[ARC5] Testing BC-442 Thermocouples

[Roy Morgan]

On Aug 14, 2013, at 4:25 PM, Leslie Smith <vk2bcu at operamail.com> wrote:

>  Hi Mark & Group,
>  I believe I've seen an article dealing with the reconstruction/repair
>  of RF thermocouples….
> 
> On Wed, Aug 14, 2013, at 12:40, Mark K3MSB wrote:...
>> 
>> For general testing of RF Ammeters I use my Variac at 20V through a 20
>> ohm series resistor.

According to my notes file on RF ammeters, there apparently are two kinds of thermocouple sensing devices: one is a "bridge" device and one is a single (?) contact device.  The bridge device in some Weston meters will not work on DC.  Here is the post:


From: "Dave Brown" <tractorb at ihug.co.nz>
To: "glowbug reflector" <glowbugs at piobaire.mines.uidaho.edu>
References: <Pine.OSF.4.43.0208071559050.358646-100000 at mail.aurora.edu>
Subject: Re: GB> Weston RF Ammeters
Date: Thu, 8 Aug 2002 12:06:51 +1200

Had the Weston 1931 Export catalog sitting here from looking up an old meter
I acquired a few days back so I looked up the 425 thermocouple series.
 Was very surprised to learn that they made the more sensitive range ones
(500 mA and down- but see caveat below)  with a thermocouple bridge and
these are NOT useable on DC.  I had always assumed they were all AC/DC as
Bob said. I'm still trying to mentally picture what the circuit arrangement
is that prevents DC operation --  I guess that's just a sign of a slow
brain!

CAVEAT--- the 500 mA range is evidently available in BOTH single
thermocouple type (i.e. AC/DC useable) AND the bridge type. (AC only)
Below 500 mA the 425 series is evidently always thermocouple bridge. (This
IS the 1931 catalog!!)
I don't believe this applies to all milliamp range RF ammeters but it
certainly did for the Weston 425 and 507 series with  FSD of 500 mA and
below, in 1931!

SO--- if at first a DC test fails- try an AC (60Hz) test before finally
giving up- especially if it  has an FSD of 500 mA or lower.
AND-- to add another dimension to all this--the 425 series were all
available on special order with external thermocouple units --so take care
with the current test setup initially--in case it's just a  low current DC
milli or micro ammeter!

73   Dave  ZL3FJ




Roy Morgan
k1lky at earthlink.net
K1LKY Since 1958 - Keep 'em Glowing!