[Since87]

If control needs to be via GPIB, (due to software) the most practical way appears to be to use the 8 bit digital I/O port of the Keithley 706. It would probably be a fairly slow interface, but there's not much need for speed here.

If RS-232 or USB could be used there would be a lot more options. Interfacing to LabVIEW or some other canned software might be more problematic though.

Originally posted by unregistered

the coolant temp is stable (±0.02°C fluctuation), but subject to drift as the outside temp changes form night to day
- the pump, chiller, and cross-flow brazed plate heat exchanger in the chiller bath are all outside
(too much heat and noise)
I suspect that with a DAQ board having a 16 bit analog output it would be simple enough to gin up an interactive chiller control
- the chiller is now remotely controlled with a precision microvolt (calibration) source, but such devices with GPIB control are VERY expensive (0.0001 VDC = 0.01°C)

Hardware wise this isn't difficult. Getting the control right could be a real problem. I expect the chiller bath has a long thermal 'time constant'. It would be a long tedious process ensuring that an automated control loop wasn't doing more harm than good. (Causing very low frequency oscillations.)

Originally posted by unregistered

coolant flow is a can of worms as the system length is ~11' and I have 3 oscillations of different periods
the flow rate is quite stable but understanding the actual range (to 'set' the mean can be vexing)
- I know of no single throttling (needle) valve whose effective range spans my flow rates (0.3 to 4 or 5 gpm),
nor with the sensitivity needed in an 'automated' controller
??
BTW, it does need to be set accurately for the 10 mountings as these are single point readings,
otherwise no, not at all critical - so long as accurately known

How about four electrically controlled on/off valves. Each one followed by a manual 'restrictor' valve. All four sets of valves would be connected in parallel. For maximum flow, all four electrically controlled valves would be opened. For minimum flow, only the electrical valve connected to the most restrictive 'restrictor' valve would be open. Intermediate flows could be achieved by opening combinations of the four electrical valves. It would require some initial setup of the manual 'restrictor' valves to get the right range of flowrates for any given DUT.

Perhaps something like these dishwasher valves could be used. (These look more restrictive than desireable, but the price is right. $9.50 per five at this moment.)

Originally posted by unregistered

air flow can be difficult to stabilize, at specific flow rates the fans may sync, the flow start pulsating, really bizarre
(the compressibility of air is quite apparent)
I started with mechanical dampners, went to huge pots from welding machines (10A of fan current is no joke), to 115VAC fans with Variacs
- don't think its worth the effort (not actually running enough hours, at least for now)

Have you seen fluctuating flowrates with a single fan or only with combinations of fans?

I haven't found a single fan that is spec'd to cover the full range of the rads in your article. It seem like what is needed, is a fan run by a stepper motor, to allow it to cover a wider range of RPM's than is achievable with brushless DC fans.

It wouldn't be difficult to create a computer controlled variable output switching supply to drive 10A worth of fan current, but it still leaves the issue of a fan that covers a wide enough operating range. Can you cover the full range with an AC fan on a variac?

Originally posted by unregistered

I have a different question for you:
in running 4 - 20mA transmitters, I have a huge settling time (as in hours) to read at the 100pA level (Setra units, Foxboro only to 1µA)
which I believe is caused by the transmitters and not the meter, a Keithley 195A
-> can I switch in and out a resistive dummy load in the transmitter current loop when its not being read ?

I'm guessing you meant nA? (nanoamps) I don't believe the Keithley 195A can give you very meaningful data at 100 picoamps.

Are these transducers powered through the 4-20 loop? (I did some looking around on the web, but wasn't sure if I was looking at relevant stuff.)

I'd say a dummy load is worth a try. (Probably even a dead short would be ok.) What would be most likely to give the current output fits, is an open circuit. I'd suggest trying a 100 Ohm resistor and set up a 'make before break' switching arrangement. (i.e. connect both the dummy load and the Keithley in parallel, and then open the connection to the dummy load as a second step.)

If the only electrical connection is the current loop, having the dummy load, (and power constantly applied) will probably make a big difference in settling time.