Worlds' cheapest GM - follow-up

[Ted de Castro <tdc@XRAYTED.COM>]

Well I got one and had it put through a few of its paces.  While this is

not a complete evaluation - it does reveal a weakness that others who

buy one ought to know about before they do too much testing.



Its cute, its functional.  It certainly does everything they claim.  Its

Russian made BTW.



It turns on by simply opening a window that reveals the display.  You

must be sure it is FULLY open or it won't turn on.  (this is done by a

small magnet and reed switch so the life of the switch ought not to be a

problem.  It comes with a 1 yr guarantee.



It has a glass GM tube with an E comp sleeve.  When you turn it on it

goes into an approx. 30 second count cycle - during the cycle it beeps

for each count and flashes a 3 bar character on the display while it

counts.  It then displays the counts integrated as the micro R/hr - thus

the sensitivity is 120 counts per micro R.



It is certainly linear up to the stated 200 µR/hr and goes all the way

up to the full display of 999 µR/hr with some small loss of linearity

(sorry - the tech didn't give me exact numbers)  Beyond that linearity

would APPEAR to fall of rapidly - BUT we can't be sure if this is just

an artifact of counter roll over - if indeed it does roll over.



All in all I'd say its a very useful instrument and would certainly have

a place in a professional operation.  Think of it as the worlds largest

and cheapest chirper if you like. But if you think of it as a detection

device rather than a measuring device and where the energy range is

appropriate - it could be quite useful - especially in those labs where

we investigate every stray count.  At a 10:1 price ratio with normal

instruments (probably less if you want enough to go directly to the

source) - they can buy a LOT of detection you simply can't afford any

other way.



The weak point however needs to be understood and considered - mine is

heading back for warrantee replacement!  The piezio chirper is driver by

a triple darlington (discrete surface mount devices) and the first two

stages do not have a current limit resistor.  So - if you put it in a

sustained high field for a few minutes - that transistor burns out!  I

didn't have enough units to determine how high for how long - but I

think it would be safe to consider this as the reason they claim use up

to 200 µR/hr!  Nor would I want to on my nickel!



Maybe after I get my replacement unit back we might tell the mfgr this

since such a small fix as adding a resistor could make a major

improvement.



As for the average person using this - they probably don't have easy

access to anything hot enough to cause this - BUT it is certainly not

impossible - a box of ceramic high voltage high power resistors will do

the job!



So - for now - that is all I know about this unit.



BTW - I didn't measure current draw to calculated battery life - I could

do so right now - but without the piezio functioning I suspect it would

be a considerable under estimate.  That can wait til I get the new unit

or until someone else checks it out.

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