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Epoxy Measuring - Balances vs Digital Scale vs Ratio Pump

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keithhickling(at)clear.ne
Guest

Posted: Wed Jan 30, 2008 8:45 pm Post subject: Epoxy Measuring - Balances vs Digital Scale vs Ratio Pump

Roger,

I used the Europa balance, but modified by using 20 lb breaking strain nylon 

fishing line as the pivot, rather than the nail as described in the manual. 

The nylon line goes through the brass tube, and is stretched tight across 

the frame. This was a suggestion from Dr Martin Armstrong, technical 

director of SP systems (now Gurit) who make Ampreg resins. He pointed out 

that if you use a nail for the pivot, the brass tube tries to roll on the 

nail, resulting in a slight movement of the pivot point, and it also causes 

significant friction, both resulting in inaccuracies.



 I tested the balance against accurate pharmacy scales and found it was 

accurate to less than 0.02 g when balancing a 100 g weight with a 25 g +- 

small weights using the 4:1 ratio (ie 40 cm to 10 cm on the balance). In 

practical terms, I can easily detect the effect of 1 drop of hardner when 

nearly balanced. I expect that using digital  scales would take longer, as 

you have to accurately measure quantities of both resin and hardner, whereas 

with the ratio balance you just pour roughly the amount of resin you want, 

then add exactly the right amount of hardner. Having now got close to the 

end, I have been amazed at the number of resin mixes needed - far more than 

I initially envisaged - so any time saved here is significant.



I also always mixed the resin and hardner back and forward between the two 

cups, rather than simply pouring the resin into the hardner as suggested and 

then reusing the resin cup next time. I had thought, and Martin Armstrong 

confirmed, that this can result in significant inaccuracies, depending how 

long you spend draining the resin from the cup each time, as well as 

allowing water (vapour) and CO2 into the residual resin in the cup.



I'll be interested to hear what others have found.



Regards,

Keith Hickling,

New Zealand.

Engine in, working on cowls.



---

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gregoryf.flyboy(at)comcas
Guest

Posted: Wed Jan 30, 2008 11:18 pm Post subject: Epoxy Measuring - Balances vs Digital Scale vs Ratio Pump

-------------

Keith Hickling wrote:



"I expect that using digital  scales would take longer, as you have to

accurately measure quantities of both resin and hardner, whereas with the

ratio balance you just pour roughly the amount of resin you want, then add

exactly the right amount of hardner. "



-------------

I have always used the digital scale. It is an Ohaus, with a resolution of

0.1 gram and does not suffer hysteresis inaccuracies like some do. Absolute

accuracy is not important, since the accuracy error will factor out in a

ratio, as long as repeatability is good. No need to bother with using the

sample weights that come with the scale to make it accurate.



With the scale, you never need 2 cups to mix the resin back and forth, and

only have to carefully measure the hardener. I put resin pumps on the

containers of hardener and resin, and leave them there(they can be obtained

from the local epoxy supply, such as Tap Plastics, or probably Aircraft

Spruce or Wicks), with a plastic bag over them when finished. Upon needing a

batch, just grab both out of the temperature-controlled storage area, pull

off the plastic bags, and start pumping. For example:



Epoxy:



Put a disposable cup onto the scale. Turn it on, and it will auto zero,

excluding the weight of the cup from the equation. Dump in whatever amount

of resin you feel like. It does not need to be measured, a wild-ass guess

will do, but write down the final amount of the weight of the resin.

Multiply this amount by 0.27 (Aeropoxy), and you have your hardener amount

to add. Add that number back to the initial amount of resin measured, and

you get the total amount of resin plus hardener needed. Simply pump hardener

into the same cup until you get close to the final amount, then go drop by

drop until you nail the reading to a tenth of a gram (resolution of the

scale), and you got it! Its very easy and takes almost no time, and no need

to mess around with pouring between the resin cup and hardener cup to use

all of the material -just keep a calculator handy, and a pencil to tap the

numbers to keep the calculator clean. Mix until you see a homogenous

mixture, and you are done. It took a little while to explain, but it goes

really fast. As a final check, keep a mixture lookup chart nearby and right

before adding the hardener, verify that the calculated hardener amount to

add is close to proper value. 

Araldite:



Araldite won't pour, and needs to be scooped out with a Popsicle stick. It

will adhere grossly to the stick.  Hence in this case, put the stick and the

cup on the scale before it zeros the value, to cancel out the weight of the

popsicle stick too. Make sure it is always in the cup during measurement.

Scoop out the araldite needed, and put the scooped amount along with the

stick, into the cup. The measurement reading taken will be for the araldite

only, since the stick was cancelled out. Make the calculation (multiply by

0.4, similar to the epoxy, above) Then pour in the hardener from a

ketchup-like bottle which has the ability to dispense from stream to drops,

depending on squeeze pressure (I purchased one from the local epoxy shop,

and it is similar). Verify calculated amount with the araldite mixture

lookup sheet (found on one of the web sites) to verify that the calculated

amount is close, then mix. 



In the build manual, I write the resin amount measured, and hardener amount

to be mixed, along with the total amount of the two, which is the important

number, since you will add hardener until the total amount is achieved, then

write how close I actually got to the calculated total amount. Almost

always, the final resin + hardener measurement (actual) is the same as what

is calculated.



It is amazing how accurate the ratio scale can be if it is built properly,

like using the fishing line technique that Fred described. And it is

cheaper. For me though, the $100.00 digital scale which you can sometimes

find for 35 or 40 works really well, is fast, and if carefully done and

verified, will always make the proper ratio.



Have fun,



~Greg Fuchs

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topglock(at)cox.net
Guest

Posted: Thu Jan 31, 2008 9:30 am Post subject: Epoxy Measuring - Balances vs Digital Scale vs Ratio Pump

Roger,



Guess I'm the odd man out, but I used the pump and loved the ease and 

speed.  Only down side I found was having to clean it up every few 

months.  15 minute job.  Well worth it, in my opinion, for the 

convenience of pulling the handle and you're done...



BTW, I got the pump with the kit, so cost was not a factor for me...



Jeff - Baby Blue

363 hrs



Roger Lowe wrote:

 	  Quote: 	
		   

 

 Hi, I've purchased a partially complete Europa a little over a year ago and had to put it storage until my garage/workshop was re-vamped in order to build the kit. It is now to a point to where I can start moving my kit into it and start working on it. 

 

 I have question about the different devices used to measure the epoxy and wondering what most builders have done in the past. The cheapest of course is to make the Europa mechanical balance which should be no problem (is it quick and easy to use during a layup?) but I'm leaning towards either a Ohaus / Sartorius digital scale with a resolution of 0.01gm (do I need that much precision or can I get away with a lower cost 0.1gm unit?) OR a ratio pump like a Glenmarc Portionator MBT-JR from A/S which is the most expensive of the all the options (is it worth the cost for ease of use?).

 

 I live in Canada so I will be using Aeropoxy resin (100:27 by weight or 3 to 1 by volume).

 

 I would appreciable any feedback/experience that you have that would help me make up my mind!

 

 Thanks, Roger

 

 

 

 

 

 

 Visit -  www.EuropaOwners.org

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keithhickling(at)clear.ne
Guest

Posted: Thu Jan 31, 2008 3:22 pm Post subject: Epoxy Measuring - Balances vs Digital Scale vs Ratio Pump

Ira wrote:

A scale that gives you 0.5% is way better than necessary since 5% is 

probably just fine.



Favid Miller wrote:

 the Aeropoxy reps at Oshkosh, they were  clearly amused by anyone bothering 

to measure to the nearest tenth of a gram -  off the record  they indicated 

that the tolerance levels for a proper cure are quite high.

I don't know who is correct, but Martin Armstrong, technical director of SP 

Systems (Ampreg) told me, as stated in the data sheets, that 2% is the 

maximum permissable error in mixing resin and hardener - any greater error 

results in measurable and significant reductions in strength. He was very 

concerned that some people get a bit "gung ho" about it. And 2% is only 0.1 

gram of hardener in a 5g / 20 g  mix. And a similar error in the opposite 

direction neasuring the resin would result in a 4% total error.



Keith Hickling,

New Zealand.

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