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Author Topic: Measuring the weight of a beehive  (Read 12558 times)
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Ontario
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my 200kg load cell have been standing on my kitchen bench with a constant load for over a week, and it is only drifting like +/- 20g (a little worse when the temperature changes, but not to bad).

Interesting, that's a good start.  It sounds more stable than I would have guessed.  What % of load capacity is on there?

How about outside in the weather?  For a year?

If you could demonstrate ~ +/- 1% of full scale accuracy, while sitting with 50% of rated load for 10 months across temperature changes of +30C to -20C, then I'll be convinced.
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Norway
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Hi
In my load-cell test I used a 20kg load. I am not sure if using a higher load will make a difference since it is the offset of the load-cell and not the gain which is causing the error.

For my next tests of the load-cell, I will probobly leave it outside for a month or two, logging the load and temperature. Maybe I can give you some better answers then smiley
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Ontario
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Load cell is actually a strain gauge attached to a beam, in such a way as to measure the elastic deformation of the beam under load.  A higher load will accelerate the effects of creep, which will be one of the modes of long term reading drift.  Affects like work-hardening, corrosion and fatigue will also be there.

My working assumption is that those effects are going to make long term readings from a constantly loaded load-cell drift in such a way that you can't really trust the reading you get, until you, at least, re-tare the load cell to a known load.  If you can prove I'm crazy, that would be great.
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Barcelona
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Very interesting project, would love to know how it has progressed!

I'm doing the very same thing, environmental monitoring of beehives, including weight. Just as you have, we've identified the weight to be the biggest issue. From what I've heard, constant weight higher than 10% on a load cell will strain it over time. I'm hoping to get to testing this within the next few weeks, but I don't know if testing will be done in time for this season.

On the electronic side of things I've ordered Moteino:s. They sell for $16, including radio at 434MHz, and should be Arduino-compatible. It will probably be a couple of weeks before they arrive, but I'll come back with some details once I've gotten to play with them a bit.

http://lowpowerlab.com/blog/2012/12/20/moteino-the-wireless-low-power-low-cost-arduino-clone/

I should mention that this will be my first "Arduino" project, very excited!  smiley-grin
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Hi Lars, what is going on in your project? I'm interested in hive scales also and did some research. Nice to see that cheap load cells seams to work. Do you have some new recommendations for the amplifier? 
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Norway
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Time for an update on my project smiley
lately I have been focusing on the wireless interface and power consumption of the sensor nodes. The ina333 op-amp workes great in my low voltage setup.
I have a prototype up and running on a breadboard(see the schematic)


To optimize the battery life, I am running a barebone AtMega328P on the internal 1Mhz clock, the bod level is set to 1.8v and is software-disabled while the processor is sleeping.
Also i am using a pnp-transistor to shut off the current to the loadcell and op-amp, so they only consume current a few milliseconds while sampling.
I am also using the Collector voltage of the transistor as my analog reference, so the voltagedrop of the transistor won't affect my measurements.
Since The Internal ocilator is'nt accurate enough, I connected a 32.768 khz crystal to the chip's clock-pins. This crystal feeds Timer2 and creates an accurate interupt every second. This way the the AtMega also works as an RTC, and I save the cost and powerconsuption of an external RTC.
All in all including the radio tranciver my crappy multimeter shows a current consumption of 2-5µA smiley-eek when everything except the 32khz crystal and timer2 is sleeping.

The wireless comunication was a real twist. I want the sensor nodes to sleep as much as possible, and there will be a lot of sensornodes trying to communicate with one "master-module".
The solution I foud, is to let the master-module decide when the sensor-node can start transmitting. So when a sensor-node is waking up from sleep, the master-module will already had started listening to that exactly node. Then when the sensor-node has sent it's data to the master-module, the master will respond by sending how long time to next datatransfer. The whole prosses of waking up the prosessor, wait for the op-amp to stabilize, take ten samples and calculate the average, sending the data, reciving the time to next datatransfer, and go back to sleep only takes about 100ms with an average current consumption of 6mA smiley-eek
If i take one measurment each hour, I would get an average current consumption of 5.25 µA smiley-cool
Anyone who know how long this thing will run on a cuple of AA? smiley-wink

the next tasks on my todo list:
1. Use the internal bandgap reference of the AtMega chip to measure the battery voltage, and make a low battery warning.
2. upload the measurements to Cosm.com using a gprs-module. (I just smoked my gprs-shield so this can take some time smiley)
3. put everything in a watertight box and do a real longtime outdoor test of the loadcell.


ps. It looks like this thread is turning into a worklog. Mabye a moderator can move this thread into a more suitable category.

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Pittsburgh, PA, USA
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I'm not sure that this will be good for beehives but if you have a mass that can move freely and an accurate accelerometer, especially at low G's, then if you apply a known force... F=MA.



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For this type of application, I have imagined two approaches:
...
(2) build a pneumatic balloon system under the load, like a hot water bottle.  Inflate it and seal it up, then just measure the pressure in the balloon.  If its hard to keep the balloon inflated all the time, then let the weight rest at the bottom and make a contact closure.  When  reading is needed, pump air into the balloon until the contact closure opens, take pressure readings for a while, then kill the pump and let the weight settle back against its stop.

That has given me an even simpler idea of how you could solve the problem. Fill the 'balloon' with (coloured?) oil and use a manometer type system (a U tube) to measure the pressure (= weight). Detecting how far up a tube the fluid level is would be far simpler than using stress gauges, which will have to be regularly zeroed. The narrower the tube, the more accurate it will be. You could use ultrasound to measure the distance of the fluid from the top of the tube. You may need a floating passive reflector for this. That way, no pump is needed and readings can be taken at any time. You will still need a 'dummy' hive to take into account ambient temperature (and, therefore, volume) variations. You'll also need to calibrate the tube (once) by adding known weights onto the 'balloon'.
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I'm not sure a "balloon" type system would work quite as expected. When weight is added to the hive the balloon will flatten out and result in a non-linear pressure reading at your manometer?

Using the example of a car tire, pressure measured in PSI, the footprint (in square inches) of the tire multiplied by the inflation pressure equals the weight supported by the tire (an "ideal" tire, ignoring sidewall stiffness, etc.). When weight is added the footprint can increase and the pressure can stay near or at the original pressure.

I might be totally off my rocker.

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I'd suggest using magnets and coils to raise the hive instead of a balloon but the fields might have a long term effect on the bees.



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Norway
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Thank you for your ideas but I have decided to use loadcells for my project. It looks like my prototype using a loadcell meets my requirements in both accuracy, power consumption, cost and simpleness. smiley
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That's great! The world does need more bees.
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I'm not sure a "balloon" type system would work quite as expected. When weight is added to the hive the balloon will flatten out and result in a non-linear pressure reading at your manometer?

Using the example of a car tire, pressure measured in PSI, the footprint (in square inches) of the tire multiplied by the inflation pressure equals the weight supported by the tire (an "ideal" tire, ignoring sidewall stiffness, etc.). When weight is added the footprint can increase and the pressure can stay near or at the original pressure.

I might be totally off my rocker.

Not off your rocker, but have you thought about a flat square 'balloon', the same size as the base of the hive? Remembering that he's starting with 100+ Kgs on it, the few grams more or less wouldn't make any difference at all to the footprint.

But after sleeping on the idea, I realised that a manometer system would need an equal weight of oil to balance the hive. 200 litres of hydraulic oil doesn't come cheap!
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I'm not sure a "balloon" type system would work quite as expected. When weight is added to the hive the balloon will flatten out and result in a non-linear pressure reading at your manometer?

Using the example of a car tire, pressure measured in PSI, the footprint (in square inches) of the tire multiplied by the inflation pressure equals the weight supported by the tire (an "ideal" tire, ignoring sidewall stiffness, etc.). When weight is added the footprint can increase and the pressure can stay near or at the original pressure.

I might be totally off my rocker.

Not off your rocker, but have you thought about a flat square 'balloon', the same size as the base of the hive? Remembering that he's starting with 100+ Kgs on it, the few grams more or less wouldn't make any difference at all to the footprint.

But after sleeping on the idea, I realised that a manometer system would need an equal weight of oil to balance the hive. 200 litres of hydraulic oil doesn't come cheap!

Or you could measure the height of the flattened balloon... I would use an inner tube to rest something on and maybe fill with dry CO2 or nitrogen.

But I think that his load cells are better than springs of any type.

I understand that putting mechanical strain on any conductor changes its resistance, even copper.




 
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2. upload the measurements to Cosm.com using a gprs-module. (I just smoked my gprs-shield so this can take some time smiley)

Thanks for this update! May be this combined board is usable: http://imall.iteadstudio.com/im120411004.html It is an Arduino, GSM, XBee, SD-Card on one board. It is cheap, but I do not know how power saving can be done.

Perhaps the new Arduino GSM shield is more power saving and battery compatible but there are some inconsistent information atm:
http://arduino.cc/forum/index.php/topic,148708.msg1156939.html#msg1156939

When <400 mA peak current is correct this would be an interesting board. But you have to pay nearly 100 Eur for the board only!
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