FSR Sensor

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yo1dog
 
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FSR Sensor

Post by yo1dog »

I am working on a project for my kegerator. One of the objects is to be able to tell how much is left in the keg at any time. The obvious solution to me was to weigh it and compare it's current weight to the full and empty weight of the keg (Cw - Ew)/(Fw - Ew).

My first thought was to use a digital scale but then I read that leaving large weights (~160lbs) on the scales for long periods of times (~2 months) can ruin them.

Adafruit sells "Force-Sensitive Resistor (FSR) - Interlink 402" and I was wondering if I could use these. Can three of these semi-accurately weigh from ~20lbs - ~160lbs?

If not, what are some other solutions?

Thanks,
- Mike

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adafruit_support_bill
 
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Re: FSR Sensor

Post by adafruit_support_bill »

FSRs are not really capable of precision measurement and do not hold up well to prolonged loads.

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yo1dog
 
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Re: FSR Sensor

Post by yo1dog »

Well that answers that. Any got another solution?

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adafruit_support_bill
 
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Re: FSR Sensor

Post by adafruit_support_bill »

Strain-gauge based load-cells are pretty much the standard for industrial weighing applications. Most digital scales are strain-gauge based.

Another approach would be some sort of liquid level sensor in the keg. But that introduces a whole other set of problems.

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yo1dog
 
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Re: FSR Sensor

Post by yo1dog »

But won't keeping a large amount of weight on it for up 2 months be harmful to the digital scale?

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adafruit_support_bill
 
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Re: FSR Sensor

Post by adafruit_support_bill »

If you load the strain-gauge beyond the yield-strength of the material it is made from, there will be a permanent deformation and accuracy will suffer. If you choose one designed for the weight range you will be measuring, there should be no problem.

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zener
 
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Re: FSR Sensor

Post by zener »

Either a strain guage as was said, or keep track of the flow time/rate. Either flow sensor or put a switch on the tap if the flow is fairly uniform. Or if you are pouring 12 Oz beers, put a dollar in a jar for each beer. When you have $165 you are empty.

meseta
 
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Re: FSR Sensor

Post by meseta »

A while ago, there was a nice writeup of an ultrasound based liquid level sensor. Apparently the water-air interface reflects ultrasound well enough, so this seems like a minimally invasive and dry way to directly measure liquid level if you're not adverse to cutting a couple of holes in the top of the keg: http://www.open-electronics.org/water-t ... h-arduino/

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zener
 
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Re: FSR Sensor

Post by zener »

There are obvious problems with cutting a hole in a keg of beer. That is not a practical solution.

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abqlewis
 
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Re: FSR Sensor

Post by abqlewis »

I'm just brainstorming here (because there is beer involved) so don't beat these up too bad.

My first (and most reasonable) thought is to spring mount the keg. Either hanging or platform, and use some reasonably hefty springs. Now you're just measuring a distance based on weight, so you could use something like the ultrasonic sensor without cutting a hole. The key is that your sensor isn't under constant strain.
Another thought is to use 10 or more strain gauges or FSRs, and sum up the results. They will still be under constant, although much lighter, stress so I don't know if this is really better.
A more outlandish and experimental idea, based on estimating LP tank levels: Ping - a small metal object pinging the keg will produce a different tone based on the amount of liquid. Ping the keg (with a servo maybe), measure the tone (maybe with a Piezo), and this might just indicate how much liquid is inside.

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adafruit_support_bill
 
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Re: FSR Sensor

Post by adafruit_support_bill »

Here's a source of reasonably priced strain gauges. I have used the smaller one to monitor the fill level of a wine bottle. :wink: One of the larger ones should handle your keg. This is a good tutorial on interfacing one to an Arduino.

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yo1dog
 
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Re: FSR Sensor

Post by yo1dog »

Thanks for all the good info!

If I go the load cell direction it will run me around $140 (for 3 30kg) per keg. I was hoping to spend less than half this, however, this does seem like the best option.

I have heard of the "Pinging" idea before. Any thoughts on this?

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zener
 
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Re: FSR Sensor

Post by zener »

Maybe you could hack something like this:

http://www.ebay.com/itm/Saga-New-330lb- ... 2314be103a

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yo1dog
 
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Re: FSR Sensor

Post by yo1dog »

I am actually about to head over to walmart and pick up a cheap bathroom scale, mostly to see if I can get it working with Arduino.

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Re: FSR Sensor

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