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Digital scale woes.. #143312
05/14/08 03:35 PM
05/14/08 03:35 PM
Joined: May 2003
Posts: 4,451
West coast of Norway
Rolf_Nilsen Offline OP
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Had a scary experience using digital scales for measuring out epoxy today. They are not always the most reliable, but fortunately we noticed before we began glassing the outside of the hull half we are working on: http://woodastic.blogspot.com/2008/05/dont-trust-digital-scales.html


Now we are considering an old "manual" set of scales with counterweights. Would be nice if we could find a set instead of being unsecure about the digital scales. Our epoxy are metered by weight (100:32) so we need some good scales are a accurate weight, without breaking the bank.

-- Have You Seen This? --
Re: Digital scale woes.. [Re: Rolf_Nilsen] #143313
05/14/08 11:51 PM
05/14/08 11:51 PM
Joined: Nov 2006
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Finland
Gato Offline
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I have used an electronic kitchen scale (30) for years without any problem. Using a 2/1 weight ratio epoxy, and I have made mixtures down to 20 grams without problems.

Re: Digital scale woes.. [Re: Gato] #143314
05/15/08 01:23 PM
05/15/08 01:23 PM
Joined: Sep 2007
Posts: 531
Lake Murray SC
FasterDamnit Offline
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So-

Sounds like you have been using a scale for a couple of years without calibrating it? Either have it done professionally or buy some NIST standard weights to do it yourself. Do you make the some proclamation when your car breaks due to lack of maintenance? Stuff fails if it is not cared for... And sometimes it fails for no good reason.


Race cheap, race faster, Damnit!

E-Scow
24' ULDB

18HT hulls plus Gcat 5.7 rig = 18GT!
Re: Digital scale woes.. [Re: FasterDamnit] #143315
05/15/08 02:00 PM
05/15/08 02:00 PM
Joined: May 2003
Posts: 4,451
West coast of Norway
Rolf_Nilsen Offline OP
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Rolf_Nilsen  Offline OP
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Whoa!

How do you maintain a "best buy" US$40 digital weight except for changing batteries? We noticed it was broken and are screening our new weight as we must have hit a bad model with the one we bought.
If my brand new car crapped on me I would definately make some proclamations to the seller. If the seller replaced the car and it had the same faults, like failing to start, I would be really pissed. But we are not talking about cars but a US$40 weight.. No big deal except for others to take care when using digital weight, which was why I wrote the post.

Not exactly the reaction I expected to the post and the story..

Re: Digital scale woes.. [Re: Rolf_Nilsen] #143316
05/15/08 05:46 PM
05/15/08 05:46 PM
Joined: Sep 2007
Posts: 531
Lake Murray SC
FasterDamnit Offline
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Why the Whoa? Just giving my .02c.
The correct use is to either have it calibrated regularly or use a set of calibrated weights before use. If the downside of inaccurate weighing is expensive in time and materials then maybe more money should be put into the balance or the weights to check it before use. Mettler/Toledo and Ohaus make nice balances and scales.

and as a helpful hint, not a slam, use the term electronic balance or scale in your article in place of electronic weight. It will make more sense.

No slings or arrows intended.

Just info. from a former metrologist.

http://www.northshorecare.com/lab-balances.html

Re: Digital scale woes.. [Re: FasterDamnit] #143317
05/15/08 07:33 PM
05/15/08 07:33 PM
Joined: Jun 2001
Posts: 115
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Kevin Cook Offline
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Rolf,
I have been using inexpensive digital scales a long time. But I always keep 2 or 3 on hand so I will always have a back up. Same thing with vacuum pumps. Half way into a big lay up job is no time for either item to quit on you.

I mix batches anywhere from 100g to 3kg and find scales with a 1g accuracy (or is the word precision) do fine. The specification on a typical bulk epoxy resin (Dow Chemical DER 330) has the epoxide equivilent weight between 176 and 183. It is this property that determines the number of cross linking sites available for a given curing agent. Curing agents too have variability in their amine weight. So, even you have a super accurate scale, what you are mixing has some variability and that dosen't do much good.

Re: Digital scale woes.. [Re: Kevin Cook] #143318
05/15/08 08:00 PM
05/15/08 08:00 PM
Joined: Sep 2007
Posts: 531
Lake Murray SC
FasterDamnit Offline
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The two aspects of mass measurement are accuracy and precision. If accuracy is not as critical then no need for a 5 digit balance. Precision is then the most important aspect and checking the balance before use or calibrating regularly ensures precision. Lastly, a triple beam is very reliable and durable if properly cared for but is not at as easy or quick to use as a digital balance.

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Race cheap, race faster, Damnit!

E-Scow
24' ULDB

18HT hulls plus Gcat 5.7 rig = 18GT!
Re: Digital scale woes.. [Re: FasterDamnit] #143319
05/16/08 01:35 AM
05/16/08 01:35 AM
Joined: May 2003
Posts: 4,451
West coast of Norway
Rolf_Nilsen Offline OP
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Rolf_Nilsen  Offline OP
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Sorry, just a knee jerk defensive reaction. Are you working with scales in your professional life at this time? Calibrating is a novel idea. The scales we have used have all been "Best Buy" kitchen appliances with a 1 gram resolution (supposedly). There are noe calibration features on these, at least the manuals dont say anything about it and there are no extra buttons/dials/screws to work on. Perhaps putting a known weight like a piece of steel on the scales before measuring epoxy is better than just mixing epoxy and looking at the results?

If we find a model able to measure in grams, which the two new models we have tested so far have not done, we will buy two. By doing so we are reasonably certain that the scales used will last indefinately..

Kevin, what you say about the mixing ratios are interesting. You finish it off by saying that homebuilders will not get super accurate results anyway? Taking it one step further, will the pre-pregs have better quality on the mixing ratios of the matrix? Is that another of the variables which can make a pre-preg product "better"?

Re: Digital scale woes.. [Re: Rolf_Nilsen] #143320
05/16/08 03:34 PM
05/16/08 03:34 PM
Joined: Jun 2001
Posts: 115
K
Kevin Cook Offline
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Kevin Cook  Offline
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What I meant to say is the epoxy out of the factory can vary up to a couple percent in terms of chemical bonding sites from one batch to another. Perhaps the metrology person knows statistically whether it makes sense to measure to the nearest .5% accuracy in such a case. Dosen't seem to.
Interesting question you raised about prepreg. I know a lot of the high temperature structural curing agents have a mix ratio of 8:100 or less. Looks like they would have to be more careful. Prepreg processors are very touchy about about their resin formulations. You never see that given in their product specifications.
Since I used about 70 gallons of resin for my boat, I did some internet research. If you google "Dow Chemical Liquid Epoxy Resins" you will find a great tutorial on epoxy chemistry. I explains how to calculate your own mix ratios from the chemiacal specification sheets. Most homebuilders use "secret recepies" from a formulator and they don't tell you what it consists of. So, if you have a half gallon of WEST resin and a quart of SP curing agent there is no way to determine the mix ratio. The formulators like it this way!! Industrial epoxy is different - it comes with data sheets that allow you to calculate mix ratios with any hardner so long as the curing agent also has a data sheet.

Re: Digital scale woes.. [Re: Kevin Cook] #143321
05/16/08 03:51 PM
05/16/08 03:51 PM
Joined: May 2003
Posts: 4,451
West coast of Norway
Rolf_Nilsen Offline OP
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Rolf_Nilsen  Offline OP
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Thanks Kevin. It seems logical that with a mixing ration of 100:8 you need some really good quality control on your supplies!

The good news is that we have tracked down a weight accurate to 1/10 of a gram. Will make it easier to get our mix more consistent. Will also get some weights so we can check the scale before use.

Now, what are triple beam scales?

Re: Digital scale woes.. [Re: Kevin Cook] #143322
05/16/08 07:15 PM
05/16/08 07:15 PM
Joined: Aug 2007
Posts: 190
lesburn1 Offline
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Posts: 190
Quote
Rolf,
I have been using inexpensive digital scales a long time. But I always keep 2 or 3 on hand so I will always have a back up. Same thing with vacuum pumps. Half way into a big lay up job is no time for either item to quit on you.

I mix batches anywhere from 100g to 3kg and find scales with a 1g accuracy (or is the word precision) do fine. The specification on a typical bulk epoxy resin (Dow Chemical DER 330) has the epoxide equivilent weight between 176 and 183. It is this property that determines the number of cross linking sites available for a given curing agent. Curing agents too have variability in their amine weight. So, even you have a super accurate scale, what you are mixing has some variability and that dosen't do much good.


I have to agree! I was involved in a thread over at SA recently about vacuum pumps. And while I built my own vacuum pump and reservoir, I keep a vacuum generator (as a backup) plumbed into the system at all times!!


lesburn1.blogspot.com

A-Cat USA 49
18Sq 49

member- Royal Society for Making Cool Stuff
Re: Digital scale woes.. [Re: Rolf_Nilsen] #143323
05/16/08 07:32 PM
05/16/08 07:32 PM
Joined: Jun 2004
Posts: 502
Port Noarlunga, SA, Australia
D
Darryn Offline
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Quad beam scales also available, good resolution,
http://www.balances.com/triple.html

I check calibration of my digital scales using some weights I made and checked on scales calibrated at a lab.
Darryn

Re: Digital scale woes.. [Re: Darryn] #143324
05/16/08 07:59 PM
05/16/08 07:59 PM
Joined: Sep 2007
Posts: 531
Lake Murray SC
FasterDamnit Offline
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Rolf,
Yes, I work on automated blood testing systems and use balances regularly. I also used to work on them directly in a Precision Measurement Laboratory.

The definition of calibration is to simply compare the ability of a measuring device to a known standard. In this case, some known weights. Click the link in my previous post for options on purchasing some. I bet the basic units you use have little or no adjustments. At the price that is OK- you just need to be ready for the day it goes out of spec or quits.

Re: Digital scale woes.. [Re: FasterDamnit] #143325
05/17/08 04:39 PM
05/17/08 04:39 PM
Joined: May 2003
Posts: 4,451
West coast of Norway
Rolf_Nilsen Offline OP
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Rolf_Nilsen  Offline OP
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