Bourdon Tube Tip Travel

Connect a felt tipped pen to the tip of the tube and have it draw on a piece of paper!

The best plan would be to use a co-ordinate measuring system to get the co-ordinates of the free end of the tube, holding the pressures constant for different pressures. If you have no access to a measuring system you can use a milling machine to make the X, Y, measurements. I had done this many years back---- not to measure the motion of a Bourdon tube of course, but to check the dimensions of components obtained from elsewhere.

Not a bad idea. It would definitely work to give the coordinate information I'm looking for but I worry for the repeatability of this form of measurement. Not repeatability in that it will be off a bit next time... but in that some one might need to use the milling machine

I should note, and this goes for a few of the other replies also, that we're hoping, if I'm successful in accurate data, to use this with Quality and Manufacturing to test incoming Bourdon tubes for acceptable deflection. Usually we don't find out until the gauge is assembled and then we've wasted material AND labor. So I think if we take the basic idea of the milling machine and whittle it down into a 2-dimensional micrometer-like idea then it could work, though may be more complicated than is need.

Thanks for the reply!

Seeing as a bourdon tube is always operated over its linear range - so giving a nice linear scale reading.

I don't see why you would need to empirically test the tube?? It will follow a linear path until you over stress it and then it will be permanently deformed.

Just look at the gearing used and the scale markings and write them down!!

John.

Yes, a bourdon tube is linear in deflection-scale (if you understand what I mean) but it's far from linear in the direction it takes while deflecting. I was fooled by the FEA software at first because I used a linear FEA solver and it gave a linear path of travel. Then I used a non-linear FEA solver and it showed a much more accurate curve.

I'm not sure I know what you mean by gearing and scale markings though...

Thanks for the reply!

Oh I was referring to the typical use of a bourdon tube in a pressure gauge.

The gearing inside and the linear scale markings show just how much the end of the bourdon tube is magnified up to a linear scaling of pressure applied.

John.

Oh ok, I see what you're saying. But there are linkages and adjustments inside the movement of the gauge that can diminish the effect of the tube displacement to make sure the gearing moves at the correct scale. So that will essentially tell me rotary displacement of the last gear, but not necessarily the tube.

I appreciate that you do want to verify the FEA results. It is the right way.

Now with respect to the measurement it should be done without contact since any contact will introduce resistant forces and influence the results. The method would depend on the uncertainty you want to reach. An optical method with triangulation devices (used for many measurements in labs or industrial environment) will be quite precise.

If you can accept a slight force it is possible to use 2 LVDT spring preloaded with a flat plate maintained in contact with a ball or cylinder bonded to the tube end at 90° from each other.

The problem you are confronted with is the fact that the tube end has not only a displacement but also a rotation so that the target you use for the measurement and the type of device has to consider this aspect as well.

I doubt that the trajectory is a straight line in the x-y plane.

Use shadow technology! Put a beam of light in front of a known piece and place a piece of glass with paper taped to the outside surface behind! Mark the paper with a pen on the known length. This is your scale. Replace the object of known size with the Bordon tube at the same test distance and as the tube does its thing, you plot the tips shadow on the paper! Thus you can play with the scale of the final plot by changing the distance between the lamp and the tube (make sure to do a scale with the known object) giving you more magnification!

This idea was thrown around in the beginning but we weren't too certain about the accuracy/precision of the measurements this would provide or the repeatability of the experiment. Though it would definitely give a nice visual plot. I like the procedure you have mapped out, that's very helpful.

Thanks for the reply!

You can place a simple go/no-go shadow trace template on the glass to make the operation almost fool proof as long as you have a fixed datum or reference point! A Shadow graph with a digital readout is quite accurate! It all depends on what you call accurate! You can get 100x magnification on most shadow graphs. Another option would be to use the same style setup they use to film and Analise crash test dummies! Put a reference point on the tip and use a good quality computer camera to time lapse the cycle and trace the reference point! High magnification lens close up.

I completely agree with the non-contact approach. The contact sensor idea has been shot down already for that exact reason but I've kept it on the table as a last resort. By using 1 LVDT for one coordinate by measuring the movement of a precision linear bearing guide pulled by a linkage attached to the tube tip. The linkage is connected to a rotary potentiometer which is clamped to the top of the precision linear bearing. That gives, with a few calculations, us information needed to calculate the second coordinate measurements.

We've been looking at the laser displacement sensors (the triangulation devices you recommend) and, while a little bit more expensive, they are very appealing (and they look really cool too). To maintain a constant reflective surface for the 2 sensors needed for the measurements, I've designed a lightweight anchor-of-sorts to loosely dangle from the tip if the bourdon tube is mounted vertically and allowing gravity to maintain the orthogonal surfaces. The problem with that would be sudden movements, vibrations, and swinging of the anchor, but we may need to live with that or just let the anchor settle before reading. I'm glad you came up with this too.

Thanks for replying!

Attention at the bearing of the "anchor" its friction has to be very low.

Since you use gravity the "mass" has to be far away from the bearing to reduce the angular error due to friction.

Thanks, I'm very much aware of that. It's an interesting balancing act between keeping the COG away from the axis of rotation AND also keeping the total mass low enough to have negligible affect on the tip displacement. If we can get the 3D printer in soon, I can make it out of ABS plastic and perhaps cover the sides and bottom with metallic tape to weigh it down lower as well as give it a reflective surface. At least that would be the easy way out.

Buy a guage and take it apart. The bourdon tube assembly is complete and the housing is only for appearance and protection. You can pressurize the exposed bourdon tube assembly and measure the displacement or movement of the bourdon tube. Tom

Thanks for the reply!

I guess I should have mentioned in the original post that I work for a company that makes pressure gauges. We have a supplier that sells us the tubes, so the company's expertise is in the movement of the gauge, not the tube. The company's gone through a lot of changes and threw out the paperwork involving all analysis of the pressure gauge line (tube, movement, all of it. Yay management!) so I'm working to rewrite it. I'm a coop at the company til the end of August, so that's why I called it a project.

So I already know how to get it to move and I have plenty of gauges, but obtaining the data is the challenge.

Very few people has worked on TIP TRAVEL ANALYSIS of bourdon tube. It has lot many asumptons in modeling.

I would like to see accuracy of Bordon tube design and compare with real output.

Good subject though.

The simulation with FEA has advantages and limitations. It would be good to make a mechanical computation model considering the tube as a curved beam and compute with classical equations the displacements. This has the advantage to make very fast the optimization of the different dimensions for a given case and then with a FEA model refine the values. As for the measurement I would consider for very low pressures a non contact solution with optical means since the forces can have an impact on the results and a for instance a LVDT for higher pressures for which the stiffness of the tube makes it less sensitive to the contact forces (springs). If you have difficulties with the curved beam model I can give you some support.