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tom_b
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tom_b
Joined: 19 Aug 2003
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Posted Mon May 17, 2004, 5:23pm
Subject: Junk Multimeterzzz
 

I was excited to finally get a multimter and thermocouple wire to test my temp at the PF, did so with results uniformly on the high side and then tested it in boiling water - 217F. It is about 7 degrees off (I'm at 1500 or 2000 ft altitude) and within spec 3% of range from freezing to measured temp.. No instructions on recalibration.. Anyone have a moderately priced unit they can recommend or.. any other guidance?
thanks,
tom
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PJK
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Posted Mon May 17, 2004, 6:23pm
Subject: Re: Junk Multimeterzzz
 

Hi Tom,

This is the one I use.
http://www.web-tronics.com/digtherwkpro.html

I think they are claiming +/- 2% accuracy which is just a bit better than what you are getting with your meter.

Phil

 
Philip J. Keleshian
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brokencup
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Posted Mon May 17, 2004, 6:52pm
Subject: Re: Junk Multimeterzzz
 

PJK Said:

Hi Tom,

This is the one I use.
http://www.web-tronics.com/digtherwkpro.html

I think they are claiming +/- 2% accuracy which is just a bit better than what you are getting with your meter.

Phil

Posted May 17, 2004 link

I have that one too. I think it is a very good value.

Bob
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tom_b
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tom_b
Joined: 19 Aug 2003
Posts: 537
Location: Maui, Hawaii

Espresso: Rancilio Silvia/PID
Grinder: Nuova Simonelli MCF
Roaster: USPS/Popper
Posted Mon May 17, 2004, 9:32pm
Subject: Re: Junk Multimeterzzz
 

Thanks-I remember seeing that model, lost the link. Do you get 212F in boiling water (assuming you're at sea level), or varying from 208-216?

I tried the one I posted about yesterday again, got 210-211, then 213, (but was 217 yesterday) tried yet another model and got 206-207.. hmmm.. My cheap dial thermometer is looking better than that, but is too slow..

tom
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barjohn
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Posted Mon May 17, 2004, 9:43pm
Subject: Re: Junk Multimeterzzz
 

The problem with these units is as follows:  If you are trying to achieve 1 C accuracy, but your instrument is only capable of +- 2C you can never achieve a valid reading.  In the Navy we require the test instrument to be 4 times as accurate as the instrument under test (called a 4 to 1 test accuracy ratio).  That ratio is maintained all the way to either the NIST standard or the limits of physics whichever comes first.  Since Schomer recommends being within the 1C range or better (though he states that no semi-comercial machine he has seen can achieve that result) you need a far more accurate instrument.  For example the Fluke 52-II is accurate to .05% or .1C ($219). A B&K type 630 ($89.00) is accurate to +.3% of reading +1C or +1.285 to + 0.715 C.  Not quite what we would like but much better than the meter shown with an error of +-.75%+4F (error +5.5 to +3.05 at 203F).  To be really accurate you need to take into account the thermocouple's error rate as well.  A good instrument is calibrated with the measuring device so that the error is the total error.

Probably more than you wanted to know but you can waste money on testers that won't do the job if you don't know what it must be capable of measuring.  There is nothing worse than have several temperature measuring devices and no two agreeing with the other.  Which one do you believe and count on?

 
John

Veritas en Trashitas
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brokencup
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brokencup
Joined: 18 Feb 2004
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Posted Tue May 18, 2004, 4:57am
Subject: Re: Junk Multimeterzzz
 

Barjohn is correct in that a $20 insturment may not be suitable for precise PF readings.

I use mine in my roaster at temps from 200F to 400F as a back up to the on board themometer. It works well for this purpose.

I want to point out that, there is accuracy and then there is repeatability  Repeatability may be more important to you than accuracy because if you always get 208 degrees for boiling water then you have a reference point. I don't know the repeatability of measurememts with this thermometer with it's $20 innards, but the several times that I stuck it into a pot of boiling water it came up around 211F which is about right for Atlanta (elevation 1000ft). Just a guess, but I think the short term repeatability would be good. That is if you read boiling water and then your portafilter within 2 minutes of each other the relative temps would be meaningful.

Repeatability is probably a function of the ambient environment including temp and humidity. Conditions that are fairly constant in a kitchen. But might include battery condition and aging.  None of these factors would preclude obtaining useful information about your PF if you want to risk another 20 bucks.

Barjohn's approach is good  'Metrology', whilst mine is 'Mr Wizard' at best.

Bob
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