Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Plate temperature measurement
Plate temperature measurement
Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 1st September 2019, 07:09 PM   #11
Duke58 is offline Duke58  United States
diyAudio Member
 
Join Date: Jan 2009
Radiated heat...
  Reply With Quote
Old 1st September 2019, 07:53 PM   #12
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
So it seems that much, but not all, of the IR from a hot anode is absorbed by the glass, then some is re-radiated and some is convected. Hence pointing an IR thermometer at a valve will not tell you the anode temperature, and might not tell you the glass temperature accurately either.

There was a thread on here last year about measuring glass temperature. I forget the conclusion, but it was agreed that it was difficult. Glass is not a good black body radiator and its IR properties vary with temperature/wavelength; it is a poor heat conductor so measuring with a thermocouple is not easy either.
  Reply With Quote
Old 1st September 2019, 08:55 PM   #13
Mark Tillotson is offline Mark Tillotson
diyAudio Member
 
Join Date: May 2018
Location: Cambridge UK
Quote:
Originally Posted by rayma View Post
Is the tube's glass envelope transparent to infrared light, or not?
I'd say not much, since the glass gets pretty @#&^%#$! hot
from the radiation from the plate.
Very few materials are fully transparent to far infrared. Glass absorbs heavily, all you can measure with a thermal camera or themometer is the temperature of the outside of the glass (and only imperfectly because glass also reflects heat from the surroundings.

The materials that are broadly transparent to far IR heat radiation are the same materials used for thermal camera lenses, basically germanium, silicon, calcium fluoride. zinc selenide, and others.
  Reply With Quote
Old 1st September 2019, 10:52 PM   #14
trobbins is offline trobbins  Australia
diyAudio Member
 
trobbins's Avatar
 
Join Date: Feb 2009
Location: Melbourne, Oz
Some good references:
http://support.fluke.com/ircon-sales...62_ENG_A_W.PDF
1961 RCA, page 261 and nearby.
http://www.lirkorea.com/Landinstrume...ermometers.pdf

I measured the thickness of the glass for a horizontal sweep power tube, and it's circa .027 inch or 0.7mm.

Last edited by trobbins; 1st September 2019 at 10:56 PM.
  Reply With Quote
Old 2nd September 2019, 10:26 AM   #15
pcbampmaker is offline pcbampmaker  New Zealand
diyAudio Member
 
Join Date: Aug 2019
Quote:
Originally Posted by pcbampmaker View Post
it's very unlikely that this emission would be lambertian, because the emitting surface is very smooth, so the expectation would be to have most radiation coming off at a normal angle to the surface.
This is in fact also discussed in the Application Note mentioned by TRobbins, when they say that the angle under which instrument sees the surface makes a big difference in the reading

Quote:
Originally Posted by pcbampmaker View Post
Looking at convective transfer data between air and glass it seems that the convection coefficient for "air in the house" is h=10-15 W/m^2K (figures used in architecture to estimate windows heat loss). So the convective heat flow through the envelope should be h*A*\Delta(T). A is ~0.01 so we have one/one and a half watt per 10 deg C difference between envelope and air around it. To move 18W we'd need the envelope at approx 200C (20+180) if it stayed in continuous contact with fresh, room temperature air. If we instead say that the air touching the glass is more like 60C (because it doesn't flow fast enough), the heat exchange rate goes to 14W approx.

Putting the glass radiation and convection transfer mechanisms together we get that glass at 132C radiates away 11.6W and convects away 6.45W. I wonder how one could measure the actual temperature of the glass to verify what's going on here...
On second thought I don't think this idea of using 60C air is sound. I've convinced myself that the whole point of the convection coefficient is to account for air convecting upon contact with the hot surface, and that it's the "room" temperature of the air to use, not the temperature of the air very close to it (which is why I was using 60C). So, putting that back to 20C, we'd get that the envelope of a 6L6 at 113C radiates 9.6W and convects 8.33W, for a total of 17.9W.
  Reply With Quote
Old 2nd September 2019, 10:53 AM   #16
pcbampmaker is offline pcbampmaker  New Zealand
diyAudio Member
 
Join Date: Aug 2019
Quote:
Originally Posted by trobbins View Post
The tube glass is quite thin,
0.7-0.8mm according to my caliper, unfortunately the one 6L6 I have at hand that is broken has all the markings rubbed off, and I can't tell what brand it was.

Quote:
Originally Posted by trobbins View Post
and the plate is circa 400-500C for a 6L6 at rated dissipation (there is a nice assessment of this in RCA 1962),
Well so yes: my spreadsheet says you need 400C of avg temperature to get 31.67W of emission. 430C eyeballing away the "stitching" sides, and only keeping the area of the anode proper. It'd probably be better to reduce it further because the beam forming structure I'll bet creates a substantial shadow on the plate.

Quote:
Originally Posted by trobbins View Post
(if you look up the transmission characteristic of common glass to wavelength (there is a nice IRCON app note AN109 on this)
Well so. To this: I had posted a like before, from which I thought you could see clearly the absorption wasn't much. However, upon looking for better information I found that Wikipedia says tube envelopes are made of borosilicate glass (see here) because of its low thermal expansion coefficient, which I guess helps keeping the pin sealing tight. Further this Wikipedia page says that common commercial names for this are: Borcam, Borosil, DURAN, Pyrex, Supertek, Suprax, Simax, BSA 60, BSC 51 (by NIPRO), Heatex, Endural, Schott, Refmex, Kimble as well as BK7 and B270.
It also says

Glasses containing 1525% B2O3, 6570% SiO2, and smaller amounts of alkalis and Al2O3 as additional components have low softening points and low thermal expansion. Sealability to metals in the expansion range of tungsten and molybdenum and high electrical insulation are their most important features.


This plot from the Crystran manufacturer shows that transmission falls to essentially 0 at about 5um

Click the image to open in full size.

Quote:
Originally Posted by trobbins View Post
and do some mental gymnastics for the wavelength emission spectrum of a circa 500C black body. The plate radiation that gets absorbed by the glass then mostly transfers by convection.
So yes, if these plots are more representative of our situation, it seems we'd expect the envelope to be at about 110-120C, for a tube dissipating approx 18W.

Quote:
Originally Posted by trobbins View Post
You would need to check your FLIR wavelength response, and compare that to the glass characteristic to see how much the FLIR is seeing the glass as opaque.
I got the plot here: I hope to have some time tomorrow to scan it and implement what I think the instrument computes in python, just out of curiosity.

I guess the only thing that remains unexplained is this assertion that all heat is convected away at this point... Even at 170C for the glass you get 30W total dissipation of which 16.65W due to radiation and 13.35W to convection.
  Reply With Quote
Old 2nd September 2019, 10:58 AM   #17
pcbampmaker is offline pcbampmaker  New Zealand
diyAudio Member
 
Join Date: Aug 2019
Quote:
Originally Posted by Tubelab_com View Post
Simple dumm blonde experiment......most literature states that steel will begin to exhibit a red glow somewhere between 400 and 500C, in fact the number 460C is often quoted.
I'll look into this when I hook up my python workbook, but my spreadsheet says that a 6L6 anode at 460C (average temp, admittedly) is dissipating 36W, isn't suprising you'd get (mild) replating when you're just 20% above spec? I mean, at 450V this thing is passing 80mA. It's hot, but does it feel hot enough?
  Reply With Quote
Old 2nd September 2019, 11:08 AM   #18
Sorento is offline Sorento  Europe
diyAudio Member
 
Sorento's Avatar
 
Join Date: Aug 2017
from "Kohl 1960 Materials and Techniques for Electron Tubes"
  Reply With Quote
Old 2nd September 2019, 11:37 AM   #19
pcbampmaker is offline pcbampmaker  New Zealand
diyAudio Member
 
Join Date: Aug 2019
Quote:
Originally Posted by DF96 View Post
So it seems that much, but not all, of the IR from a hot anode is absorbed by the glass, then some is re-radiated and some is convected. Hence pointing an IR thermometer at a valve will not tell you the anode temperature, and might not tell you the glass temperature accurately either.
(PDF) The Potential of Thermophotovoltaic Heat Recovery for the Glass Industry

This paper says (note 14 and 16, page 8) that molten glass is a grey-body radiator of \sigma = 0.9. But the paper below

A Review of Radiant Heat Transfer in Glass | ROBERT GARDON

which I haven't found yet, says in the abstract that heat transfer in transparent glass is complicated by the fact that the "flesh" of the glass participates in the radiant transport. However, I'd suspect that if the absorption at the wavelengths we're looking at is effectively 100% at any thickness of interest, volumetric effects in radiative transport can be ignored, and the material can be thought of as it was opaque, like you would with a piece of obsidian, for example. If that's the case the readout of the glass temperature shouldn't be that far off...

Quote:
Originally Posted by DF96 View Post
There was a thread on here last year about measuring glass temperature. I forget the conclusion, but it was agreed that it was difficult. Glass is not a good black body radiator and its IR properties vary with temperature/wavelength; it is a poor heat conductor so measuring with a thermocouple is not easy either.
Actually I was thinking about this too. I've seen some 6L6's that has a flattish top instead of a curvy, dome-like one. If the top is flat enoug, maybe using a drop or two of hot glue or wax or oil on the glass and sticking a small thermocouple to it might be a way to get a passable reading.
Of course the top is probably the coldest place of the whole envelope, but still, it might be a start... I wonder what's the melting point of blutack Jokes aside, anything that will be compliant enough to conform to both the glass and the probe's tip should do if you give it a minute or five to stabilize

Wasn't it on Morgan Jones's book to use a piece of single-strand wire to hold a heat probe to the side of a tube? I'll be if it's there for a few minutes the read would be fine.

However I myself got into knots wondering how much of the reading is from radiation and how much if is from conduction...
  Reply With Quote
Old 2nd September 2019, 11:40 AM   #20
Sorento is offline Sorento  Europe
diyAudio Member
 
Sorento's Avatar
 
Join Date: Aug 2017
more to read online:

Design manual of natural methods of cooling electronic equipment;
Washington, Dept. of the Navy, Bureau of Ships, 1962
Prepared by James P. Welsh.

vacuum tubes are discussed on pp. 57 ff.

Design manual of natural methods of cooling electronic equipment; ... - Full View | HathiTrust Digital Library | HathiTrust Digital Library

---------------------------------
and also interesting to read online:

Design manual of methods of forced air cooling electronic equipment.
Washington : Dept. of the Navy, Bureau of Ships, 1958.
Cornell Aeronautical Laboratory.

electron tubes on pp. 70 ff.
(the authors mention an "instrumented tube" with a thermocouple welded to the plate)
(not for sale anymore ... ,-)

Design manual of methods of forced air cooling electronic equipment. ... - Full View | HathiTrust Digital Library | HathiTrust Digital Library

Last edited by Sorento; 2nd September 2019 at 11:45 AM.
  Reply With Quote

Reply


Plate temperature measurementHide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
CNC custom aluminum plate, case, heatsink, front plate SunnyCNC engineering Vendor's Bazaar 0 16th September 2017 02:27 AM
nearfield measurement vs listening position measurement pointless?? charlie2 Multi-Way 21 5th November 2015 05:15 PM
Simplified pentode Loftin-White with plate-plate feedback homeskillet Tubes / Valves 17 3rd August 2013 04:58 AM
Dynamic displacement vs resistance vs temperature measurement for ribbon transducer thadman Equipment & Tools 16 22nd December 2009 04:00 AM
Dynamic resistance vs temperature measurement for ribbon transducer thadman Multi-Way 0 3rd December 2009 07:29 PM


New To Site? Need Help?

All times are GMT. The time now is 05:58 PM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 14.29%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Copyright ©1999-2019 diyAudio
Wiki