[ARC5] Testing BC-442 Thermocouples

[J. Forster]

I have not researched 'bridge' type, but here is what I suspect:

Ordinary:

O-------------o------------O   ---> CURRENT
             /\
             ||
             M

Bridge:


O---------o---o------------O   ---> CURRENT
          |   |
          | M |

The bridge type would contaminate the thermal voltage wqith the IR drop in
the 'span' of the bridge section, between the connection points.

-John

==================





>
> 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!
>
>
>
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