# Pressure Sensing Mat- Help Needed Please.

## Main Question or Discussion Point

Do you think that a mat could be made as sensitive as a barometer?

My idea is designing a inflatable mat that could determine weight of say 10grams placed on it and display an accurate reading of the weight shown on a LCD panel.

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You could do that. But why not just use a force gauge?

The idea using a mat with air inside was of more interest to me. If it can be done, can anyone please point me in the right direction?

First figure out the requirements of the scale. How much weight can it hold, how accurate does it have to be, etc. Next I would start looking at the physical design of the bag or what ever it is your going to be inflating with what ever gas.

berkeman
Mentor
Do you think that a mat could be made as sensitive as a barometer?

My idea is designing a inflatable mat that could determine weight of say 10grams placed on it and display an accurate reading of the weight shown on a LCD panel.

Many times in product design, you need to look at the overall thing you want to do, and consider how to optimize the accuracy and sensitivity and cost and whatever else is important, subject to the constraints you have for the design. As an engineer, you should always be striving to optimize your designs in whatever ways are important. To come up with a sub-optimum design is generally a bad thing. You have the computational tools, generally, to calculate things through, so doing lots of thought experiments and paper designs is part of what we do....

So in this project, you need to ask yourself what the accuracy constraints and concerns would be with using an inflatable mat to measure weight via pressure changes. What is the pressure change inside your mat for a 10g weight being added on top of the mat? What kind of accuracy, resolution, and pressure ranges are available with off-the-shelf pressure transducers that you might use in your project? What kind of preamp would be appropriate, and what would you expect your signal-to-noise ratio (SNR) to be with that 10g weight that you are trying to measure?

Now think of other ways to do the same weighing funtion. How do the various types of bathroom scales work? They are not optimized for small weights, however. But you should at least calculate what the SNR is for light weights for your inflatable scale versus some other options, like a 4-footed rigid platform, with strain gauges at each of the feet (or 3 feet might be better, if you loads allow -- Quiz Question -- why would 3 feet give you a better SNR than 4?).

If you are measuring only light weights of a limited weight and size, then maybe something more like a spring balance mechanism, with one spring, one hinge, and an optical encoder for displacement might be better still...

Think about what you really want to do, and then spend some time calculating through the various transducer and measurement options. Try to be as quantitative as possible, with an end goal of something like bst SNR versus size/cost/convenience, etc.

vk6kro
Maybe something on the following page would help:

http://www.dealextreme.com/search.dx/search.scales

There is an amazing range of digital scales there.
For example, the following scale that reads 0 to 20 grams to a precision of 0.001 grams:

http://www.dealextreme.com/details.dx/sku.10515

Costs US$23.57 delivered. Or this one: http://www.dealextreme.com/details.dx/sku.15002 100 grams at 0.01 gram resolution..... US$12.27 delivered.

Not much point in trying to make one yourself at those prices.

Very honest store too.

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It’s become more than obsession of mine now to make this work if it can. I’m not sure if a product like this would have its use or not, never the less, I would still like to mat it if possible.

Berkeman:
“What is the pressure change inside your mat for a 10g weight being added on top of the mat?”

I have asked this question before on this forum, but I still have no idea, if any of you could help answer it I would appreciate it very much. The mat size is 12” x 12” and 2” thick when inflated. Can anyone here help me work out the pressure difference when 10grams is applied to mat of air?

I understand I need some sort of digital reading from the mat when the weight is applied to it, I can work on this more when I know what type of pressure sensor I will be using.
Thanks again for all your input.

vk6kro
I guess the problem would be that the inflated mat would change pressure because of temperature and barometric pressure a lot more than by putting 10 grams on it.

The idea is a non starter, I think, but if you wanted to continue with it, you could connect the output of the mat to a U- Tube with water in it via a short hose and try to see the change in displacement with different loads on it.

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Lets say that the mat would only be used for 1-2 minutes at any one time and we were to use a guage type pressure sensor, this would automatically calibrate the barometric pressure there we should get an accurate representation of the 10g on the mat. The idea of using a U tube would work to a degree, but i would like to use something more accurate that could give a digital output. Any suggestions?

Thanks again all.

Suppose you had a mat that is 12" by 12", and filled to 1 atm =14 psi = 100kPa (kilopascals) = 100,000 newtons per square meter. The total downward force of the air on the mat is 9300 Newtons. This is of course balanced by the inflation of the mat. Now place 10 grams = 0.01 Kg = 0.098 Newtons force on the mat. The change in the downward force on the mat is about 1 part in 10^5. If the mat were rigid (non-flexible), the pressure inside the mat would rise that much. I suggest you get some toy balloons and experiment.