From:	KEKUX::"b.hawes1@physics.oxford.ac.uk" 10-OCT-1996 23:34:38.69
 
OXFORD UNIVERSITY ESPI MEASURING SYSTEM NOTES
 
This is the first draft of what I intend to become a user guide to our service.
With apologies for any rough edges which will be dealt with in the next edition
...  Brian Hawes  12 Sept 1996
b.hawes1@physics.oxford.ac.uk
 
1.  ENVIRONMENTAL CHAMBER
 
 
The present chamber is approximately 1900 mm wide, 500mm high and 300mm deep.
Viewing is through a double glazed window which covers the whole of the front
surface.
 
The rear surface is a reference plate o
nto which the samples can be mounted. This plate is perforated by a rectangular
grid of M8.0 holes on 150mm centres. Thermally insulated stand-off pillars are
provided to bring the sample away from the insulation on the back wall and to
enable full illumination. These have M8.0 tapped holes on the front surface.
Mounting can be e.g. to a sub-plate carrying the sample which mounts onto these
insulators, or single point.  Beware however that a plate of aluminium can move
and distort more than the sample it c
rries, and  kinematic design is sometimes necessary.
 
We have excellent workshop facilities at Oxford, but they cannot always respond
instantly to requests for work.
 
Small items can be mounted anywhere in the chamber, though we prefer to work
near the centre for ease of illumination.
 
The cooling is provided by a NESLAB RTE140 unit. It can operate over a wide
temperature range, but in practise, we can achieve -12 Celsius in the chamber.
The upper limit is about +40 Celsius.
The chamber is continuous
ly purged with dry air, dew point approximately -20 Celsius, to avoid
condensation on the sample or window.
 
 
2.  MODULE COOLING
 
 
A second NESLAB RTE140 provides cooling for the module under test. The working
fluid is, again 45% Ethylene Glycol. This is not ideal because the viscosity
increases as the temperature falls and flow can become quite low if pipe bore is
narrow. We can achieve little more than 100 cc/minute at -10 C in 3mm bore
pipes. It might be possible to use other fluids, Glycol was cho
sen for the first trials because of cost and safety considerations.
The flow is provided in the chamber through 3.2mm bore silicone rubber tubing.
It is normally easy to connect this to most things.
The NESLAB will cool the module flow down to the freezing point of the solution
-20 C.
 
3.  SAMPLE PREPARATION
 
It is not possible to get good ESPI fringe patterns unless the surface of the
object is diffusely reflecting. This means that we have to spray on a coating of
white powder. The material we use is
 white Talc in a Trichloroethane  propellant. There is no adhesive in this and
the powder wipes off easily, although it can never be removed completely from a
complicated object.
It is best to minimise the number of wires and other obstructions which pass
over the front surface: these can confuse the fringe patterns if too many are
present.
 
 
 
 
 
 
 
4.  THERMOMETRY
 
n.b. at the time of writing this system is electrically completed, but no
software has been produced.
The plan is to use LabView.
Ou
r standard thermometry is based on 100 ohm Platinum resistance thermometers and
four terminal
measurement. The thermometers are available as very small thin-film devices on
ceramic substrates. 
Much time can be saved if  items for test can be brought to Oxford with the
thermometers fitted and wired.
Measuring is  carried out with a Keithley 2001 DVM and Keithley 7001 switch
system with two model 7067 ten-channel four wire scanner cards. 
This gives a total  of 20 four-wire measuring channels. Although s
oftware is not yet developed, it is
possible to scan sequentially through the channels, or a subset, manually.
The channels are wired to four 25 pin D-type plugs in the environmental chamber.
Fit sockets to the thermometer wiring.
Pinout is as follows:
 
1	CH1 Sense  H
2	CH1 Sense  L
3	CH1 Source L
4	CH1 Source H
5		GND
6	CH2 Sense  H                 
7	CH2 Sense  L                 
8	CH2 Source L                 
9	CH2 Source H
10		GND
11	CH3 Sense  H                 
12	CH3 Sense  L        
         
13	CH3 Source L                 
14	CH3 Source H
15		GND
16	CH4 Sense  H
17	CH4 Sense  L
18	CH4 Source L
19	CH4 Source H
20		GND
21	CH5 Sense  H
22	CH5 Sense  L
23	CH5 Source L
24	CH5 Source H
25		GND
 
These connections correspond to the pinout on the Keithley scanner cards.
There are four such connectors, giving a total of 20 four terminal thermometer
channels.
 
Wiring is best done with e.g. 40swg double silk covered wire. This is to avoid
conducting significant heat into or out
 of the small thermometer element. Avoid any stress on the connector joints:
solder a small diameter screw, 1 to 1.5mm diameter, to one of the ground pins,
carry the wire through a short loop and tie the bundle to the screw (waxed
dental floss is good for tying bundles of fine wire).
 
A fifth 25-pin D-type plug is provided in the chamber. This is uncommitted, and
intended for heaters or anything else.
 
Connection from the internal connectors to outside instrumentation is provided
by screened ribbon cabl
es.
 
Other instrumentation is available, but are more limited : e.g. we have three
indicators for 3-terminal measurement of Platinum resistance thermometers. We
would like to make the Keithley system our standard.
 
5.  HEATER SUPPLIES
 
We have two 32-volt 2-Amp programmable power supplies for module heaters. It is
not difficult to arrange more, or different supplies of some kind if necessary.