Checking Epoxy Pumps


Everyone has read it…the epoxy manuals always suggest you test pump dispensers for accuracy.

 You know, just to make sure they are dispensing the proper ratio of resin to hardener.  Well, In the past, when I built my stitch n glue kayak, I just assumed they were correct.  Yesterday, however, I decided to test my new dispenser pumps (purchased from Pygmy boats).  I was shocked at the results.  Here’s what happened:

  1. I noticed the restrictor tube looked a little short for the hardener pump, ie. it was not 1/2 the distance of the resin pump (I am using System Three epoxy, a 2:1 ratio system).  This got me concerned, so I decided to accurately measure the results.  I’m glad I did.
  2. I pulled out my ultralight backpacking digital scale and first measured the weight of a disposable plastic cup at 15 grams.  I then tested both dispensers and measured the results, after subtracting the weight of the cup.
  3. After completing the test twice, I got the exact same results.
  4. One depression of my hand dispensers gave me 29 grams of resin and 10 grams of hardener.  That is a 3:1 ratio not a 2:1 ratio.  Wow, I’m so glad I tested this before glueing up my mast staves.
  5. I shortened the restrictor tube for the hardener by 1/8″ and did the test again.
  6. After repeating the test twice, I now got 29 grams of resin and 15 grams of hardener.  That’s more like it.


Lesson learned, I will never trust hand despisers again without completing a thorough test for accuracy.

Hold the press…Pygmy boats just called and reminded me that the 2:1 ratio is by volume not weight.  Oops…my entire test was performed on weight not volume.  This has now demonstrated what often happens to me.  The harder I try, the more trouble I create for myself.  Oh well, we’ll see if it sets up.  I was off almost 20%.  So, let me modify the lesson learned.  Don’t out think yourself…otherwise you will create more problems than  you solve.  I’ve reinstalled the short tube I cut off and will move forward and sin no more.  

(Wait a minute…even when recalculating for volume, the pumps are still off a fair shot.  After weighing all this out in my head, I plan on using the suggestions offered by those commenting below.  I will use the pumps mainly as a dispensing tool so I don’t spill, but measure using the ideas below, especially for small amounts).  Now…that’s my story and I’m sticking to it.  


6 thoughts on “Checking Epoxy Pumps

  1. I used to use pumps but had lots of problems. I had much better success using a balance beam scale. I’ve gone through over 7 gallons of epoxy and it’s been spot on. After 7 gallons I have what looks like exactly twice as much resin as hardener left over. I use this scale from Duckworks:
    Except instead of washers for a weight, I use a cup filled with water. Epoxy and water weigh close to the same amount, so if I want a half cup of epoxy, I use a half cup of water as a weight. And instead of the fancy base, I simply ran a couple drywall screws through the beam and balance the scale on the tips.

    • Joel, Thank you for this suggestion. I’m going to need to wrap my head around this system. I definitely need to come up with something I’m comfortable with day in, day out. Thanks again,

    • Joel – I love that idea. So simple and elegant. I have always used the pumps, but like the idea of being able to mix arbitrary amounts. And I like your use of a cup of water vs. the washers – very intuitive. Have to give this a try! Thanks – Dave

  2. I bought a bunch of clear plastic disposable cups (several packs of 20 or more, all the same brand/size etc). I took out 2 cups, and placed one inside the other. Then I used a syringe to add (for a 1-2 mix) 5ml of water to the inside cup, and make a mark at the water level on the outside cup, then I add 10ml more water and, directly above the 5ml mark, make a mark at the new water level. Above them I write “15ml”.

    I then empty out the water and repeat, only using 10 and 20ml (and writing “30ml” above), 25 and 50ml (writing “75ml”) etc around the cup until I’m up to about 200ml total (ie nearly a full cup). I rotate the cup 20 degrees or so, so each pair of marks is distributed around the cup, and easy to see.

    The ouyside cup is now a measuring tool.

    A bit of work ot set up initially, but once done, this cheap, simple tool allows me to accurately and quickly measure a large or small amount of epoxy, simply and reliably by volume, over and over.

    Once I’ve finished all the marking etc on the outside cup, I keep the outside cup and, whenever I need to measure some epoxy, add a new “inside cup”, pour Part A up to the first mark of whatever total volume I want, and then Part B up to the second mark. Take the inside cup out, stir the goop to mix and voila!

    Discard the inside cup when finished, and keep the outside cup for the next jobs…

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