[Amps] Muffin Fan on Swan Mark 1

[Ian White, G3SEK]

2 wrote:
>An IR thermometer,  a manometer, and some horse sense are more
>trustworthy  than computer modeling -- i. e., if the thermometer says
>it's too damn hot, and the computer app says it's not, it is.
>

They used to say that about antennas too, until MININEC and NEC became 
available free to anyone who wants them. Certainly there are big 
pitfalls in computer modeling, but antenna modeling has also shown that 
"horse sense" isn't always reliable either. In the end, you need both, 
in order to check each against the other.

Exactly the same would apply to thermal/flow modeling if we had the 
tools to try it.


Unfortunately IR thermometers are notoriously UNtrustworthy, unless they 
are calibrated for emissivity and - for this application - also have a 
very small spot measurement size.

The temperature readings depend on the emissivity of the surface. Cheap 
IR thermometers assume a fixed emissivity of 0.95, which is a poor guess 
for bright metal and even worse for glass. IR thermometers with a 
variable emissivity setting need to be calibrated against the same 
material at a known temperature (otherwise you can change the emissivity 
setting and make the temperature read whatever you like).

And then there's spot size. Even a good $500+ IR thermometer has quite a 
large spot size, typically 0.5-0.75in across, and that's much too big 
for a small object such as a tube pin. Specialized instruments with a 
smaller spot size  are going to be much more expensive.

Also the covers of the amp affect the air flow pattern, so they need to 
be in pace for valid temperature readings. You point the IR thermometer 
at the hot surfaces inside... exactly how?

Sorry, IR thermometers have their uses, but determining tube pin and 
seal temperatures probably aren't two of them (unless you're in the 
giant tube league).

I'm afraid it's back to temperature-sensitive paints, crayons and labels 
- try Omega Corp in the USA for a very good selection.


Earlier,  I'd asked:
>>> Steve, do you know if there is anything freely available on the net for
>>> heat and mass transfer modeling, equivalent to the way NEC-2 is freely
>>> available for antenna modeling?
>>>
>>> //Ian, no I don't.  I honestly doubt there's anything very good available,
>>> or my company probably would not have invested $60K in Flowtherm!
>>> -WB2WIK/6

Sorry to hear that...


-- 
73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                           'In Practice' columnist for RadCom (RSGB)