Consistency of NuArc 26-1K

Hi folks,

I posted the following message at another site, but so far have not gotten a clear explanation for the problem. So let me see if anyone here might want to comment.

This week I have been printing some multiples of a negative in palladium, exposing with a NuArc 26-1K. I have experienced a lot of difficulty in printing exact duplicates, even though the integrator appears to be working as it should. The situation is that I am printing one after the other (exposures of about three minutes) , with just enough time between each printing to change the negative from one piece of sensitized paper to the next. What I am finding is that each printing results in slightly greater density on the print than the last, with the last of four about 1/4 stop more dense than the first of four. I develop all four prints at the same time so processing is not an issue.

I thought for a while that the problem was a coating one, but nothing I did to get greater consistency in coating helped. Then I decided to try to print the negatives with my BLB exposure unit, which is also controlled by a light integrator. Result. Four prints of identical density. And just to be sure I did it again with the BLB unit with the same result.

So I appear to have a problem with the NuArc, but why? If I only print once and then not again for 15-20 minutes I get perfectly consistent exposures with the NuArc. What appears to be the case is that as the bulb heats up the response of the integrator is changed. But why?

Would like to hear from other NuArc 26-1k users to know if you have been able to make exact duplicates printing the way I described above.

Sandy

Sandy, could it be the sensor. It seems like since the bulb is already warmed up you are getting more light even though the integrater its not suppost to make a difference . Its the only thing that makes sense to me.

Steve

magic823 said:

Sandy, could it be the sensor. It seems like since the bulb is already warmed up you are getting more light even though the integrater its not suppost to make a difference . Its the only thing that makes sense to me.

Steve

Click to expand...

But how could that be? Bad sensor?

The sensor is positioned in the upper part of the unit so that it receives light only from the lamp, and it does not move during exposoure. It just seems that as the lamp heats up, the sensor slows the count down.

Might have to contact the folks at NuArc about this.

BTW, just wondering if you remember what happened to the brush that Sharon was using to coat for VDB and kallitype? That was a real Richeson brush (and these fellows are not cheap) and I forgot to take it home. If by any chance you folks took it back to Washington with you, just enjoy. But if it is still there at the Formulary, maybe you can locate it next time you are there and hang on to it.

Best,

Sandy

What may be going on is the sensor is not sensitive in the same wavelengths as the platinum print. As the bulb heats the UV output is rising faster than the visible light. Might take a filter on the sensor and some adjustments in the calibration to even things out.

glbeas said:

What may be going on is the sensor is not sensitive in the same wavelengths as the platinum print. As the bulb heats the UV output is rising faster than the visible light. Might take a filter on the sensor and some adjustments in the calibration to even things out.

Click to expand...

Interesting. That did not cross my mind, but it sure makes sense.

The UV sensor that I have on the BLB bank of lights is very specific for the wavelength of those tubes. This could explain the greater consistency. Plus, I don't think the BLB tubes change tempeature during exposure as much as the NuArc HID lamp.,

Sandy

Sandy,

PM sent about the brush.

Steve

As I was reading thru the thread I was thinking the same thing as Gary.
John

Sounds like a function of heat. If the system is left on prior to exposure (a while, whatever that is) to warm up all parts in the circuit, does that help? I know this is not scientific, but resistance in circuitry varies as a function of heat at times. Somewhere in the loop, resistance must be changing which allows a larger amount of current to flow. No idea which part of the circuit it might be (bulb, transformers, integrator, switches, etc.) so leave the thing on a while and see if it doesn't stabilize after all comes up to a working temperature. tim

Sandy

I have PM'd you with the details for the tech who specializes in these units... I suspect it could be a power supply issues related to heat.

What about a cooling fan and a hose as some larger enlargers use?

vet173 said:

What about a cooling fan and a hose as some larger enlargers use?

Click to expand...

Well, the Nuarc 26-1k has a cooling fan? It comes on as soon as you power up the unit.

Sandy

noseoil said:

Sounds like a function of heat. If the system is left on prior to exposure (a while, whatever that is) to warm up all parts in the circuit, does that help? I know this is not scientific, but resistance in circuitry varies as a function of heat at times. Somewhere in the loop, resistance must be changing which allows a larger amount of current to flow. No idea which part of the circuit it might be (bulb, transformers, integrator, switches, etc.) so leave the thing on a while and see if it doesn't stabilize after all comes up to a working temperature. tim

Click to expand...

I spoke with the technician today at Nuarc and made several of the modifications he suggested, which involved primarily checking the filter of the light senor and re-positioning it. Unfortunatley none of these changes altered the situation.

There is no question in my mind at this point but that you are correct in that heating is allowing a larger amount of current to flow. Leaving it on for several minutes does result it in stabilizing, but having to do this negates much of the advantage of a light integrator. At this point I am actually thinking of attaching an external light integrator and probe, which will be a practical solution for the problem, though not a very elegant one.

BTW, other owner of Nuarc 26-1k might want to check the consistency of their units. I have a good friend who prints with the Nuarc 26-1k and the timer also slows down as the machine heats up.

You can check your machine by doing two timed counts, one you start with the unit cold, the other you start about two minutes later. Just set the unit on 40 units (which is about 4:30 minutes) here, and time it at 30 seconds, 20 seconds, 10 seconds and at the end. There should not be significant difference in timing between the first and second tests.

Sandy