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EX -- mass scale recommendations?

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Gunkie

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I'm just beginning an initial foray into EX motor development. I need a mass scale to measure to 0.1g precision. Any recommendations? Balance/beam scale or electronic? Upper range? 1000g? 2000g? More?

Thanks!
 

llickteig1

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This information was gathered from some local EX practioners a few months ago. I'm no expert, but I recently went through the information gathering process myself. This applies to a person mixing their own propellant and testing small batches of new formulations.

For the accuracy and size you need, an Ohaus triple beam scale is the best value. It allows you to measure accurately down to 0.1 grams for small test batches, and with the extra weight set up to 2610g for your production runs. This capacity is adequate, because if you try to make bigger batches you will need an industrial mixer to keep from burning-up your mixer, or you will have trouble packing such big batches.

Electronic scales are nice, but many of the cheap ones out there are of poor quality. A mechanical balance will stay more accurate over time over a cheap digital. Comparable quality, accuracy, and capacity similar to a triple beam in an electronic scales are much more expensive.

There are usually many Ohaus triple beam balances on eBay for very reasonable price, if you're inclined to purchase that way. I got mine for about $55.

HTH, --Lance.
 

Gunkie

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Thanks for the information!

I was beginning to think along the same lines (beam scale). I figure if I'm going to become more involved in EX rocketry, I'll invest in a high-end electronic scale and a bigger, more-heavy duty mixer. For starting out, I beginning to think that the beam scale is the best option.

Thanks again!
 

powderburner

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There are a couple up for auction right now on ROL in the 'Tools and Equipment' section.

If you buy from an individual, whether on ROL or ebay, make sure they know how to take the 'beam' part OFF and cushion it separate from the base for shipping. If the two main pieces are left together, the banging and bouncing of shipping will damage the pivot points and ruin the accuracy of your new purchase!
 

AlexNUMB

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Hi Scott--

Looks like you really are getting ready to take the plunge!

I have used triple beams for mixing before (at a friend's shop) and didn't really enjoy the experience.

With a triple beam, there is a high "fiddle" factor. i.e. you need to fiddle with the balance to tare the weight of your container, then it doesn't display exactly how much you have, etc.

I recommend an electronic balance with AT LEAST 4000g capacity and .1 resolution. Here's why:

Your scale/balance is your most important tool. You will be weighing a lot of materials out on this. You want it to be as quick and as accurate as possible.

Additionally, if you are using a Kitchenaide, you will want to weigh alot of your liquid chemicals (DOA, R45, Tepanol, etc) directly into the bowl. You want your electronic balance to be able to tare the weight of the bowl.

I use an Ohaus Scout Pro SP4001 (4000grams x .1g readability). I bought it from http://www.discountscales.com/specs/ohaus/scout

A friend has the Acculab vi4800 model, and it works well also, but is a little more expensive.

I can't emphasize enough how important it is to have a good scale. A good scale and proper weighing technique allows you to have consistant, repeatable results.

Don't let cost scare you off. You knew you were going to spend some money to get going... spend it here and spend it on a mixer. :)

--Alex
 

powderburner

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Sounds like AlexNUMB knows what he is talking about when using scales to measure the ingredients for a rocket recipe. He also makes a valid point that beam scales do indeed have a high 'fiddle factor' and require more care in set-up, more time to use, and certainly more patience (you have to wait for that indicator/pointer at the end of the beam to stop swinging up and down, and it starts all over again when you change the quantity of material being weighed). All his experience at working around the tare weight of a container is important to pay attention to also.

Looks like the electronic scale is the more practical way to go, at least for making motors.

I use my little (500 g capacity) scale for weighing airframe components. This scale was designed for ammunition reloaders who want to measure powder accurately, and has two beams. Measuring sensitivity goes down to 1/10 grain (7000 grains = 1 pound). I don't deal with tare weights, and my scale doesn't take long to set up or use. If this type of scale fits your needs, you can usually find one on ebay for $10 or so.
 

AlexNUMB

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Originally posted by powderburner
Sounds like AlexNUMB knows what he is talking about when using scales to measure the ingredients for a rocket recipe.
Pet peeve: It's not a recipe. We're not cooking anything. It's a formula. :)

--Alex
 

Gunkie

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OK, so you have me thinking about going big on a scale. I'm not planning on building anything beyond 38mm J for quite a while. I really just want to 'test the water' before building L, M,... powderburner made a very good point in that a beam/balance scale takes time to settle. If I'm mixing five items and the propellant begins to set up in 20 minutes, those moments waiting for the scale to zen in on a value will be critical.

I'm beginning to lean toward an electronic scale which adds at least $100 to my scale expenses. I looking along the lines of a 2000g model rather than a 4000g model. Give me some reasons to spend $300+ for the 4000g scale. The 2000g scale costs $200+.

THANKS TO EVERYONE!! Great comments and well thought out posts. It has really helped with this decision.
 

daveyfire

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Originally posted by Gunkie
Give me some reasons to spend $300+ for the 4000g scale. The 2000g scale costs $200+.
You'll be needing it later... trust me... :D I thought the same thing. "Oh, I'll just start out with a few small test motors." Then the plans grew and grew and grew... I think it's best off just to take the plunge in the beginning, pay the extra $100, and get the 4000g. You never know what motor you'll be building someday!
 

powderburner

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daveyfire is almost certainly right, you probably will want to move up to bigger motors after you get started----so you might as well go ahead and buy the Morton-Thiokol plant out in Utah so you can mix a buzillion pounds at once

Or, if you need to weigh 5 pounds of ingredient 'X' but you only have a 4 pound capacity on your scale, then weigh 2.5 pounds twice. This is obviously more trouble, and introduces twice the amount of weighing measurement error, but it IS a way to squeak by with a smaller scale. Just a thought.

And my earlier comments about watching a beam balance teeter back and forth, and requiring extra time to weigh your components, is only a comment. If you need to weigh/measure a bunch of components for mixing a batch of propellant, and there is a limited amount of time for working with the mix once you have started, then the smart time to weigh is BEFORE you start mixing, not during the mix. Take all the time you need to weigh the components, and keep them in separate containers until it is time to mix them in.

If you have a preference for electronic versus mechanical, then by all means go with it. If you are on the fence, then mechanical can usually be purchased a little less expensively. The experts who have chimed in here make it sound like electronic is much handier and they are probably right. I would just point out that you could make do with either, if you happen to end up with a limited choice.
 

AlexNUMB

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Hey Scott--

No problem, happy to help. I'm sure you'll find that from most "EX" folks, also.

The reason I advocate a 4000gram scale is, again, due to the tare weight when dispensing liquids into your mixing bowl.

The liquids used in propellant making can be pretty thick (especially R45 and Tepanol). Weighing them in a seperate container then transferring them to the mixing bowl can throw your results off. Thick liquids stick to the sides of your weighing container resulting in less of the actual chemical getting into the bowl.

I use a 6 quart pro model Kitchenaide. The bowl weighs somewhere around 1600grams. If you are doing any kind of sizeable batch (1 kilo or more) you are going to be weighing 200-300 grams worth of liquid in to your bowl. AT that tare weight, your accuracy and capacity suffers greatly.

My typical batch size is pushing 2800grams. In that size, we use ~1900 grams of AP. Weighing out 1900 grams of AP on a smaller capacity scale is doable, but if you ever get up to 98mm, 115mm, 152mm, etc, it's going to start taking some time.

I know the cost of these units is not trivial. Again tho, you will thank yourself in the long run.

The other 2 pieces of tooling you are going to want next is a vaccuum pump and a mixer. You can mix by hand for your first few (small) batches. However, as you get to the point where you want to do more, your going to notice your arm getting really tired...

The vaccuum pump will allow you to degas your propellant more easily. This results in densly packed propellant that is easy to replicate from batch to batch. Non degassed propellant can be unpredictable due to air bubbles n' stuff.

Not trying to scare you off, just trying to give youa reasonable picture of what to expect to need as you continue on this path. :)

--Alex
 

Gunkie

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First, thanks for everyone's help with this decision. I truly owe you folks a non-catoing rocket motor or ignitor or beer or something.

Secondly, I've purchased a scale based on the comments made here and a non-rocketry friend who works in a pharma lab in NJ. I purchased a My Weigh i2600 electronic scale with AC pwoer and calibration weights. With shipping, it cost a grand total of $178. My lab friend (my rock climbing partner) has used My Weigh scales for the past two years or so and has been very satisfied with their quality. Prior to that he used Ohaus models and was very satisfied with those. I went with the 2600 gram limit only because it was cost effective.

So it's done. Thanks agian and I hope to be making motors within the next few weeks. I already have a Kitchen Aid 4.5 quart mixer at my disposal.
 

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