Dummy Load possibilities - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

Radio Wrangler · 27th Feb 2019, 9:37 am

Quote:

Originally Posted by merlinmaxwell

Engine oil will be fine and cheap.

Engine oil on sale today has all sorts of additives which are not good in this application, some rather aggressive detergents and such.

Someone in an ARRL publication did a lot of work on this and found apart from intended transformer oils,'Turbine oil' is good..... The stuff Aeroshell flog you to top up the lubricant tanks on jet engines.

In ARRL handbooks of your there are several designs for using these resistors and even one with flared ground cones around the resistor, nit just to connect to the far end but also to create a matched transmission line down its length, making the big resistor good to UHF

David

m0cemdave · 27th Feb 2019, 2:59 pm

Bird dummy loads (which have a similar shape resistor, but carbon film on ceramic tube, not solid carbon) use a silicone oil. I have found a note that says it has an average kinematic viscosity of 20 cSt (centiStokes, a cgs unit) and that Dow Corning 200/20 is suitable, but rather expensive.

That should give a guide as to what else would be suitable.

Bazz4CQJ · 27th Feb 2019, 4:20 pm

Quote:

Originally Posted by Radio Wrangler

Someone in an ARRL publication did a lot of work on this and found apart from intended transformer oils,'Turbine oil' is good..... The stuff Aeroshell flog you to top up the lubricant tanks on jet engines

As I recall, the jet engine stuff is pretty expensive, operating happily from -40 to +200'C, it add's a whole new dimension to the term "fully synthetic"!

B

John M0GLN · 27th Feb 2019, 4:48 pm

What about the oil in domestic oil filled radiators? I've no idea what it is but Wikipedia says,

"The oil has a relatively high specific heat capacity and high boiling point. The high specific heat capacity allows the oil to effectively transfer thermal energy from the heating element, while the oil's high boiling point allows it to remain in the liquid phase for the purpose of heating, so that the heater does not have to be a high pressure vessel."

Hopefully it would be pretty innocuous in case of leakage having been made specifically for domestic use.

John

G4XWDJim · 28th Feb 2019, 11:34 am

Quote:

Originally Posted by Radio Wrangler

In ARRL handbooks of yore there are several designs for using these resistors and even one with flared ground cones around the resistor

My ARRL hand books 1956 and 1963 are probably a bit too days of yore to include that info but I found something similar in a 4th edition RSGB book using a half size version of my resistor. I have no plans to use it on VHF so simple connections will be fine.

My dummy loads have nearly always been lamps or a string of. I'm only thinking of making this because I've had the resistor lying around for many years. At my low frequencies I could probably even calibrate it using AC mains with near enough accuracy. At the moment I don't even have an HF transmitter until I make one so that might not be too soon.

Jim

27th Feb 2019, 2:59 pm	#22
m0cemdave Octode Join Date: Feb 2009 Location: Bletchley, Buckinghamshire, UK. Posts: 1,223	Re: Dummy Load possibilities Bird dummy loads (which have a similar shape resistor, but carbon film on ceramic tube, not solid carbon) use a silicone oil. I have found a note that says it has an average kinematic viscosity of 20 cSt (centiStokes, a cgs unit) and that Dow Corning 200/20 is suitable, but rather expensive. That should give a guide as to what else would be suitable.

27th Feb 2019, 4:48 pm	#24
John M0GLN Octode Join Date: Jan 2004 Location: Southport, Merseyside, UK. Posts: 1,156	Re: Dummy Load possibilities What about the oil in domestic oil filled radiators? I've no idea what it is but Wikipedia says, "The oil has a relatively high specific heat capacity and high boiling point. The high specific heat capacity allows the oil to effectively transfer thermal energy from the heating element, while the oil's high boiling point allows it to remain in the liquid phase for the purpose of heating, so that the heater does not have to be a high pressure vessel." Hopefully it would be pretty innocuous in case of leakage having been made specifically for domestic use. John Last edited by John M0GLN; 27th Feb 2019 at 5:06 pm.