Durst vacuum table and GRANE/GRAHAL glasses

[NormaN]

Hello

I'm back with questions about thirst G139.

To all who have a thirst vacuum table:
Can it be that the table area in the middle drops about about 1 mm? Has the table been "overvacuated"? Or is this intentional (a kind of correction due to Field Curvature)?

To all which use the "Grane / Grahal" image stage as a negative holder: What is the correct geometry and the thickness of the glasses? Appended is a sample picture which I found. It is not clear whether to see the original glasses in the picture ...

Thank you all for your support

[Drew Wiley]

That device has nothing to do with vacuum. It's the negative carrier. The film is held flat between two sheets of totally flat sheets of optical glass. Precise registration and even holding tight focus itself would be impossible if any kind of sag or bowing were involved.

Vacuum is ideally used at the easel plane, on the baseboard below the lens. And yes, it's important not to have to much vacuum draw or the top platen might indeed get distorted inward. You need not only a mild rotary vacuum pump (not peristaltic), but a "bleeder valve" in the vacuum line allowing excess Hg force to be relieved. All of that is really quite simple to improvise; no need for anything brand labeled Durst, or photo anything either.

[NormaN]

That device has nothing to do with vacuum. It's the negative carrier. The film is held flat between two sheets of totally flat sheets of optical glass. Precise registration and even holding tight focus itself would be impossible if any kind of sag or bowing were involved.

Vacuum is ideally used at the easel plane, on the baseboard below the lens. And yes, it's important not to have to much vacuum draw or the top platen might indeed get distorted inward. You need not only a mild rotary vacuum pump (not peristaltic), but a "bleeder valve" in the vacuum line allowing excess Hg force to be relieved. All of that is really quite simple to improvise; no need for anything brand labeled Durst, or photo anything either.

Hello Drew

Thank you for your hint

I really wanted to ask two questions. Have not clearly separated it (excuse my bad English, I use the Google Translator). On the one hand, I'm looking for the dimensions for the glasses in the Grane / Grahal holder.

The topic with the vacuum table makes me more worries:
I used a pump that has been included. I have the pump turned on while the red knob was closed (so the pump went to 80% and then blocked) is it perhaps possible to reverse the process and re-inflate the table again? Sounds a bit silly?

[NormaN]

This is the pump

[Drew Wiley]

Well, it's important your vac easel itself is completely flat, and remains so during use. If it's bowed in now, yes, you could trying reversing the air and see if the top pops back up. But if it were properly made to begin with, with support struts inside, a problem like that wouldn't have occurred to begin with.

I'll have to go measure the correct Grane glass size next time I'm in the lab.

[NormaN]

Well, it's important your vac easel itself is completely flat, and remains so during use. If it's bowed in now, yes, you could trying reversing the air and see if the top pops back up. But if it were properly made to begin with, with support struts inside, a problem like that wouldn't have occurred to begin with.

I'll have to go measure the correct Grane glass size next time I'm in the lab.

Thank you Drew. Would it be possible for you to measure also the thickness of the glass?

I do not understand the inner construction of the vacuum table: the table area is bent inside but the round pins are flush with the table surface.

I suppose that no one was so curious to take the table apart :-)

[Drew Wiley]

Well, there was an original Durst glass thickness, which does not need to be precisely duplicated. I could measure both types - original and well as more recent substitutes.

I don't know how the top of that vac table could be slightly concave unless it was somehow crushed under weight during storage or shipping. It shouldn't be that way.

Your pump is of the traditional rotary vane type once common to the graphics industry, probably made by Gast. You will still need a bleed-off valve to get rid of excess vacuum force, or else you can just create a careful small leak somewhere in the line.

[Drew Wiley]

OK - here are some specs for the Grane 138 :
1) both pieces of glass should be about 145mm X 180mm. Note they have eased edges and slightly rounded corners. But before you order any actual glass, cut a piece of cardboard to this size and make sure it fits correctly when the red adjustment knob is at its tighter position within your specific carrier.
2) Durst glass is 1.57 mm thick; but any nominal 2 mm thickness glass will do. I prefer AntiNewton glass for both sides. If you can't find original Durst glass for this specific carrier, which will probably be impossible these days, order the stock 2mm glass from ScanTech in the Los Angeles area. They can cut and finish to any size. There seem to be other custom sources too.
3) Life gets a quite a bit more complicated if you plan to use the Grahal-Grane system for actual pin-registered-in-carrier multiple exposure purposes. That should get discussed separately.

[NormaN]

OK - here are some specs for the Grane 138 :
1) both pieces of glass should be about 145mm X 180mm. Note they have eased edges and slightly rounded corners. But before you order any actual glass, cut a piece of cardboard to this size and make sure it fits correctly when the red adjustment knob is at its tighter position within your specific carrier.
2) Durst glass is 1.57 mm thick; but any nominal 2 mm thickness glass will do. I prefer AntiNewton glass for both sides. If you can't find original Durst glass for this specific carrier, which will probably be impossible these days, order the stock 2mm glass from ScanTech in the Los Angeles area. They can cut and finish to any size. There seem to be other custom sources too.
3) Life gets a quite a bit more complicated if you plan to use the Grahal-Grane system for actual pin-registered-in-carrier multiple exposure purposes. That should get discussed separately.

Thanks Drew
Good idea with the cardboard box.
At the moment I look in parallel for the variant Grane and a Nega 138 (here you probably have more chances to find original glasses). The company Kienzle and Mr. Heiland (from him I ordered the LED cold light module) may be able to organize something, but need the dimensions.
You can do a lot with the G139. (also stupid see my mishap with the vacuum table) I am thinking of converting the 5x7 to 8x10 in the near future....

Didn't you do that too?

[Drew Wiley]

If you can find a decent condition Durst 138 chassis, there's nothing better for up to 5X7. I have adapted them to 8X10 cold light use, but that doesn't allow for a Grane or Nega 138 carrier. Pin-registered Durst carriers for full 8x10 L184 series enlargers are getting scarce. I have a nice set, but have also made my own custom pin-registered 8x10 carriers. The Heiland folks should be able to accommodate you in that respect if you later decide to move up to 8x10 film capacity, but it will probably be expensive. They can probably source you your Grane glass too if you prefer dealing with them.