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PICList Thread
'[PIC]: Measuring weight'
2001\04\30@164045 by Drew Vassallo

picon face
>I'm thinking about ways to measure weight, on the cheap.  The idea is to
>have a rack in a cooking oven, with one rail of the rack support supported
>on a spring, to measure the weight of the object being cooked ( never mind
>why we are interested in that... long story)

Keep in mind most electronics that you might select likely have a relatively
low upper operating temperature (~250 deg. F), which I'm sure you're aware
of.  As far as I know, it would take a while to cook a pie that low :)  But,
your readings will vary from calibration (at room temp) to operating temp,
if you really are using this in an operating oven.

Why not just buy a spring scale and put it underneath the pie tin?  Or a
hanging spring scale with the plate suspended beneath it?  I don't know how
much room you have.

Seems to me that a spring situated vertically on each corner support of the
rack could be easily calibrated for distance vs. weight and a simple
arrow-type indicator attached to the rack which is read against a scale on
the side wall of the oven.

Note: I haven't seen a PIC-related question posed here :)

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'[PIC]: Measuring weight'
2001\05\01@080654 by o-8859-1?Q?K=FCbek_Tony?=

so if I might summarize a bit we have the following:

You 'need':

a) an device to measure weight, not particular accurate.

b) it's to be placed inside an oven.

My guess:

a) You are interested in an 'relative' measurment
  i.e. the weight delta over time.

b) To determine when something is 'done'. :)

Some notes:

Mechanics inside an oven is *not* suitable, so all the
clever devices using levers, shafts etc. will probably only
work in an 'lab' ( read: not normal 'oven' ) enviroment.
If to be used 'profesionally' i strongly recommed placing
the measuring device outside the oven.
Also to take into account the an normal oven has to be cleaned
at times ( duh ! ) and it should then contain mostly flat surfaces.
Having plates, levers, bolts etc would make it close to impossible to clean.

I guess an option could be an strain gauge, accurate however
could have trouble with large temperature variations
( as most of the other techniqes ).
You could always use an wire vibrating at resonant frequency
( by piezo or the like ) and measure the frequency change
when load is appiled to the wire ( i.e streched ).


Tony Kübek, Flintab AB            

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2001\05\02@165331 by John

Hello Lawrence & PIC.ers,

Y'might want to try the basic principle used in many laboratory balances.

The platen has a very small range of movement, with NO springs.
Powered-down the platen rests at the bottom of its range, powered-up it is
raised by a solenoid till it interrupts a slotted type
opto-coupler. There is no hysteresis with this arrangement, the opto can
look through an angled appendage attached to the platen.
The solenoid current is controlled by your PIC, in closed loop, so that the
opto output current remains constant at some convenient set-point value,
i.e. the platen remains at a fixed position.
When the platen is loaded externally (someone
dumps material on it, or in your case Mrs. Jones pie)
the closed-loop increases the solenoid current and brings the
opto output back to set-point again.
The compensating solenoid current is directly proportional to the
load on the platen.

This arrgt. is real simple and capable of tremendous accuracy, even if
know you don't need to weigh pies to +/- 0.001 gm.

       best regards,   John

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