Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!iuvax!cica!ctrsol!sdsu!csun!csusac!sactoh0!americ!erk
From: erk@americ.UUCP (Erick Parsons)
Newsgroups: sci.electronics
Subject: Re: How to convert weight to volts?
Message-ID: <1102.AA1102@americ>
Date: 12 Jul 89 23:33:53 GMT
Followup-To: sci.electronics
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>From: kuusama@etana.tut.fi (Kuusama Juha) Message-ID: <7903@etana.tut.fi>

>I'd like to build a small (pocket-size) weight scale, with a range of
>about 1 to 20 kilograms and accuracy about 1%. If I get voltage, I can
>surely handle the rest. But what to use as the converter?
>The catch: I prefer the converter not to cost more than $20.
                                   ^^^^^^^^^^^^^^^^^^^^^^^^^
Good Luck :-)

Its been a while since I've been in the Scale business but I will try
to give you a somewhat intelligent answer.

The only type of transducer that I've seen are strain gauge transducers.
Basically a Wheatstone bridge affair. The x-ducers are quite expensive
due to the fact that almost all are made to conform to state and federal
specifications which are .1 % error. Yes that's 1 tenth of 1 percent or
1 pound in 1000 allowable error. They come in differant flavors for 
differant applications ie. 25 LB 50 LB 100 LB all the way up to 1000 LB
total capacity. Prices vary and I think that they were about 250.00 up.

These units would put out about 5 or 10 microvolts / graduation of rated
capacity and thus require some pretty sensetive equipment to use fully.
They are excited by about 12 - 15 VDC highly regulated and sensed supply.
most are linearized mechanically though the meathod lately is to linearize
them via uP lookup tables which could possibly have driven the price down
to a more reasonable level )but I doubt it :-( 

If you are interested in buying one your nearby scale shop should be 
able to set you up. Who knows... the prices may be within reason by
now.

BTW all of the instruments that I've seen use a dual slope conversion
which is rather slow but extremely accurate. ~ 2500.00

Your real best bet would be to buy a ready made scale such as an
O Haus diet scale or such. There are a million of them on the market 
lately but I wouldn't bet my life on the accuracy or linearization of
such beasts, 1 % though doesn't sound too unreasonable.


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  Erick Parsons    //    Words for the wise:   *If it works don't fix it.*
  Sacramento Ca   //        mail to:   ...pacbell!sactoh0!americ!erk
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