[time-nuts] Achievable temperature stability for Thunderbolt

EWKehren at aol.com EWKehren at aol.com
Tue Jan 18 14:12:02 UTC 2011

Looking at ways to optimize GPS-Rb-OCXO performance, using dewar, foam and  
fan cooling, using a fan was an eye opener. I have now come to the 
conclusion  that for the temperature environment we are working in, a fan is the 
best way  for all three. I am using a 5X5X1 cm fan drawing 50 mA at 12 Volt, 
results are  amazing. Realizing that Tbolt and Rb's have to be cooled not 
heated I powered up  a FRS with out heat sink and monitored the back plate.  
With 22 C ambient  it increased on the bench to 58 C. Placing the fan 5 cm 
away, the temperature  dropped to 38.9 C in 2.5 minutes! I repeated the test 
using a laptop heat pipe  with the fan running at 5 Volt and 170 mA the back 
plate temperature dropped to  30 C. Presently running a HP 10811 with similar 
results. I am convinced that I  will be able to get better than .1 C control 
in all three applications.  Having large temp. fluctuations outside this 
time of the year I will take  advantage of the large fluctuations. I think the 
challenge will be at higher  ambient's, it will determine the set point. 
Right now I am looking at the 40 to  45 C range. A small amount of air flow 
goes a long way. Noise and vibration is  small and manageable. 
Any comments? Fan power is lower than heating how ever all three devices  
power consumption is increased overcoming the lower case temp.
Bert Kehren
In a message dated 1/18/2011 6:42:29 A.M. Eastern Standard Time,  
holrum at hotmail.com writes:

Generally you have to do some playing with the fan position and  internal 
baffling and thermal mass for best performance of the temperature  
controller.  You don't want direct airflow onto the tbolt.  Also you  probably don't 
want an large/aggressive fan...  a little airflow can go a  long way.  

Some thermal mass can  be useful...  I use a  2 kg stainless steel 
calibration weight (but probably need to do some testing  without it.  Warren's 
setup seems to be a little better than mine,  but he probably does not need to 
deal with the heating/air conditioning  extremes that I do)

One thing that I have is an ESI SR104  hyper-accurate 10K resistance 
standard.  These have two temperatures  where their resistance is exactly 10K.  I 
have it (and a Fluke 732B 10V  precision voltage reference) mounted in a box 
with a tbolt set to one of those  temperatures...  cheap temperature 
controlled cal lab in a cardboard  box.                    
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