C-41 DIY kits: How does ISO count for developer exhaustion?

[Buzz-01]

So yesterday I've successfully developed my first color negative film, using a Compard/Rollei Digibase Ready To Use kit.
I've developed it at 30°C rather than 37.8°C so that the processing times would be a little longer and therefore more forgiving for any possible timing errors I'd be making during pouring in and out the chemicals etc.
To the eye the negs look good, will scan them somewhere in the next couple of days and find out how well everything really went.

Now, in the instructions it says that the kit can be used for 14 rolls of 36 exposures, extending times somewhat every 5 rolls.
First roll was a 24 exposure Kodak ColorPlus 200, so that should count for approx. 2/3 of a roll I guess (as I assume the 14 roll yield is based on film surface area).
On several places of the Internet, I've read that people say that higher ISO films will more extensively exhaust the developer than lower ISO films (maybe even bleach and fix too?), but the kit's instructions don't mention anything about this.
I've also checked other manufacturer's instructions (Cinestill, Tetenal), but so far have not found any information whether this is true.

Since days are getting shorter I've loaded up one of my cameras with Lomography 800, so I'll also want to develop some high ISO film with the kit.
Does anyone know from either personal experience or information which I might have not found myself, whether I'd need to count high ISO film as more than one film in terms of C41 kit yield?

 

[MattKing]

Unlikely - although subject matter (lots of light subjects leading to dense negatives vs. lots of shadowed subjects leading to almost transparent negatives) might make a small difference.
I would strongly recommend using the proper temperature. Changing the temperature leads to crossover and colour casts, with crossover being much more challenging to correct for when post processing, and almost impossible to correct for when printing optically.
Purple highlights and green shadows look awful!

 

[Mr Bill]

Unlikely - although subject matter (lots of light subjects leading to dense negatives vs. lots of shadowed subjects leading to almost transparent negatives) might make a small difference.

Actually this can make a BIG difference.

Does anyone know from either personal experience or information which I might have not found myself, whether I'd need to count high ISO film as more than one film in terms of C41 kit yield?

Like Matt indicated, the level of "exposure" can have WAY more effect on exhaustion of the developer than the type of film. Although film type definitely can have an effect.

If you want some hard data on a few film types, look at Kodak's Z manual for C-41. Somewhere in the section on "continuous" machines there will be "replenishment rates." I'm guessing this may be a new idea for you - in essence, commercial processors do not generally use a developer a bit and then discard it. Rather they use it a bit and then add a bit of extra-strength developer (known as "replenisher") to restore it to its original strength. Then just keep on using it indefinitely. Anyway, if you find the section of the Z manual that gives "replenishment rates," you'll find one rate for (generally) lower speed films and a higher rate for higher speed films. So very roughly the comparison of suggested replenishment rates are proportional, in a reversed way, to the "capacity" of the developer. Note that in the real world processors done stick to those exact replenishment rates; they "test" the extent of development and then change the replenishment rates as needed.

But again, differences in film type are pretty insignificant compared to the overall "exposure" on the film. As an example, completely unexposed film essentially doesn't "use up" developer at all.

Personally, I would say that for the highest quality processing, by which I mean closest to the development specifications made by the inventors of the C-41 process - Eastman Kodak, you should follow the guidelines given in the Z manuals. All C-41 developers use fundamentally the same formula, so if another maker states a greater capacity, this is really more a statement about their quality tolerances than it is about any special capability of the developer.

As a note, back in the day when everything was optically printed, rather than scanned, this was much more important. The results of film development had to match just right to the photo paper characteristics for the best quality. With scanning it is POSSIBLE for a sufficiently skilled person to correct nearly anything.

As a note I spent years overseeing the process quality control in a large lab running miles per day of of our own film - almost exclusively Kodak pro portrait films, and equipped with an onsite chem lab capable of analysis of the major developer components. So I have, in the distant past, been intimately familiar with all of these aspects. And I'm probably more finicky about the color quality than most people, having dealt with this for a living for a lot of years. Doesn't mean that you have to be - if you're satisfied then it's good enough.

If this is "too much information" then the simple answer is: don't worry about the film type.

As a note, I'm probably seen here as a person who tends to give too much info. I want to say that what I'm doing is explaining things the way I would have wanted to hear it in my younger days. These things were "hard-won information" in my younger days; I'm just passing it on for any younger "me" that might be on the forum. You might find, a couple of years down the road, that you occasionally come to an understanding about something, and think to yourself, "now I realize what that guy was trying to tell me." Best of luck in your explorations.

 

[Buzz-01]

Thank you both for your replies. Regarding temperature, it was my first time processing C-41 and as I had seen some acceptable examples from others at even 25°C, I thought I'd just give it a try.
Optically printing color at home is not really feasible for me so that will be scanned anyway (probably making things less critical for me) but on a sidenote I do plan on optically printing some B&W someday (I also shoot B&W film and I do have an enlarger with a B&W head etc just never used it yet).
The roll was just a test roll for a recently acquired camera so nothing too important. I haven't seen them scanned yet, so it's too soon for me to tell if things went well.
Next time I'll probably be confident enough to work at 37.8°C though.

Having a background in engineering, I can appreciate the detailed explanation. I always want to try and understand what's going on and why something works the way it works.
The fact that dense negatives will "use up" developer much quicker than a heavily underexposed negative does make sense to me, but then again would the underexposed negative be gentle on the developer but more exhausting to the bleach/fix?

I always read that the exhaustion of chemicals does not happen from one film to another, but what about after storing it for a longer period of time?
The chemicals were delivered in softpacks, in which I keep the chemicals and push out as much air as possible before tightening the lid.
But how would I know whether it went bad in case I would have not used the kit for let's say three months or so?
Would the negatives become totally unusable (so I might develop a test roll first or make some 8- or 10-exposure test rolls by re-reeling some film on empty canisters), or would things probably still be good enough for correcting after scanning (and then get new chemicals ofcourse).
As you might see I'm not afraid to do some experimenting and I do have a lot of questions which will probably be answered in time by growing experience, but as there is a lot of knowledge and experience around here I might just ask anyway...

 

[Chan Tran]

When we used the Noritsu processor it has automatic replenishment and it does take into account of long and short roll but it doesn't know the ISO of the film.

 

[Mr Bill]

The fact that dense negatives will "use up" developer much quicker than a heavily underexposed negative does make sense to me, but then again would the underexposed negative be gentle on the developer but more exhausting to the bleach/fix?

Hi, you're welcome, and glad to hear that you're looking for the tech details.

Regarding exhaustion of developer vs bleach: a big difference between them is that development is done to a specific degree; it is not a complete vs incomplete process. If you were to extend development time, it would keep getting a little more, and a little more development for a long time. So the manufacturer has decided how much is enough, and the whole system is designed around this. Bleach, on the other hand, is a "go to completion" process. Fixer too, for that matter. The goal of the bleach is to "bleach" ALL of the metallic silver (that was "developed") back into a silver halide. Then the goal of the fixer is to dissolve and remove ALL of the silver halide (fixer cannot dissolve metallic silver, thus the necessary bleach step). So in the case of bleaching and fixing, the manufacturer makes sure you have more than enough capacity to do the complete job. It is necessary to go far enough to complete the job, but continuing to bleach beyond this point makes no difference - all of the silver has already been "bleached." The manufacturers put such a large safety factor into the bleach that most people will never see a problem.

A bit about how the bleach works: the C-41 bleach is mainly iron held in solution by a chemical known as a "chelate." The iron goes between two "oxidation states;" initially it is able to "oxidize" the developed (metallic) silver, and in doing so, it gets "exhausted" (it has been chemically "reduced"). A second important thing happens: the bleach also contains a halogen - bromide ion. When the metallic silver is oxidized it readily combines with bromide ion to become silver bromide. So clearly the bleach is losing its supply of bromide ion.

In summary, two things happen to the bleach: it gradually loses its ability to oxidize (bleach) the metallic silver, and it gradually loses its supply of bromide ion. But... the ability to bleach is restored by exposing it to air, reoxidizing it. (In commercial processing we will aerate it by bubbling air through it via a "sparger;" in amateur processing it is probably sufficient to just pour it in and out of tanks, etc., OR perhaps shake it up in a large jar, exposing it to air.) With respect to the bromide being used up, there is such a large surplus that most people won't ever run into this. But if you do, it's not very gradual; it happens fairly suddenly. But this is not such a huge problem provided that you realize what happened; it is possible to rebleach (and fix, etc.) the film in a good bleach, and everything will be fine.

Back to your actual question, "would the underexposed negative be gentle on the developer but more exhausting to the bleach/fix?" An underexposed negative means that there will be very little silver developed (developed means that the silver halide was converted to metallic silver). Consequently the bleach will not have to bleach much silver, so the bleach is not much used-up in the process. In any case the fixer needs to remove ALL silver present, so the fixer doesn't care if it was developed or not. Fixer is seen as "exhausted" when it contains a certain concentration of silver; the rate of fixing slows down as this happens. Also, a second thing happens: when silver builds up in the fixer we end up with various silver compounds in the film that don't wash out so easily, so the long term stability of the image may suffer. So the life of fixer is largely based on reaching a certain silver concentration, and a typical way to test for this is to measure how long film takes to "fix" in used fixer, compared to new fixer.

One other thing that I didn't point out, as you are developing film the film is wet when it enters the bleach. This gradually dilutes the bleach, reducing its effectiveness. But, as I mentioned, bleach has such a large safety factor that this is not much of a problem in amateur processing. Another aspect of this is that the developer has a fairly high pH value, about 10, and the bleach pH is relatively low. (pH is loosely a measure of whether a water-based solution is acidic or "basic," the opposite of acidic. A pH of 7 is neutral, higher than 7 is basic, or alkaline; lower than 7 is acidic.) So the carried-over developer will tend to move the bleach out of its aim pH range. In commercial processing this is counteracted by the bleach replenisher, which will have a lowered pH value. And, commercial machines use use squeegees to strip surplus solutions off the surface of the film. In small scale use, a lot of people seem to use a stop bath between developer and bleach; this will counteract the tendency to raise bleach pH.

Hopefully this is understandable; feel free to ask questions!

 

[Mr Bill]

I always read that the exhaustion of chemicals does not happen from one film to another, but what about after storing it for a longer period of time?

Well, they're all somewhat different, but the most common thing is that they go bad from oxidation. Bleach is essentially immune from this, but developers and fixes can suffer. The manufacturers will typically give suggested life spans; if you look at Kodak data these tend to be conservative. So it's probably pretty safe to go by their numbers. But if you happen to have a some really valuable shots, it's probably worth testing the chems.

Developer, especially a color developer, has a somewhat delicate balance of things. The developing agent is readily oxidized on exposure to air, so this is a bad thing for developers. The main way to protect against this is, aside from being in air-tight containers, is to use a "preservative," a compound that is hopefully oxidized in preference to the developing agent. B&W developers can typically use a large amount of preservative, commonly sodium sulfite. But in color developers the actual amount of preservative is important, and must be kept relatively low - it plays a role in formation of the color image. So color developers are more touchy.

Fixers are more rugged, but they typically also use sulfite as a preservative. In general, if a fixer still has spare sulfite ion it's ok. But once the sulfite is gone, the fixer may "come apart," being seen as "sulfurizing," with perhaps a sediment or cloudy appearance, and maybe a bad smell. You don't wanna use it, even if it does clear film.

But how would I know whether it went bad in case I would have not used the kit for let's say three months or so?

The most sensible way to test a developer is to actually develop a test piece of film. Now, if the developer turned dark, or some other obvious problem, you probably don't want to use it. But to really be sure, develop a small piece of film which you have an idea what it should look like.

What photofinishers do is to process something called a "process control strip." This is something you buy from the manufacturer; they have a precisely controlled exposure and come with a "reference strip" processed by the manufacturer. What you do is to first use a densitometer to read test patches on the reference strip. Then you process your own control strip, read it, and compare to the ideally-processed reference strip. If you are out-of-tolerance you make whatever adjustments are needed.

Now, I don't think someone processing at home wants to go to that much trouble. But something you might consider is to shoot a roll of a specific test image. Process a short piece in the new developer as a reference, then save the rest of the roll. Then, if there is ever a question about the quality of your developer, pull off another strip and process it. You can compare the two visually, side by side on a light box to see if there is a visual difference. Note that film gets what they call a "latent image shift" over time, most of which happens in the first few hours or days. So you would ideally let your reference roll age for several days before using it, then keep it in cold storage, which greatly slows down the shift (make sure to protect against condensation when warming up).

Some people do what they call a simple clip test in the light. Basically they just want to know if the developer is "dead" or not. So they clip off a piece of exposed 35mm film leader and develop for whatever time in the light. It should get dark. Ideally, then, you compare to a reference test from a fresh developer. It's nowhere near as good as an actual test image, but ok as a test for "dead" vs "not dead."

 

[Buzz-01]

This makes it even more clear to me, as I did not yet understand what the bleach step between dev and fix would do (having done only B&W in the past).
As with black and white, I did know the incomplete process of development vs the complete process of fixing, and this is why you can push or pull film in development.
But now it also makes sense as in why the color negs are fully transparent compared to the silvery look of b&w negatives...

I'll think a bit more of what might work for me for testing my chemicals from time to time, things will also depend on how much color development I will be doing within the next couple of months.
Most likely the most convenient method for me might be shooting a test roll and develop a piece of it every time it's been a while between developing sessions.

 

[Adrian Bacon]

Unlikely - although subject matter (lots of light subjects leading to dense negatives vs. lots of shadowed subjects leading to almost transparent negatives) might make a small difference.
I would strongly recommend using the proper temperature. Changing the temperature leads to crossover and colour casts, with crossover being much more challenging to correct for when post processing, and almost impossible to correct for when printing optically.
Purple highlights and green shadows look awful!

+1

Run it at the right temperature. Unless you have a 5 or 8 roll tank, hitting 3:15 +-a few seconds isn’t that hard. In fact, you’ll have a much more difficult time keeping the agitation consistent if doing by hand as C-41 is not BW. You’re supposed agitate every 15 seconds if doing it by hand. Your second challenge will be getting the temperature in the tank right once the chemistry is in. No matter how much you pre-heat the tank the temperature of the developer going in needs to be much higher than you would think because the reels and film will suck a bunch of heat out of the developer to get up to processing temperature.

The best way to test for this is to have some already processed test rolls, put them on the reels, and put them in the tank, and do your normal development regime, except with straight water instead of developer. After 1 minute of “developing” pull the lid off the tank and measure the temperature of the water in the tank. Make a note of it, note down the ambient room temperature, and what the temperature of the “developer” was before going into the tank. Don’t be surprised if your temperature in the tank is way low.

Repeat and adjust the temperature of the “developer” going into the tank until the temperature in the tank at the 1 minute mark is correct. Use dry room temperature film and reels for each test run. The ambient room temperature will effect this a bit, so keep track of that. Once you have that down, the hardest part is just getting the agitation down.

 

[Buzz-01]

Next time I will definitely try processing at 37.8°C/100°F.
I'll run the process with regular tap water brought up to temperature and a test roll, it will indeed be very interesting to see how temperature varies during processing.
As per the instructions of the kit I do pre-soak the film before developing with water at working temperature, to minimize temperature drop of the developer.

I also do a quick wash (2 quick fills with water at working temp) between developer-bleach and between bleach-fix, to minimize cross-contamination.
However, the manual of my kit states continuous agitation for the first 15 seconds and then once every 30 seconds, rather than once every 15 seconds. But indeed I also read people advising to do agitation once every 15 seconds.
How big is the influence of this difference and how would it show on color negative film?

As for the results of my first test roll, the scans look quite nice.
No strange color casts that I can notice after scanning, other than some minor individual adjustments needed per image to get them to my likings. But perhaps my scanner already compensates for any color cast that might be present, I'm not sure about that.
So far I'm happy with what I've got!

 

[blockend]

No matter how much you pre-heat the tank the temperature of the developer going in needs to be much higher than you would think because the reels and film will suck a bunch of heat out of the developer to get up to processing temperature.

This has always been my doubt with C41 home processing, and I've done a lot. We know large surface areas dissipate heat effectively, which is why air-cooled motorcycle engines get by with finned cylinders alone. Depending on the ambient temperature, three-and-a-quarter minutes of chemistry running past the huge surface area of several films and the spirals that hold them, must represent a massive potential heat loss. It would be interesting to compare before and after temperatures of chemistry entering and leaving a typical developing tank, my guess is the difference would be easily sufficient for colour changes in the negative.

Re. film longevity, there is a home colour development group on Flickr, and some of the numbers claimed exceed manufacturer's advice by several factors. It's partly how high your technical standards are, and how good you are at estimating time increases for tired developer. I've found age from oxidation kills C41 almost as fast as exhaustion from use. Optimum development numbers will be a backlog of film processed in a new kit over a short period, with regular checks on temperature. It'll probably need an extra pair of hands to keep things moving. Home development of an occasional roll of colour negative film is barely worth the effort, it will be as cheap to send the film to a lab, though someone will be along to disagree!

 

[Adrian Bacon]

This has always been my doubt with C41 home processing, and I've done a lot. We know large surface areas dissipate heat effectively, which is why air-cooled motorcycle engines get by with finned cylinders alone. Depending on the ambient temperature, three-and-a-quarter minutes of chemistry running past the huge surface area of several films and the spirals that hold them, must represent a massive potential heat loss. It would be interesting to compare before and after temperatures of chemistry entering and leaving a typical developing tank, my guess is the difference would be easily sufficient for colour changes in the negative.

Re. film longevity, there is a home colour development group on Flickr, and some of the numbers claimed exceed manufacturer's advice by several factors. It's partly how high your technical standards are, and how good you are at estimating time increases for tired developer. I've found age from oxidation kills C41 almost as fast as exhaustion from use. Optimum development numbers will be a backlog of film processed in a new kit over a short period, with regular checks on temperature. It'll probably need an extra pair of hands to keep things moving. Home development of an occasional roll of colour negative film is barely worth the effort, it will be as cheap to send the film to a lab, though someone will be along to disagree!

I run c-41 in a JOBO and don’t pre-wet. I have to have the dev going into the tank at 40.2C with a 21C ambient room temperature and the tank pre-heating on the jobo for at least the processing time. After 1 minute of developing, my temperature in the tank is 37.9C.

 

[Adrian Bacon]

I do pre-soak the film before developing with water at working temperature, to minimize temperature drop of the developer.

That will help minimize the temperature loss, though depending on your tank size and how much chemistry you have going in, and how many rolls of film you process at a time, and the ambient room temperature, you’d be amazed at how fast heat gets lost and you’re low on temperature.

For agitation, C-41 was designed for continuous agitation. Can you do every 30 seconds? Sure. The contrast won’t be the same though. All that being said, you should do what works for you. If you’re getting acceptable results for you doing what you’re doing, then here’s not much point in changing anything unless you want to be more official.

 

[blockend]

I run c-41 in a JOBO and don’t pre-wet. I have to have the dev going into the tank at 40.2C with a 21C ambient room temperature and the tank pre-heating on the jobo for at least the processing time. After 1 minute of developing, my temperature in the tank is 37.9C.

I sink develop, with tank and chemistry standing in warm water for ten minutes prior to development. The variables are too great to contemplate, but if the dev is going in at around 40 degrees, I'm good. Blix temp is however close I can get. Strangely, I've found negatives turn out at least as nicely as lab processing, so there's some latitude.

C41 kits were designed for photojournalists back in the day to process films in their hotel room.

 

[Buzz-01]

My tank is a one-reel Paterson System 4 so I can only process one film at a time. Until now this is enough for me, as it's solely a hobby and I rarely shoot more than one roll a week or so. Usually much less.
It's more about the fun and convenience (in terms of speed, lab turnaround time is a week) to do it at home rather than sending it out to a lab, even though the break-even point with this ready-to-use kit is very low at at 7 or 8 films compared to the lab I use.
And the lab is cheap at 3 euros per roll!

I also develop around the sink and with B&W I don't take any extra heating efforts other than making sure my chemicals are around 20°C when I start. Since ambient is also around 20°C, things stay at working temp during the whole process.
For the next time I'll do color I am thinking of using a bowl (as used for washing the dishes) with warm water to keep the tank at or at least close to the correct temperature during development.
The extra water inside the bowl will allow less heat to escape, it would need some testing to see how much that helps ofcourse (but hey, it's only 3 minutes and 15 seconds). A test run using water as "developer" will tell!
Heating up the chemicals is also done in a large bucket of hot water and it took about 10 minutes to reach 30°C, so that will take maybe 20 minutes to reach 38-40°C.
For the small amounts of film I'm currently developing, it's just not yet worth the effort to me of buying extra equipment, and i'd probably never need extra equipment as long as I'm happy with the results I get.

 

[MattKing]

For the next time I'll do color I am thinking of using a bowl (as used for washing the dishes) with warm water to keep the tank at or at least close to the correct temperature during development.

Consider an insulated picnic cooler instead - something like this: https://www.amazon.de/dp/B000IBWI42...s=p_89:Coleman&rnid=669059031&s=sports&sr=1-3

 

[Adrian Bacon]

My tank is a one-reel Paterson System 4 so I can only process one film at a time. Until now this is enough for me, as it's solely a hobby and I rarely shoot more than one roll a week or so. Usually much less.
It's more about the fun and convenience (in terms of speed, lab turnaround time is a week) to do it at home rather than sending it out to a lab, even though the break-even point with this ready-to-use kit is very low at at 7 or 8 films compared to the lab I use.
And the lab is cheap at 3 euros per roll!

I also develop around the sink and with B&W I don't take any extra heating efforts other than making sure my chemicals are around 20°C when I start. Since ambient is also around 20°C, things stay at working temp during the whole process.
For the next time I'll do color I am thinking of using a bowl (as used for washing the dishes) with warm water to keep the tank at or at least close to the correct temperature during development.
The extra water inside the bowl will allow less heat to escape, it would need some testing to see how much that helps ofcourse (but hey, it's only 3 minutes and 15 seconds). A test run using water as "developer" will tell!
Heating up the chemicals is also done in a large bucket of hot water and it took about 10 minutes to reach 30°C, so that will take maybe 20 minutes to reach 38-40°C.
For the small amounts of film I'm currently developing, it's just not yet worth the effort to me of buying extra equipment, and i'd probably never need extra equipment as long as I'm happy with the results I get.

If you were to buy anything extra, get a Sous Vide cooker for your tempering bath. It is very good at very accurately maintaining your tempering bath temperature. You basically put it it the bath, plug it in, put your chems/tank in, set the temperature you want and go do something else for half hour or so. It will get everything up to temp as quickly as it can get there and maintain it for as long as you want with no other effort on your part. They’re not very expensive and make developing life so much simpler.

 

[Buzz-01]

A sous vide would be the first thing I would probably put on my list for the future. But as long as developing stays limited to one roll at a time / session, I don't think I'd really need it. But time will tell...
The picnic-cooler is a smart idea though, didn't think of that and I already have one at home!

 

[MattKing]

A sous vide would be the first thing I would probably put on my list for the future. But as long as developing stays limited to one roll at a time / session, I don't think I'd really need it. But time will tell...
The picnic-cooler is a smart idea though, didn't think of that and I already have one at home!

And the picnic cooler + a Sous Vide would mean you are golden!

 

[Buzz-01]

I developed a test roll and was surprised with how easy the developing step was at around 38°C. Unfortunately I could not use the picnic cooler for now but tried the dish washing bowl instead.

I put my chemical softpacks in a large bucket with 45°C water to warm up and covered the bucket with a lid. This kept the temperature inside high enough to warm up the chemicals.
When the chemicals were warmed up and I was ready for action, I half-filled the dish washing bowl with 40°C tap water and started processing.
I put the developing tank in the dish washing bowl as much as possible between filling/agitating and emptying, to keep the temperature as stable as possible with this simplistic setup.
Pre-soaked the film twice with 40°C water, both times for 2 minutes (so 4 minutes in total).
Then the developer went in the tank at 39°C and came out after 3:15 at 35.5°C.
I left the other chemicals inside the bucket until just before I needed them.
So at 2:45 during developing, between agitation steps, I filled a measuring beaker with bleach to be able to fill the developing tank quickly enough with the bleach and later I did the same with the fixer.
This way all chemicals would stay up to temperature inside the bucket for as long as possible.
I took notes but don't have them at hand, I believe the bleach went in at 37-36°C and the fixer went in at 34°C.
I kept a timing chart for both bleach and fixer nearby, and read the required times for the specific temperatures just after pouring them into the tank.
As bleaching and fixing are finite processes, I increased those times a little just to be on the safe side.

After drying, the negatives look quite good. I quickly scanned and roughly post-processed one image and I'm very pleased with the result!
No scientific analysis of how they would be when printed the analog way, but since I'm not able to do that anyway the quality of the scan is what really matters to me anyway.

In the overall process, the chemicals did still cool down quite a bit, but I think all went well enough for me to have a workable flow, at least for processing one roll at a time only.

 

[Adrian Bacon]

I developed a test roll and was surprised with how easy the developing step was at around 38°C. Unfortunately I could not use the picnic cooler for now but tried the dish washing bowl instead.

I put my chemical softpacks in a large bucket with 45°C water to warm up and covered the bucket with a lid. This kept the temperature inside high enough to warm up the chemicals.
When the chemicals were warmed up and I was ready for action, I half-filled the dish washing bowl with 40°C tap water and started processing.
I put the developing tank in the dish washing bowl as much as possible between filling/agitating and emptying, to keep the temperature as stable as possible with this simplistic setup.
Pre-soaked the film twice with 40°C water, both times for 2 minutes (so 4 minutes in total).
Then the developer went in the tank at 39°C and came out after 3:15 at 35.5°C.
I left the other chemicals inside the bucket until just before I needed them.
So at 2:45 during developing, between agitation steps, I filled a measuring beaker with bleach to be able to fill the developing tank quickly enough with the bleach and later I did the same with the fixer.
This way all chemicals would stay up to temperature inside the bucket for as long as possible.
I took notes but don't have them at hand, I believe the bleach went in at 37-36°C and the fixer went in at 34°C.
I kept a timing chart for both bleach and fixer nearby, and read the required times for the specific temperatures just after pouring them into the tank.
As bleaching and fixing are finite processes, I increased those times a little just to be on the safe side.

After drying, the negatives look quite good. I quickly scanned and roughly post-processed one image and I'm very pleased with the result!
No scientific analysis of how they would be when printed the analog way, but since I'm not able to do that anyway the quality of the scan is what really matters to me anyway.

In the overall process, the chemicals did still cool down quite a bit, but I think all went well enough for me to have a workable flow, at least for processing one roll at a time only.

Sounds like you’re on your way! As I said before, you’d be amazed how at how much temperature you lose. Now that you have an idea of what your temperatures are, you can adjust from there.

The only step that is particularly temperature sensitive is the first step. The other steps do need to be about 100 degrees, but can vary quite a bit. For the developer step, I’d recommend spitting the temperature differential evenly, so if the developer is going in at 39 and coming out at 35.5, that’s a drop of 3.5, so If it were me, I’d leave everything else the same, but boost the developer going in to 41. It would then come out at ~37.5, everything else being equal.

 

[Buzz-01]

Indeed, I expected the tank to stay up to temperature a little more than this. I'll try starting at 41°C next time and measure the steps again.