amplified 5220 84 10-Dec-1996 Active
A-A-59120 Gauge, Surface Plate Flatness Checking 5220 84 22-Sep-1997 Active
A-A-50767 Analyzer, Surface Finish 5220 IS 15-Aug-1983 Active
A-A-50685 Level, Precision Master, Single Face 5220 IS 8-Jul-1989 Active
A-A-59572 Gage, Comparator, Electric 5220 84 29-Mar-2004 Active

RC99 · Feb 1, 2016

Demon73 said:
How would you about recording these point to point readings, and what might they you about the surface being measured?

Your diagram is incorrect to start with. Between 8 points you only get 7 readings. Also you have not leveled the machine, at least one section should read level.

Anyway lets move the level diagram half way to the left, between the numbers and all a 0 point.

so point one relative to point 0 is dropping two divisions, point two to point one is dropping the same amount, point three to two, dropping one and a half, and so on and so on.

Then you can plot that on a graph.

it would look like, down, down, still down but not so much, still down but not so much, still down but not so much, still down but not as much as the last three, steeper drop down, less steeper drop down.

Mark Rand · Feb 1, 2016

Peruse this article by J.C Moody. It relates to autocollimators, but the principle is the same. You're measuring in units of 50 microns/metre per division or 10.3"/devision, with each reading relative to the last one. Cranking it though a laborious series of worksheets like Moody does will integrate the changes in angle into a map of height changes. If you have access to an electronic spreadsheet on a computing machine, you can automate a lot of it. Moody had pen and paper...

Hope that helps somewhat.

PS:- do it in the morning or your head'll explode.

TGTool · Feb 1, 2016

Surveying does nominally the same thing with a level circuit. (A circuit means that you make a closed loop and your the last point is the same as the first. Same as double checking zero with an indicator. In surveying though, you have all the numbers in the field log and when you do all the adding and subtracting, if you don't come out to zero you know there's an error somewhere. If the math checks okay then there's an error in reading someplace.)

Comparatively, the level readings above would amount to an open loop, just like machine control. No error checking. So the mapping would be like surveying. Your first point is the origin, that is the end of the level at the end of the way or whatever. The first reading is the reading on the level telling you that the opposite end is +x or -x higher or lower than the origin. Note that. That's the difference in elevation between points 0 and 1. Move the level heel to toe and the second level reading is the third point referenced to the second. If you know what each division of the level represents PER LENGTH OF THE LEVEL in thousandths or millimeters, you can start mapping the surface on graph paper or whatever. It makes sense to me to use units of height since that's what I want to know about the surface I'm checking.

Heavey Metal · Feb 1, 2016

dcsipo said:
it is a measuring device,it measures angles relative to level.

US Federal Spec A-A-50685:

1. COMMERCIAL ITEM DESCRIPTION LEVEL, PRECISION MASTER, SINGLE FACE
The General Services Administration has authorized the use of this commercial item description as a replacement for Type IX, class 1, single face, precision master level in superseded specification GGG-L-211C.
1.1 This commercial item description covers a single face, precision master level used to calibrate other levels.
2. REQUIREMENTS.
2.1 The item shall be made of steel or close grain cast iron of a design to preclude warping or deflection due to temperature change. It shall also consist of a single faced stock, one adjustable level vial, one f~ed cross test vial, a vial holder, a cover and a case. The vial shall have a cover or top plate of heavy nonferrous metal or plastic to shield vials from heat of operator's hand. It shall be 15 inches long and have vial accuracy of 0.0005 inches per foot of single faced stock. Concavity and convexity of single faced stock shall not exceed 0.0002 inches per foot. The vial shall be a glass tube with a vial
sensitivity of 10 seconds, maximum. The vial shall have no less than three
auxiliary lines, permanently marked, spaced l/10 inch apart on both sides of both bubble defining lines. The angular value of these lines in either degrees or inches per foot shall be permanently marked on the level.
2.2 The level, including all parts and accessories, shall be free from burrs, rust, external sharp or rough edges, imperfections, and any other defects which may impair serviceability, durability, appearance, or operational performance.
3. QUALITY ASSURANCE.
3.1 ~ The contractor shall certify that the product offered meets the salient characteristics of this description and that the product conforms to the producer's own drawings, specifications, standards and quality assurance practices, and is the same product offered for sale in the commercial marketplace. The government reserves the right to require proof of
such conformance prior to first delivery and thereafter as maybe otherwise provided for under the provisions of the contract.

It lives in this pile of GSA specs....

Document Number Document Title FSC P reparing Activity Date Superseded By Document Status
GGG-B-461A Blocks, Step, Machinists' 5220 AR 17-Aug-1966 Active
GGG-G-61 Gages, Plug and Ring, Plain and Thread (Inactive) 5220 AR 23-Aug-1932 Active
GGG-P-61A(2) Parallels; Steel and Cast-Iron, Precision-Ground 5220 84 6-Jul-1966 Active
GGG-P-441A Plates, Angle, Adjustable and Solid 5220 PA 9-Apr-1965 Active
GGG-P-453A(1) Plate, Surface, Cast Iron 5220 99 8-Apr-1965 Active
GGG-P-463C(1) Plate, Surface, (Granite) (Inch and Metric) 5220 NBS 10-Sep-1973 Active
GGG-B-121B Bar, Sine; Plate Sine (and Fixtures) 5220 AR 4-Jun-1964 Active
A-A-58089 Comparator, Bench Type, Electronically Amplified 5220 84 10-Dec-1996 Active
A-A-59120 Gauge, Surface Plate Flatness Checking 5220 84 22-Sep-1997 Active
A-A-50767 Analyzer, Surface Finish 5220 IS 15-Aug-1983 Active
A-A-50685 Level, Precision Master, Single Face 5220 IS 8-Jul-1989 Active
A-A-59572 Gage, Comparator, Electric 5220 84 29-Mar-2004 Active

Wrong

Clue

The level rests on 3 points and there location is uncertain.

Demon69 · Feb 1, 2016

Thanks for the look in chaps. Is late now so will try and get some in tomorrow

Everyones welcome to comment, be good if we could leave the handbags at the door though.

Cheers all :cheers:

dcsipo · Feb 1, 2016

Heavey Metal said:
Wrong

Clue

The level rests on 3 points and there location is uncertain.

The level gives you the tilt of the main axis of its base as it rests on the three points, how is that not a measurement?
Based on your assessment a micrometer is just a tool to approximate a distance....all measurements have some degree of noise , all you are saying a level is a noisier measurement tool than say a auto collimator, or an em microscope or an optical comparator, but all those "measurement" tools are just approximations of what is really there, an agreed upon methodology to compare an object's attribute to the same attribute of another thing. A level measures the tilt of the major axis of its base in respect to gravity within the plane of the three points it touches on a surface. The goal is to push the three points out as far as possible to the extremes of the base of the level so the base and the plane of the three points measured will have the least amount of differential. A ground vial has good repeatability and accuracy, that is only limited by the errors in the curvature of the glass.

Even my thumb is a measuring device, the question is how much accuracy, and repeatability i demand.

dee
;-D

TGTool · Feb 1, 2016

Okay, first case in considering post #3. If the machine were leveled first and these are the readings as the level traverses the feature then they would show a continual slope down to the right, easing a little part of the way but never beginning to slope back up.

Second case. If we don't know that the machine was leveled, then the level could or should have been adjusted to center the bubble in the first location. Then it shows a slight rise towards the middle, then a slight dip and a moderate recovery at the end.

Assuming the first post information of .05mm/meter sensitivity and a 250mm long level, then for this level each division represents a rise or fall of .0125mm. The readings either show that the feature declines to the right by .159mm, or it shows that the center is high by about .006mm, we don't know which.

Pete F · Feb 1, 2016

For God's sake man, are you still going on about this? LET IT GO! :nutter:

When you first proposed this question, you showed a level in two positions and said that the difference in each position is 1 division. As others have quite rightly pointed out that one division will be from your datum, which should be level. You implied by the drawing that you were looking at a surface plate with a symmetrical error. So if you were to put the level perfectly in the centre of that hole so it was bearing on the ends, that would form your datum. You'd preferably grab the datum from somewhere else that wasn't worn, but let's just assume your whole plate is one big bowl and just run with that. It then goes up to position 1 and is 1 division out, one side. You then take the level and put it in position 2 and it will be 1 position out the other side. The difference between those positions is a total of 2 divisions. You've changed the diagram and story from that you first presented to now say the bubble went out half a tick each side, or a total of delta 1.

As others have said, the machine, plate, whatever should be levelled somewhere. You should be able to put a level on a point and it will read zero, that's your datum. You don't start with the whole lot on the piss and then just keep subtracting away until you run out of divisions on your level!

RC99 · Feb 2, 2016

The info you require about using a level to determine straightness is spelt out for a beginner in Michael Morgans book "Scraping for Beginners". I am not sure why there seems to be an embargo against it. It is a fantastic book for beginners, the best easy to read book on scraping you will find.

Demon69 · Feb 2, 2016

dcsipo said:
just hope Pete wont jump on this one ...I am not certain what I am looking at. The problem is that the only thing you know is that between the two ends of the level you have about 10 arc sec of tilt change for every division move. It is unknown if the left went up or the right went down, the only thing you can do is to record the angle of the 15" line relative to level. Lets say you start at the edge of the surface and you are 0 degrees, i.e. level. so then you move over to the right 6" and now you are at -10 arcsec then move another 6" and at +5 (if you trust your eyes) then another 6" back to 0 and then -10 and so on. what i mean is the you cannot rely on the elevation number you know at the end of the level, you have to back calculate the intersections from the points of the center of the level with the angle of the tilt.

View attachment 161936

If you look at the diagram above also notice the in some instances two level positions may not even be at the same height .

dee
;-D

Sorry dee, I could have made that clearer. The reason i dont like the idea of flat levels for this sort of thing is as in your drawing.

Demon69 · Feb 2, 2016

RC99 said:
Your diagram is incorrect to start with. Between 8 points you only get 7 readings. Also you have not leveled the machine, at least one section should read level.

Anyway lets move the level diagram half way to the left, between the numbers and all a 0 point.

so point one relative to point 0 is dropping two divisions, point two to point one is dropping the same amount, point three to two, dropping one and a half, and so on and so on.

Then you can plot that on a graph.

it would look like, down, down, still down but not so much, still down but not so much, still down but not so much, still down but not as much as the last three, steeper drop down, less steeper drop down.

8 refers to the number of level readings, i make it 9 points to measure from. Check image in previous post.
Re being level. For sure levels are used to 'level' things

. Im more interested in how can check, interpret and record results as to the condition of a surface.

Would be interested to see how you jot down the numbers and use them.

Demon69 · Feb 2, 2016

Mark Rand said:
Peruse this article by J.C Moody. It relates to autocollimators, but the principle is the same. You're measuring in units of 50 microns/metre per division or 10.3"/devision, with each reading relative to the last one. Cranking it though a laborious series of worksheets like Moody does will integrate the changes in angle into a map of height changes. If you have access to an electronic spreadsheet on a computing machine, you can automate a lot of it. Moody had pen and paper...

Hope that helps somewhat.

PS:- do it in the morning or your head'll explode.

Thats spot on mate. Que the brain ache! :willy_nilly:

TGTool said:
Surveying does nominally the same thing with a level circuit. (A circuit means that you make a closed loop and your the last point is the same as the first. Same as double checking zero with an indicator. In surveying though, you have all the numbers in the field log and when you do all the adding and subtracting, if you don't come out to zero you know there's an error somewhere. If the math checks okay then there's an error in reading someplace.)

Comparatively, the level readings above would amount to an open loop, just like machine control. No error checking. So the mapping would be like surveying. Your first point is the origin, that is the end of the level at the end of the way or whatever. The first reading is the reading on the level telling you that the opposite end is +x or -x higher or lower than the origin. Note that. That's the difference in elevation between points 0 and 1. Move the level heel to toe and the second level reading is the third point referenced to the second. If you know what each division of the level represents PER LENGTH OF THE LEVEL in thousandths or millimeters, you can start mapping the surface on graph paper or whatever. It makes sense to me to use units of height since that's what I want to know about the surface I'm checking.

Thanks TG

RC99 · Feb 2, 2016

On a 10 second between division vial, that has the contact points 250mm apart, a single division represents 0.012mm.

If it was a 200mm between points then that would be ~0.01mm(just under actually)...

Depending on what end of the bubble we are looking at as that is not mentioned, but lets say the dot represents the left side of the bubble then:

If we class the first reading as 0-0 on a 200mm between contact points 10 second level

second reading, still 0

third reading depending on what end of the bubble we are looking at as that is not mentioned, but lets say the dot represents the left side of the bubble then it has dropped down approx 5 microns.

fourth reading, dropped down at the same angle do down another 5 microns, so we are at 0.01mm below the first reading now.

fifth reading, still the same dropped another 5 microns, now 0.015mm below first reading.

sixth reading now dropped a full 0.01mm, so 0.025mm lower then first reading

seventh reading it has levelled off but still 0.025mm below first.

eighth reading dropped another 0.01mm again so 0.035mm below first reading.

Clive603 · Feb 2, 2016

Classic winding yourself into confusion by overthinking. Get yourself a decent clinometer and have a play with it. That way you can directly convert a bubble shift into an angle and, if its like mine pictured in the attachment below, derive the actual distance moved to create the tilt. No you can't have mine but if you are ever in the area you can come and have a play!

dcsipo
An ordinary level isn't really a measuring device. High quality ones can be used for calibration purposes under specific conditions but that isn't the same thing. Reversing it and deriving the offset from true zero via two opposed readings allows some accuracy and repeatability but the whole process is more than little unstable.

Clive

Understanding and using bubble level readings.

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