AC/ DC convertor, Croydon precision instruments - UK Vintage Radio Repair and Restoration Discussion Forum

samthelemming · 2nd May 2008, 1:01 pm

I bought this recently and couldn't wait for it to arrive.
Now its here and the first hour "glow" of getting a parcel has worn off (not completely, with something like this the glow is always there :P) I wondered how it is used. I have taken lots of pictures.

Has anyone any information on it?

I'm also at a loss to what a "vacuo junction" is

, in the pictures it looks like there is a valve holder, but the valve was replaced with something that looks like a acorn valve.

Any comments welcome

(I have more pics if needed for any reason)

(I know I said exam revision from now on, but this is just something to do in between studying

)

ppppenguin · 2nd May 2008, 1:28 pm

Looks like a thermocouple used to standardise AC voltages (perhaps currents) against a DC standard. The idea is that you pass the unknown AC through the thermocouple's heater. Then you pass a known and adjustable DC through it until it gets to the same temperature. The DC power and RMS AC power are then identical. Subject to errors caused by inductance and capacitance.

The acorn valve is the thermocouple.

Ed_Dinning · 5th May 2008, 4:42 pm

Hi Sam, I would agerr with Penguin. It would have been used in a standards lab where instrumnets were calibrated using DC against a Weston standard cell. This was the "transfer standard" that allowed AC instruments to also be checked. Well before the days of 12 digit DVM's etc. We had one in the old standards lab at Reyrolle.

Ed

samthelemming · 10th May 2008, 6:28 pm

All very interesting stuff, especially for 99p (I love a bargain

)

What use could I put it to, or would it be better to look at? (I like having things that do something practical, however old, bit like giving life to something unloved)

Thanks
Sam

coil1234 · 11th May 2008, 10:38 am

Hello Sam,
Be VERY carful when applying an input voltage or current to this, the thermocouple junction is easy to burn out and is NOT repairable.

So don't apply any greater voltage (dc or ac) than the range switches are set to, in fact I wouldn't be surprised if the T/C junction is already damaged ! Hopefully it won't be.

As previous posters have stated these were used in electrical standards labs for ac measurements using ac/dc transfer methods before the days of accurate ac digital multimeters, however for the type of measurements we are likely to make a dmm will be adequate.

One advantage of your converter is that it will measure the True RMS value of the ac waveform of whatever shape.

Probably best to not use it and just admire the quality of workmanship

Regards Mike A

coil1234 · 5th Jun 2008, 9:56 pm

Found an old Cropico Catalogue at work which gives the specifications for this instrument.

Regards Mike A

2nd May 2008, 1:01 pm	#1
samthelemming Retired Dormant Member Join Date: Nov 2007 Location: Carmarthenshire, Wales. Posts: 80	AC/ DC convertor, Croydon precision instruments I bought this recently and couldn't wait for it to arrive. Now its here and the first hour "glow" of getting a parcel has worn off (not completely, with something like this the glow is always there :P) I wondered how it is used. I have taken lots of pictures. Has anyone any information on it? I'm also at a loss to what a "vacuo junction" is , in the pictures it looks like there is a valve holder, but the valve was replaced with something that looks like a acorn valve. Any comments welcome (I have more pics if needed for any reason) (I know I said exam revision from now on, but this is just something to do in between studying ) Attached Thumbnails Last edited by Darren-UK; 2nd May 2008 at 2:30 pm.

2nd May 2008, 1:28 pm	#2
ppppenguin Retired Dormant Member Join Date: Dec 2003 Location: North London, UK. Posts: 6,168	Re: AC/ DC convertor, croydon precision instruments Looks like a thermocouple used to standardise AC voltages (perhaps currents) against a DC standard. The idea is that you pass the unknown AC through the thermocouple's heater. Then you pass a known and adjustable DC through it until it gets to the same temperature. The DC power and RMS AC power are then identical. Subject to errors caused by inductance and capacitance. The acorn valve is the thermocouple.

5th May 2008, 4:42 pm	#3
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,195	Re: AC/ DC convertor, croydon precision instruments Hi Sam, I would agerr with Penguin. It would have been used in a standards lab where instrumnets were calibrated using DC against a Weston standard cell. This was the "transfer standard" that allowed AC instruments to also be checked. Well before the days of 12 digit DVM's etc. We had one in the old standards lab at Reyrolle. Ed

10th May 2008, 6:28 pm	#4
samthelemming Retired Dormant Member Join Date: Nov 2007 Location: Carmarthenshire, Wales. Posts: 80	Re: AC/ DC convertor, croydon precision instruments All very interesting stuff, especially for 99p (I love a bargain ) What use could I put it to, or would it be better to look at? (I like having things that do something practical, however old, bit like giving life to something unloved) Thanks Sam

11th May 2008, 10:38 am	#5
coil1234 Pentode Join Date: Jan 2006 Location: Almeria, Spain Posts: 221	Re: AC/ DC convertor, croydon precision instruments Hello Sam, Be VERY carful when applying an input voltage or current to this, the thermocouple junction is easy to burn out and is NOT repairable. So don't apply any greater voltage (dc or ac) than the range switches are set to, in fact I wouldn't be surprised if the T/C junction is already damaged ! Hopefully it won't be. As previous posters have stated these were used in electrical standards labs for ac measurements using ac/dc transfer methods before the days of accurate ac digital multimeters, however for the type of measurements we are likely to make a dmm will be adequate. One advantage of your converter is that it will measure the True RMS value of the ac waveform of whatever shape. Probably best to not use it and just admire the quality of workmanship Regards Mike A