need help in designing a contact pressure sensor.

I think you don't want to measure the pressure as they're pulled apart but rather the pressure of their being forced together. If I had to do it, I'd put force sensors on the bolts (maybe 6) holding the flanges together and, in the electronics, sum the forces and divide by the area. You could use force washers for that and you could even guarantee some uniformity, always a good idea for flanges.

First question is which the distance between the flanges?

In fact you cannot measure the real pressure if your transducer does not cover the whole contact surface. If pressures are not important then it is possible to use the volumetric variation of resistivity of some materials called the Bridgman effect (in fact resistors same efect as in carbon microphones). Usual strain gages have a low factor and are not an optimal choice. This effect is used by different manufacturers for area pressure sensitive sensors. An other possibility is to use the radial deformation under an axial load (Poisson's effect) and use strain gages for sensing it. The Bridgman effect is used for pressure transducers in very high pressure environments as several tausend bar.

You can get pressure sensitive films which are designed to do exactly what you want such as this or this, I'm sure I've also seen a version which gives an electrical output so that the pressure map can be displayed on a PC but I can't track that down at the moment.

If your project however is to design a sensor of your own, this will not be much help.

Hello Nigh,

I had read these comments a while ago, and realized that I know a company that manufactures this film you were looking for, it just came to me, here it is.

Sensor Products Inc.

300 Madison Ave.

Madison, NJ 07940 USA

Tel: 1-973-884-1755

http://www.sensorprod.com/index.php

All the best, Mirco.

That's the stuff.

The idea of placing anything on the flange face is a bad idea. Proper installation of the flange faces requires a good metal to metal seal. Normally, a precise torque tool is used to connect the two flange faces together-applying the proper torque to each bolt diametrically opposed. If the flanges connect piping or other pressure containing lines, and you want to know if the seal leaks, put a pressure transducer in the line upstream-then if you see a pressure loss if the flange seal fails. One will find a host of pressure transducers and guages in the market that have alarm settings to alert you when there is a pressure loss.

Michael Leiwig

Oil States Industries Inc.

"The idea of placing anything on the flange face is a bad idea"

Agreed but these indicating films are not designed to be permanently installed. You would cut a 'gasket' from the film to fit between the flanges, bolt the flanges up then dissasemble the whole thing to see the result.

Since you are a university student, the first thing to understand is that you will not be measuring pressure in your project. That is only what you will present after conversion to the customer.

Since you are limited to strain gauge, your most obvious choice would be the wheatstone bridge configuration. There is error in this choice but it is the cheapest and easiest. The higher the amount of force the more accurate it will be. The other 2 choices would be more difficult to get a hold of and work with: PRT and Anderson loop. I've personally contacted Dr. Anderson in the past concerning a project I was working on for detecting the weight in a car seat. He was very willing to share his idea which was used extensively at Edwards AFB at the time for measuring the strain to a wing of an airplane.

Your idea on metal strip is good, but will probably require testing to determine best location. Remember the output from the strain gauge will be minimal. In most sensors I've developed using capacitive and Piezo Resistive technologies, the output range will come out around 0.1 to 0.2v. This will then need to be amplified into a more usable range, generally 0.5 to 4.5v and then for those who only think in amperage converted to 4 to 20ma to become a transmitter. At this point based on the material and area you use to sense with, these values will need to be correlated to pressure to give you your pressure output. Good luck.

I am sure most of readers will criticize my comment but I think it has to be made.

I have the feeling that comments are made without reading and understanding the question.

The question is not to measure pressure in the system but the CONTACT pressure between flanges when they are tightened. Of course everybody has his own experience but I presume we have not to show what we know but try to make the choice from what we know which is as near as possible to needed answer to the question.

Personally I understood the question.

He is being required to 'design and test' a contact pressure sensor. Whether he attaches to a bolt, nut or strip of metal as he was thinking, it will be the testing he does that will show the best and most direct approach. The project is for him to learn, so there is no better way than for him to test his idea (if plausible and it is) and try to make it work.

Whether he is does it in a tensile or compression approach really isn't the issue. It is about detecting and measuring the strain and relating it to a pressure output one would assume considering he wasn't asked to design and test a strain gauge.

yea both you and nickname understood the question. however because there is little time and resources available here at UWI, i must have my final design at the end of the semester without and testing. in other words based entirely on theory. so thats why i wanted feedback on my metal strip idea.

another thing is it probably wont be a metal strip but a strip of some other material because it will be operating in industrial conditions( that is a flange joint in an industry). so temperature will play a factor. Or maybe it can be insulated.

Another thing is that although the strip will not be in contact of either of the flange faces but instead on the thickness of the flange, is it advisable to bond the strip directly onto the thickness of the flange?

Or should I consider to build like a ring that can go over the thickness and the strip will be bonded to this ring. Of course the ring will have to be attached to the flange but is not permanent. The only query with this is if the variations in the contact pressure or closeness of the flanges will be translated to a strain in the strip. ie would this system be sensitive enough?

thanks Sonave Sunsets u cleared up some grey areas in your last post. the part about signal amplification and stuff..

Lomas

As long as your strip is flexing in axis with the direction of force I would say you are okay with either the strip or ring. You might even want to incorporate part of nick names' idea of having multiple measurements. Depending on whether the flanges are clamped or bolted, the exertion of forces most likely will be uneven between sides thus taking multiple measurements and averaging the outputs would give you a more realistic result. That is great thing about the Anderson loop. It is essentially like a thermocouple where he uses tw0 dissimilar wires coupled at the end and bonded to a surface then connected to a modem-type chip that in parallel can take and process thousands of readings.

As far as temperature is concerned, I'm not sure what your limits are on the project, but must all pressures sensors made today use EEPROMS where you test the sensors at say -40C to 125C (automotive) and compensate for thermal drift. This is one of the main reasons many quit using the Wheatstone bridge for pressure sensing, but now with MEMS technology, they are etching the wheatstone bridge into a silicon wafer (PRT) and combining it with the programmable output making it very stable over temperature with very little drift.

If you are not able to use a metal strip w/ temp compensation then make sure the material you use maintains a good propagation of strain while providing good thermal characteristics.

For instance, ceramic tends to be very fragile but is widely used in non-contact pressure sensing applications specifically due to its very stable nature over temperature. Most thicker diaphragms tend to hold up to 4x their designed pressure range, but thinner ones specifically designed to detect less than 1psi can handle over 100x their designed pressure range. Depending on how direct you want to sensing element to absorb the force exerted (1:1 ratio as compared to say a 10:1 ratio) could help determine the type of material use.

Now I'm not saying to use ceramic but it wouldn't be a bad idea to check out its thermal and elastic characteristics and search for materials with similar attributes that could handle the contact.

Applying a thermal insulating coat could be an option as well especially if you use a wheatstone bridge as you sensing element. It is greatly affected by temp without compensation, but coating should be both thermally and electrically insulative.

Hoped this helped some.

Thanks for the help.

i have another question. i am using the strain gauge on the strip of metal. so the next step now is the provide a bill of materials for the university. i have decided to connect the strain gauge in a wheatstone bridge configuration with voltage excitation source to convert the change of resistance into a voltage signal. now i would like to know what type of equipment i would need. is there a handbook that i can get this info?... i know i need to amplify the signal, and do some conditioning, then after calibration, relate the voltage signal to the contact pressure and display this value.

any ideas what equipment i would need and where to look for the strain gauge?

i am thinkin once i settled on a the strain gauge, from its ratings, i can then get the other equipment ... is that correct?

Lomas

You will be able to find a supplier in your country by Googling strain gauge (or gage if you are in the USA). They will be able to help you with power supplies, amplifiers etc. I'm sure I remember from long ago when I was installing strain gauges that, if you connect them into a Wheatstone bridge, you extend one arm of the bridge to the indicating equipment to compensate for temperature changes in the connecting cable. I'll try to dig out the information or perhaps someone more knowledgeable about this will jump in to advise.

Let us be logical and analyse what is the need and how can we reach it.

What generates the CONTACT pressure between flanges?

From one side the total effect of tightening the bolts (which is as was mentioned NOT uniform due to different objective reasons I will not insist here about) and the traction force applied by the pressure in the system.

The best way to measure a force (pressure is a specific force) is to place the sensor in the force loop. If not possible to determine the value of the pressure by the influences it has on other directions (transversal for instance since one is obliged to use strain gages).

A disturbing effect is due to the friction on the surfaces which introduces a displacement constrain.

If a plate is placed between the 2 flanges with strain gages in radial and tangential directions the specific deformations in those 2 direction will be quantified and can be used to establish a relationship between contact pressure and system pressure.

When the system pressure is nil the signal is only depending on the contact pressure. If system pressure is increased the resulting pressure between flanges is reduced and thus the signal will decrease wit respect to the one with the plate (sensor) free before tightening the bolts.

The deformation will not be important so that strain gages with a higher gage factor must be used. Normal metal gages have a gage coefficient of é to 4.5, semiconductor gages can go as high as 150 but are limited in temperature and require a much more complex temp. compensation.

Due to the imprecision of flanges and non uniform stiffness of joints the plate should be metallic in order to be tough and accept local stress points which ceramic does not tolerate.

The use of FEA allows an optimization of the plate thickness which has to be such that the strain gages will not be loaded directly by the joint.

This sensor is in fact not a sensor which can be designed with the principles of pressure sensors as ceramic or mems. It is in fact a force sensor for a very specific geometry and application.

My name is Mike Ford. I'm an Application Specialist here at Sensor Products, Inc. in New Jersey. We specialize in solutions and technology to measure tactile surface pressure. Feel free to email me directly with any questions you have. mford@sensorprod.com. Our website is www.sensorprod.com

Hi Mike,

Nice to see someone from the sensor products group in the forum- Do you have a location / reseller here in Ontario Canada?

Mirco.