Print consistency: mains voltage

polyglot · Jan 16, 2013

I did some prints last night and then again tonight; identical exposures (according to my electronic timer) and brand new developer in each case.

The densities are different. Our mains voltage varies a bit and I have a traditional lightbulb enlarger, so my suspicion is that that has caused the exposure error of about 0.15 stops - quite visible in the final print.

So I can see that having some sort of brightness-feedback system as is required for (highly variable) cold heads might also be of some value when printing with tungsten.

Steve Smith · Jan 16, 2013

Is it possible that your timer varies a little bit? It depends on how many cycles of a clock count it uses to count seconds and would have more effect on short times than long.

I made a timer a few years ago and made the mistake of using a free running one second oscillator to advance the counter. At the time of pressing the start button, the next pulse could be anything from immediate to one second away so it had an accuracy of +/- one second. Linking the start switch to a clock reset would have cured this but I didn't get round to doing that!

If it is the mains voltage, would it be possible to replace the bulb with a low voltage type and use a regulated dc power supply?

Steve.

markbarendt · Jan 16, 2013

When I first set up my darkroom this issue popped up. I was on the same circuit as my home office in a converted closet. I unloaded the circuit of other things and that helped, still and yet when the laundry is in process or the dishwasher, I still get it.

The other issue I get is related to how well the enlarger is warmed up. I've got a Beseler PM2L meter and adjust my exposure for each print with it. I can actually watch the exposure needle swing as the bulb warms up.

Michael W · Jan 16, 2013

I know that Gordon at point light uses a voltage stabiliser for all his enlargers.

polyglot · Jan 16, 2013

Steve: the timer is good to 10ms (on a ~25s exposure in this case), i.e. half a mains cycle since it uses a triac. See timer link in my signature.

I note that later DeVere enlargers have stabilized supplies; mine is the 1978ish model (Mk.I) 504 that precedes such niceties. Certainly a regulated DC supply would solve the problem, the only drawback being $$$ for a 12A 24V supply.

I think my solution will be "make a quick test strip before blithely exposing that 16x20 I setup 18 hours ago". I only got into this situation by fogging (during development) the final sheet I made last night and giving up in disgust; came back to redo it tonight and had my little issue. I suspect the airconditioner.

Monophoto · Jan 16, 2013

I presume that the bulb in your enlarger is a conventional incandescent bulb.

The light output from incandescent bulbs varies SIGNIFICANTLY with voltage and is likely to be the cause of your exposure problem. A 5% reduction in voltage (which is within the range of variation that a utility may apply during normal operations) results in a nearly 20% reduction in light output. A 10% reduction in voltage (which is unusual, but possible) can reduce light output by up to 70%! And as the light output varies, the color temperature of the lamp also changes - the output becomes noticeably more yellow as voltage drops.

The impact on timer accuracy not nearly as profound. As you say, if your time looks at zero crossings, then the timing rate is controlled by frequency rather than voltage magnitude, and frequency is unlikely to change dramatically (unless you are in India where frequency is notoriously variable). Voltage variations can affect the timing accuracy of motor-based timers, but I would not expect that magnitude of error to be nearly as significant as the magnitude of lamp output for a given variation in voltage.

Mike Wilde · Jan 16, 2013

I lucked into an old ferroresonant transformer for the right price. It is bolted to the wall, high, and out of the way in the darkroom, and has just enough capacity to power the timer and enalrger light.

They might be rare as hens teeth, but find them , and they work.

Alex Muir · Jan 16, 2013

I have this problem in my own darkroom. I have a heater with a thermostat built in. When the heater kicks in, the lights dim. I don't seem to have a problem with the Leitz enlarger which has a built-in transformer, but I'm sure the simple Durst condenser model is affected. Trouble is I need the heater!

Chan Tran · Jan 16, 2013

polyglot said:
Steve: the timer is good to 10ms (on a ~25s exposure in this case), i.e. half a mains cycle since it uses a triac. See timer link in my signature.

I note that later DeVere enlargers have stabilized supplies; mine is the 1978ish model (Mk.I) 504 that precedes such niceties. Certainly a regulated DC supply would solve the problem, the only drawback being $$$ for a 12A 24V supply.

I think my solution will be "make a quick test strip before blithely exposing that 16x20 I setup 18 hours ago". I only got into this situation by fogging (during development) the final sheet I made last night and giving up in disgust; came back to redo it tonight and had my little issue. I suspect the airconditioner.

You can get this one which is not exactly cheap but reasonable in price.
http://www.automationdirect.com/adc...ules_(RHINO_PSM_-a-_PSP_Series)/PSM24-REM360S

adelorenzo · Jan 16, 2013

I have a couple of heavy boxes that came with my enlarger heads. They make a humming sound when the enlarger is turned on and I belive they do something to control the power levels. Voltage regulators maybe? At any rate, I have them so I figured I should use them, although I am nowhere near that level of precision in my darkroom work.

Jim Jones · Jan 16, 2013

Like Mike, I use an ancient ferroresonant voltage regulator. Mine is a Raytheon brand, although Sola regulators are more commonly encountered. They are simple and durable. Modern electronic regulators should be more efficient and quiet.

polyglot · Jan 16, 2013

Chan Tran said:
You can get this one which is not exactly cheap but reasonable in price.
http://www.automationdirect.com/adc...ules_(RHINO_PSM_-a-_PSP_Series)/PSM24-REM360S

That's pretty much exactly the sort of thing I was considering, except more the brick form-factor than DIN-rail.

In terms of my current transformer, it could well be a ferroresonant as it has two separate secondaries and I think there's a big capacitor in there. Ferroresonance didn't occur to me at the time (I had it open to trace the schematic for the back plug) so now I'm going to have to check.

adelorenzo: they are transformers. They give you isolation from the main supply and step the voltage down from 115 or 240V to (probably) 24V. They may or may not be ferroresonant. You must use them for their voltage-converting role if nothing else.

silveror0: shouldn't matter for regulation purposes. If it's always on (timer after stabiliser) it will use more power, but that's not an uncommon arrangement.

Sirius Glass · Jan 16, 2013

My enlarger has a voltage regulator and I have never had that problem.

pjbc · Jan 16, 2013

Doesn't a computer UPS work as voltage regulator/stabilizer?

Sirius Glass · Jan 16, 2013

pjbc said:
Doesn't a computer UPS work as voltage regulator/stabilizer?

No, it is a backup power supply, not a voltage regulator. It will do nothing in a brown out until and unless the voltage drops below a setting.

cliveh · Jan 16, 2013

polyglot said:
I did some prints last night and then again tonight; identical exposures (according to my electronic timer) and brand new developer in each case.

The densities are different. Our mains voltage varies a bit and I have a traditional lightbulb enlarger, so my suspicion is that that has caused the exposure error of about 0.15 stops - quite visible in the final print.

So I can see that having some sort of brightness-feedback system as is required for (highly variable) cold heads might also be of some value when printing with tungsten.

This has probably nothing to do with your equipment, but a difference in developer temperature and ambient temperature difference.

ic-racer · Jan 16, 2013

polyglot said:
I did some prints last night and then again tonight; identical exposures (according to my electronic timer) and brand new developer in each case.

The densities are different. Our mains voltage varies a bit and I have a traditional lightbulb enlarger, so my suspicion is that that has caused the exposure error of about 0.15 stops - quite visible in the final print.

So I can see that having some sort of brightness-feedback system as is required for (highly variable) cold heads might also be of some value when printing with tungsten.

Variacs are pretty inexpensive. You already own a voltmeter. If it is day-to-day variation in the line voltage then just set your voltage with the variac at the beginning of each printing session. As you probably know, the automatic regulated power supplies that will keep the voltage fixed will be pretty expensive.

polyglot · Jan 16, 2013

cliveh said:
This has probably nothing to do with your equipment, but a difference in developer temperature and ambient temperature difference.

I process prints in a Jobo at 24C. If it's consistent enough for E6, it's more than consistent enough for B&W prints that are developed to completion. And in fact the ambient temp was higher on the day with lower density.

ic-racer said:
Variacs are pretty inexpensive. You already own a voltmeter. If it is day-to-day variation in the line voltage then just set your voltage with the variac at the beginning of each printing session. As you probably know, the automatic regulated power supplies that will keep the voltage fixed will be pretty expensive.

A variac actually turns out to be $$$ in AU, even compared to a switchmode supply

One nice thing to discover from all this though is that a normal photographic incident-lightmeter serves (with the dome removed and the photodiode exposed) as a perfectly good enlarging meter. You just need to ignore the absolute readings and work in differences of stops. I reckon I'm going to have to write down a couple of tables of print zones vs meter readings, which should save me a shirtload of time on test strips. Certainly saved me a bunch of time the other night when printing one image at three different sizes.

Jim Jones · Jan 17, 2013

silveror0 said:
Interesting discussion. Does anyone know the best location in the system to place the voltage regulator, i.e., between the AC outlet and the timer vs between the timer and lamphouse? Does it really matter? . . .

My regulator is between the timer and the enlarger to reduce power consumption and that annoying hum.

Mike Wilde · Jan 18, 2013

It might be nice to putthe ferroresonant downstrem of the timer, from a sound perspective, but the light output could be non-linear to time for shorter expsoures.

Ferroresonant units stabilise output by building and storing a surplus of energy in the form of an extra winding that will keep the magnetic core that powers the output winding at a constant output level over short perionds of time. It does take time to build up that field, and while doing so, the enrgy is not available to go out as ouput power.

Kind of like adding extra mass tuned to adjust the resonant freqency of the suspension of a motor vehicle. Makes for a smoother ride, but the extra mass means moderatly slower acceleration from a stop.

Also when you stop the power, the ferroresonant dumps the extra energy stored in it, but not necessarily in the same voltage profile that it absorbed it in at at the start of the process.
So the light may have a differnet intensity/colur at the end as extra field collapses.

Steve Smith · Jan 18, 2013

But surely as it's AC, we are only talking about one cycle worth of time?

Steve.

polyglot · Jan 18, 2013

Depends on Q.

My understanding is that the regulatory behaviour is from saturation (i.e. the core is grossly undersized) and that the extra resonant winding exists just to reduce additional harmonic content, i.e. prevent it producing just square waves. So the quantity of energy stored by the resonant coil would represent probably a cycle or two's worth at nominal load. But it could well be many cycles, depending on the damping factor of that resonant system.

Ken Nadvornick · Jan 18, 2013

A Zone VI Compensating Enlarging Timer paired with a Zone VI Compensating Developing Timer.

Test strips and prints are freaking identical. Every single time. Even weekends apart, if I use fresh developer from the same batch.

So consistent it's unnerving.

For perspective, I've even run tests on the developing timer, varying a water bath by 0.1F against my standard mercury thermometer. Moving between 67.9F 68.0F 68.1F I've observed the addition or subtraction of ~2 seconds per minute. Rezero and it goes away. Move in the opposite direction and it follows exactly as expected. Back and forth. Consistent as hell.

If the tones are wrong, I can't point fingers. I know it's my fault every time.

I would expect RH Designs more modern hardware might be even better.

Ken

ic-racer · Jan 18, 2013

Another inexpensive solution is a second-hand baseboard color meter. I got a free Melico color meter a while back. The 'white' channel nulls with a good sized wide swinging needle that is easy to read and center.

Print consistency: mains voltage

Privacy & Transparency

Privacy & Transparency