> I've been asked for a thumbnail model of how the
> thermocouple I've superglued to the common lead
> of a laser diode to measure its temperature rise is
> impacting the measurement.


Starter for 10 points.
No guarantees of any relevance whatsoever ... :-)

The lead will be affected by the temperature of the diode (one can 
hope).

It will be lower due to

- Conduction down the lead to and past the measuring point. A thermal 
temperature divider will act.
- Radiation from the lead.
- Convection from the lead due to air currents.

Adding the thermocouple will (perhaps) -

- Add conduction down the thermocouple leads. This will decrease the 
downstream thermal resistance so the temperature will drop. How much 
depends on the thermal division ratio and the fraction that conduction 
forms of the total heat loss model.

- Add radiation from thermocouple parts that rise in temperature. But 
may block some existing radiation.

- Add more area for convection to act on but may block existing air 
access to higher temperature parts.

Overall I'd *guess* that the addition would lower the temperature 
somewhat.
An indication of what is really happening may be able to be obtained 
as follows.

Add sensor.
Let temperature stabilise and note reading.
Remove sensor. Leave LASER diode operating.
Attach sensor to a variable temperature heat source (such as a 
temperature controllable soldering iron) and adjust temperature source 
until sensor reading is as it was before.
Transfer sensor from heat source to LASER diode as rapidly as 
possible.
Note sensor reading history.

If the temperature is higher without the sensor you may expect the 
reading to initially rise and then to settle back to the same value as 
before.

If the temperature is lower without the sensor you may expect a dip.

How large such dips or peaks are will depend on sensor time constant, 
time to transfer and how much movement through the air affects the 
sensor. Doing dummy runs where you take the sensor off the heat source 
for several seconds and then replace it may give some indication of 
sensitivity to movement in air.

As an alternative you could try IR measurement or a very small 
thermocouple using fine appropriate wire or an RTD sensor - which can 
be very small.

IR measurement tends (in my experience) to best have a target area all 
at the same temperature as if there are several sources you get some 
sort of averaging. Very small thermocouples may need smaller wire than 
is readily available but a keen enough masochist could probably worry 
a small amount of appropriate wire into a thinner cross section. It 
may be possible to take a weld joined wire pair and extrude it by 
careful hammering or squeezing with arcane instruments. RTDs can come 
very small and are quite accurate.


        Russell



 

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