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scale vrs pump


dust

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Originally posted by Rui

....and found that the advertised 1g accuracy is rather optomistic...... but the display just tricks you into thinking its more accurate.

I suggest that everyone wondering about scales and pumps read the following web pages on the difference between "accuracy" and "precision". There seems to be some confusion regarding the definitions of these terms with respect to the needs we have when mixing epoxy.

 

http://www.hcc.hawaii.edu/distance/sci122/SciLab/L5/accprec.html

 

http://www.wl.k12.in.us/es/concepts/shosys25.htm

 

When using a scale, we care about accuracy and precision, so that we get the appropriate mixing ratios - we don't really care about precision alone, because whether I mix 8.1 OZ of epoxy or 8.2 OZ or epoxy is meaningless.

 

With a pump, the accuracy of the RATIO is important, but the accuracy of the weight of each pump is meaningless. Precision doesn't factor in, because we're not measuring anything.

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Well however you put it (I guess the correct way of thinking is that the thing is precise since it does give the same reading every time, just not accurate since the reading is wrong) two 14g cans should not equal 30g when placed together on the scale. I guess I would need some standard weights to actually measure the accuracy, but from this single test it looks like the accuracy gets worse the more weight you put on. But then if you measure accuracy as a percentage this makes sense, but I wasn't expecting to see such an obvious error.

 

It comes down to expectations, if the scale measures in grams then I would expect the error in the equipment to be at most 0.5g. Obviously this is not the case with the one I have.

Rui Lopes

Cozy MkIV S/N: 1121

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all the stories about the pump failing and other problems associated with it have scared me away from it's use. I'm not willing to dish out $275 on a pump that seems to have problems just for a little added convience. On ebay I found a 500 gram 0.1 gram accurate digital scale for $40. do a search on ebay for "500 GRAM mini SCALE" and you will find many scales for the same price. Since I'm using MGS I can set my pot time. I like the idea of being able to ration the slow and fast hardner depending on the layup I'm planning. Not possible with the pump. Once it's full of resin and hardner you cannot easily change the slow/fast ratio. Just make up a chart showing the weights you need for a certain pot life for easy reference. I'm thinking of buying a few condement dispensers, like a one gallon ketchup pumping container to hole the resin and hardners. Right now I have the West brand pumps that sit on top of my MGS bottles, but they don't screw on.

 

Tim Dale

KD5ZXM

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and my west brand pumps fail all the time, they make it about a gallon and a half. the pumps work great, end end end end end of story

 

enjoy the build

 

dust

 

now, lock this dam thread up and throw it away

maker wood dust and shavings - foam and fiberglass dust and one day a cozy will pop out, enjoying the build

 

i can be reached at

 

http://www.canardcommunity.com/

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Hey Marc,..... Thanks for the links, good point. It is nice to know exactly what we are talking about here. Those terms do get used incorrectly often.

 

We know what we mean, and mean what we say, but we sometimes don't say what we mean. That sounds confusing, I know.

 

As for Dust.......Dust was speaking about the "Sticky Stuff Pump", when he said:

 

Dust Wrote:

the pumps work great, end end end end end of story

 

It seems that it works great.........

 

:cool: Be more of what you are......:cool:

Joe

Joseph@TheNativeSpirit.Net

I am Building a Jo-Z IV StarShip.

 

What Do YOU Want?

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