The glass temperature changes dramatically depending on where the temperature is sampled, so putting a thermocouple on the top section of glass won't represent the hottest area. If a thermocouple is used to get a valid reading then it needs to be a very small thermocouple, with very fine wires, so as not to disturb the local glass temperature, or wick away heat.
The outcome of the thread so far is that the FLIR reading is suspect, so it would take some effort to make comparable measurement to clarify the situation, unless of course you can make that effort and report back?
The other issue is that the type of glass used for high temp rated output valves may be significantly different to the glass used by more mundane valves, and just the higher temperature reached by the glass in some output tubes, may vary the way an instrument such as a FLIR responds to the glass and what is behind the glass.
The outcome of the thread so far is that the FLIR reading is suspect, so it would take some effort to make comparable measurement to clarify the situation, unless of course you can make that effort and report back?
The other issue is that the type of glass used for high temp rated output valves may be significantly different to the glass used by more mundane valves, and just the higher temperature reached by the glass in some output tubes, may vary the way an instrument such as a FLIR responds to the glass and what is behind the glass.
Last edited:
I'm interested in measuring the temp of the output tube, in my case 5881/6L6. They take the most abuse, I don't think the preamp tubes are burning too hot anyway. I agree the IR measurements are a little unreliable, if inside the tube is 400-500°C and the glass exterior is under 250°C. Altho, like someone mentioned, putting a piece of paper or something solid on the tube exterior and measuring that with IR sounds like a good idea. But I was thinking of something a little more permanent, like fixing a bimetal thermometer (eg. oven thermometer) on top of the tube and leaving it there to read the temp constantly. But you say the plate is not the best place? I figured heat rises, so the top would be the best place, no?
I have an IR thermometer, and have a bimetal coming soon, so I'm game for some experiments, if anyone has any suggestions or ideas, I'd be glad to give it a go
Thanks
Many diyer IR 'guns' have a 12:1 optic, and a recommended distance to target of 15 to 100cm, so the nominal spot size on a valve would range from 12mm to 90mm diameter. Attaching a 12mm diameter sticky label to a hot output stage valve is starting to be a concern. Nomex or some similar high temp paper would be required I'd suggest.
Any physically large metal sensor will both wick away heat from the sensing point (ie. inaccurate reading), and likely not provide much thermally conductive surface contact area with the glass (unless you use a thermal paste to fill in the surrounding air gap).
If you search on valve/tube shields there are a few presentations on valve glass temperature measurements, and results on how temperature changes with location.
'Heat rises' refers to the free air convection around a hot valve, not the temperature of the glass covering of the valve.
Any physically large metal sensor will both wick away heat from the sensing point (ie. inaccurate reading), and likely not provide much thermally conductive surface contact area with the glass (unless you use a thermal paste to fill in the surrounding air gap).
If you search on valve/tube shields there are a few presentations on valve glass temperature measurements, and results on how temperature changes with location.
'Heat rises' refers to the free air convection around a hot valve, not the temperature of the glass covering of the valve.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.