teflon capacitors

[tinbergen at ksw.rug.nl]

++++++++++++++++++++++++++++++++++++
+ From:	Jaap Tinbergen		   +
+	Kapteyn Observatory	   +
+	Mensingheweg 20		   +
+	NL-9301 KA Roden	   +
+	Netherlands 		   +
++++++++++++++++++++++++++++++++++++

OUR OUTWARD INTERNET CONNECTIONS ARE COMPLICATED AND SOMETIMES
GENERATE DISINFORMATION. PLEASE IGNORE THE "FROM" OR "REPLY TO" 
ENTRIES AND USE ONE OF THE ACCESS OPTIONS LISTED AT THE END 
(PREFERABLY THE INTERNET ONE). 

=====================================================================


I wonder whether 30-year old history is worth hearing about. Here goes, 
just in case. The main point is: some polystyrene capacitors are good, but 
Teflon probably more reliable if you can get it. 

We needed high-quality integrators for PMT photometry of 0.1% precision 
over several decades of photocurrent and ended up with auto-ranging models,
published in Bull Astr Inst Neth vol 20, pp279--299 (1969!!). I still have
a few copies, if needed. 

Teflon capacitors were not yet available then (or very expensive) and we
(Walraven) selected polystyrene on the basis of HF loss angle reports and
quick trials. We ended up with our local firm Philips; those capacitors 
(5000 pF and 10, 100 times that) were fine, typical results: 

linear to better than 0.05% over 1-10 volt

leakage less than 0.02% per minute 

capacitance ratio 5000/50000/500000 constant to better than 0.1% over temp 
range 15 t0 40. 

We found dielectric absorption to be `marginal for our application' (30
second soak or longer, 3 sec discharge just before the next integration,
0.1% over a 10 to 1 dynamic range), but we state:"careful selection of the
brand used is worthwhile". Dielectric absorption at 40 degrees C twice as 
large as at 20 degrees.

I no longer have the details of those experiments, but we still use one of
those auto-ranging integrators in our optical lab; if necessary, I could 
probably do a few expts.    But I suppose the moral of the whole thing is
that if your application is critical, you just have to get a few different 
brands and models and experiment again beofre you lay in a sufficient stock 
to last you for years. 

Rgds,     Jaap Tinebergen



=====================================================================

Internet   :  tinbergen at ksw.rug.nl   

NOTE:  MOST DUTCH TELEPHONE NUMBERS WERE MODIFIED ON OCTOBER 10 1995. THE 
OLD NUMBERS REMAIN VALID UNTIL APRIL 10 1996.   MY NEW NUMBERS ARE:

Phone      :  my extension	   31 (0)50 5028815
	      secretariat	   31 (0)50 5028888
              
TeleFAX    :  			   31 (0)50 5028800
                                   
==========================================================================