Re: Somewhat defective balance Mettler AE100 - how to work around limitations?

From: Andrew Tweddle (sarason_not_me_at_alphalink.com.au)
Date: 10/20/04 

Date: Thu, 21 Oct 2004 02:14:49 +1000

Wilco Oelen wrote:
> Hello,
>
> I purchased a defective analytical balance for just a few dollars,
> Mettler Toledo, model AE100, with a weighing range up to 110 grams, in
> steps of 0.0001 gram. It is quite old already, sale was discontinued
> back in 1986.
>
> The balance powers up and does weighing, but the front panel on the
> balance does not work properly anymore. The display works (and happily
> I can read the weighed amount on the display), but the balance does
> not respond to the push/pull bar on the front. The switches on the
> panel are OK, I measured them with a Ohm-meter, but apparently the
> commands towards the back of the device (such as "calibrate" or "use
> long integration interval" or "switch off display") are not generated
> anymore. This makes it impossible for me to calibrate precisely (using
> a built-in 100.0000 gram mass) and to re-zero the balance, whenever I
> put another weighing scale on top of the builtin metal scale.
> I can measure weights by computing the difference between the weight
> of the scale and the weight of the added chemical, but I would like to
> be able to re-zero it. If I disconnect power and then connect power
> again, then it does re-zero, but the manual states that power has to
> be connected for at least 60 minutes in order to do accurate
> measurements. I doubt whether this really is necessary if weight is
> measured by computing differences, and the amount of measured
> chemicals is only in the order of 1 gram or even less.
>

Sounds like a dead switch?
> I have a few questions for you.
>
> 1)
> If I switch off power and then reconnect it a few seconds later, do I
> need to wait 60 minutes again, or does a short power-interrupt no harm
> as far as accuracy is concerned?
>
> 2)
> If I measure the weight the of built-in mass of 100.0000 grams, then I
> get an error of approximately 0.003 grams (readout somewhere between
> 99.9965 - 99.9975), when the empty weight is exactly 0.0000. If I
> measure a weight of approximately 1 gram, then I expect the error to
> be much less than 3 mg, because I think that the error is a gain
> error. I think that I do not have offset errors, because I measure
> weights by weighing the empty scale and then adding the chemical. The
> difference of the two weights is the weight of the chemical. Does this
> method indeed eliminate offset errors and can I safely assume that
> with weights of less than 1 gram, the gain error has no significant
> effect on the measurement (remember, resolution is 0.1 mg)?
>
> 3)
> Does anyone of you know the signal pinout and the protocol, used by
> the balance for communicating between the front panel and the actual
> electronics inside the balance. If I know the protocol (a 10-pin
> connector is used for passing the signals), then I'm quite sure that I
> can build a small piece of hardware and write a piece of software for
> reading and managing the device, using a notebook as an advanced
> display.
>
> 4)
> The balance also has a so-called foot-pedal interface on the back,
> with just two pins. One pin is the electrical ground. Does anyone of
> you know the protocol, used for communicating through these pins?
>
>
> I would be pleased if someone can help me. Even if I cannot
> repair/modify the balance, if someone can explain to me, how I can use
> the device with its limitation for doing accurate measurements, then I
> would already be very pleased.
>
> Thanks in advance,
>
> Wilco

I fixed a few different Mettlers a few months back. That basically had
the same interface across the few different models I looked. Since I
couldn't find a specific manual for the unit I was after I just worked
off another manual. The Interface is RS-232. My guess for the foot pedal
would be TTL. All in all It was a pleasure to work on mettler equipment
because of its consistency in the way it worked across the model range.
I have worked on many instruments, software, other equipment where
pointless differences between different models make servicing/fixing a
real PITA. I looked but couldn't find the PDF I used but you could check
out

http://www.mt.com

Andrew

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