AC pressure & temp results from Arduino Digital Control and Air-Bucket builds

[Discseven]

Charge

Continuous vac for 11 hours. Baseline charge = 20 weighed ozs.---10.4 oz liquid then 9.6 gas. 1 oz gas increments are added. Same measures are logged per add. Car is parked in sun. Doors & windows closed (except for when entering & exiting cabin for pics of control panel display, vent temp, and evap's bottom.) Analog Taylor thermometer is pig-stuck into center dash vent. Engine's at idle and fan's on mid speed except for last 23 oz log. Then, engine's at 2000 rpm and fan's on low. 09.09.18 Weather mostly sunny. Rain showers pass nearby but never head-falls. 1980 911.


Base = 20 weighed oz. charge


21 oz.


22 oz.


23 oz.


23 oz.


Invisibles

Minimum evap operating temp is set at 29 dF. Hysteresis is 3 dF. When minimum evap temp is reached, compressor cycles OFF while evap's temp momentum continues downward by 2 dF... to 27 dF area. When evap temps rise during hysteresis, compressor is set to cut ON at 32 dF. Evap's warming momentum then continues to push upward by 10 dF. This oscillating range of evap's temp is invisible at vent. When comparing vent to evap temps in logs here... they don't always make sense due to evap's constant temp oscillation and this motion being invisible with Taylor thermometer. Another factor is...

“Evap temp” is relative to where evap's temp is taken (inside evap.) In this case there are two temp sensors in evap. While second sensor was to serve only as backup, decided at last moment to wire both temp sensors into control system. One sensor's located at evap's core/center. Second is located at evap's perimeter near discharge port. DPDT switch is installed in dash enabling sensor selection on-the-fly. Invisible is fact that evap's core operates 10'ish dF cooler than perimeter location. This means when 29 dF “minimum temp” is sensed with perimeter sensor selected, core is 10 dF cooler.

Cabin's temp & humidity sensor (DHT21) is located on tub's bulkhead below ashtray area. (Location was chosen given AC's intakes being located at foot level near pedal cluster and behind center console---theory holds there being a constant flow of cabin air past this sensor's location.)

Arduino build: Arduino - Digital AC control system for '80 911
Air-Bucket turned Air-Server: AC – Air-bucket design-build for rear fender condenser

Digital Control Programming is credited to the brilliant creative bullet train that is: "Dr J." Thank you John

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[DRACO A5OG]

Oh Haute Damn Brother Karl!

R12 of R134?

[Discseven]

Hope all's well with you Brother Jim!

R134

[DRACO A5OG]

Wow, those are very good numbers, Congrats!

[KelogGes]

NICE Initial Vent temps, & Pressures, hope we get ambient temps back in the 90s here in south FL so Karl's A/C mixed technology heat exchangers system can be tweaked and fully tested in 92-95 F ambient with humidity and all driving conditions in September/October

GOOD JOB KARL!

[ganun]

Hi Karl, I've had a digital controller for about 15,000 miles now and also settled on a 3 degree delta, I don't worry about the evap display temp for the reason you mention, I just change the set temp as needed to be comfortable. Don't know if you can do it, I suppose so, which is a separate switch to turn the compressor off independently of the blower. Its purpose is to dry the evap . I hated having it soaking wet and garaging the car.
Hope to see you next Saturday at the Parkhaus.
George

[Discseven]

Yes George, "Set Temp" is adjustable. Low is 65 dF. High is 80 dF. Went away immediately after charging system and back a week ago... not feeling well since so haven't had much opportunity to literally test drive system. Am planning to be at Parkhaus.

[Pazuzu]

Originally Posted by Discseven

23 oz.

Can I ask how 75 degree air blowing over a 31 degree evaporator creates 22 degree air?

I wonder if your evaporator temp probe was being covered in ice, and insulated from the MUCH COLDER evap surface.

[Discseven]

Originally Posted by Pazuzu

Can I ask how 75 degree air blowing over a 31 degree evaporator creates 22 degree air?

I wonder if your evaporator temp probe was being covered in ice, and insulated from the MUCH COLDER evap surface.

George... Here's what's happening in nutshell... Minimum operating temp for evap is programmed at 29 dF. This "minimum" is not an absolute but relative to where evap's temp is taken. There are two sensors in evap. One's at core and other's at perimeter near discharge port...



What I found with this set up is core operates 10 dF cooler than perimeter location. (Both temp sensors were tested for matching accuracy while building digital control system.)

After some heat was evacuated from cabin, I switched---for the last 23 oz log---from core sensor compressor control to perimeter sensor compressor control. Doing this means when evap "perimeter control" cycles compressor OFF at 29 dF, core's operating near 10 dF cooler---near 20 dF.

Evap temps also have momentum---they drop lower than "minimum" for a shot period and rise higher than the hysteresis ceiling.

When I took pic of 31.2 dF evap temp, evap temp was dropping towards perimeter's 29 dF compressor cut OFF signal. Compressor had also already cycled through minimum & hystersis a few times.This matters because Taylor, while proven accurate, is relatively slow to change temp---is good if a smoothed out reading is wanted. Had multiple compressor cycles not been allowed before taking vent pic, vent temp would have read higher. Had more cycles been allowed, vent temp would have been lower.

Make sense?

Regarding sensor insulation, it's hard to know what's happening to them. Both sensors are about 1" long and set into evap so sensor area is midway down into evap. What I wonder is whether there's a cooler operating area than evap's core? Might be.

[Dr J]

Coldest temp should be right after the expansion valve. Warmest near the exit. Congratulations Karl in completing this project. Looks fantastic!

[hcoles]

Very impressive project and results. Good job.

[88911coupe]

This is what I dream of...I've done all the parts upgrades but can't get more than a 30 degree drop from ambient. If its 90 degrees about all I can get is about 58-60 at the vent. Core temp 1-2 degrees higher. I assumed I was doing something wrong in the vacuum/recharge (getting air in system) but took it to a long time, respected AC shop here in Dallas to do the vacuum/recharge...got EXACTLY the same results.

[GH85Carrera]

Originally Posted by 88911coupe

This is what I dream of...I've done all the parts upgrades but can't get more than a 30 degree drop from ambient. If its 90 degrees about all I can get is about 58-60 at the vent. Core temp 1-2 degrees higher. I assumed I was doing something wrong in the vacuum/recharge (getting air in system) but took it to a long time, respected AC shop here in Dallas to do the vacuum/recharge...got EXACTLY the same results.

How many condensers do you have? The factory supplied just two, and they are not nearly enough. I added two more (the Dual Khuel setup) and replaced all the lines. I can get very comfortable cabin temps in my 85 Carrera. When I was in Savannah, GA in August with my wife in the car she asked me to turn the temp UP because she was cold at lunch time. When I drove through Needles, CA in July coming from Monterey, CA and had been on the road for hours the AC struggled to keep me cool in the 118 degree heat outside, but I never broke a sweat. When I got to the nice "cool" area of Kingman, AZ it was 100% comfortable in the vehicle.

How new is your evaporator? I did replace mine during my upgrade.

[88911coupe]

Sorry, did not mean to hijack the thread, just wish I was getting these great results. Ive added a new high efficiency condenser (with fan) in the wheel well (still have both originals in place), replaced the evaporator with a new Rennaire model, new compressor and all new hoses. Cleaned out all condensers (seemed clean) then vacuumed and recharged several times over last two summers...by all accounts I should have really good AC.

[GH85Carrera]

Get one more condenser and fan. It is almost impossible to have too much condenser capacity.

[Discseven]

Originally Posted by DRACO A5OG

Wow, those are very good numbers, Congrats!

Thanks your way Brother Jim!

Originally Posted by KelogGes

NICE Initial Vent temps, & Pressures...

You are crazy Reid... Capt. Crazy! Thanks

Originally Posted by Dr J

Coldest temp should be right after the expansion valve. Warmest near the exit. Congratulations Karl in completing this project. Looks fantastic!

John, without your participation, I'd still be learning Arduino programming... and project might possibly be completed in 2020. You deserve much credit here for getting the digital side of system working as well as it does. Genuinely enjoy our collaboration!

Originally Posted by hcoles

Very impressive project and results. Good job.

Thanks HC! Glad you followed project to this point. (Am not done.)

Originally Posted by 88911coupe

This is what I dream of...I've done all the parts upgrades but can't get more than a 30 degree drop from ambient. If its 90 degrees about all I can get is about 58-60 at the vent. Core temp 1-2 degrees higher. I assumed I was doing something wrong in the vacuum/recharge (getting air in system) but took it to a long time, respected AC shop here in Dallas to do the vacuum/recharge...got EXACTLY the same results.

No hyjack Buck. AC frustration SUX. You're not alone. Bottom line: If you want something done right, who's gonna dig deeper than ourselves? I've got years invested pizzing around with my AC. Started as total AC idiot. Am upgrading self to AC idiot---still lots to learn. From time & dollars invested to this date, here's chips I can put on table: 1. no one magic bullet---is multitude of bullets. 2. incremental tweeks DO add up. 3. compromise is evil---evil has round trip ticket by default. 4. notes are important tool. Their value is proportionate to personal age (at least in my case.) 5. spend 10 hours doing something most shops would do in 5 minutes and much is saved. Now...

Three functional prospects for you:

Look at bottom of evap (by removing resistor and sticking camera or mirror in hole.) If bottom of evap is dirty... big opportunity to improve AC performance. (My thanks to Bob Kounterattack for this gem.)

Remove top of evap's plenum (blower housing) and check where capillary sensor is located. If it's at evap's core/center, move it: half way between core and evap's discharge port. Set that capillary into evap per capillary specs. (Guessing at even simple AC things... evil.) Check vent temps before doing this move. Make notes. If your thermostat is working, and your system is in relatively good form, move can/should result in slightly cooler vent temps. Next: move capillary sensor again---farther away from evap's core towards evap's discharge port. Again, notes---know what's happening. If you fail to be scientific about this AC stuff... evil. (Am suggesting moving capillary sensor in increments because I genuinely don't know your system. Got to be careful with this "moving" because evap could freeze if move is "aggressive" for system---you might be happy with freeze!... for short while at least.

Can prove enlightening to study Griffiths AC online intel. Charlie's got some excellent stuff on troubleshooting. Google “Porsche AC troubleshooting.” Griffiths should be first result.

Stick with it.

Originally Posted by GH85Carrera

How many condensers do you have? The factory supplied just two, and they are not nearly enough. I added two more (the Dual Khuel setup) and replaced all the lines. I can get very comfortable cabin temps in my 85 Carrera. When I was in Savannah, GA in August with my wife in the car she asked me to turn the temp UP because she was cold at lunch time. When I drove through Needles, CA in July coming from Monterey, CA and had been on the road for hours the AC struggled to keep me cool in the 118 degree heat outside, but I never broke a sweat. When I got to the nice "cool" area of Kingman, AZ it was 100% comfortable in the vehicle.

How new is your evaporator? I did replace mine during my upgrade.

Agree on factory's lacking interest where AC's condensing performance is concerned. (Not only number of units but way rear lid condenser's stock-installed... bad stuff IMHO. We have engine's fan moving great volume of air that should be focused on passing through lid's condenser. In stock form, air's not focused through lid condenser. To do this, engine bay, rear lid, and condenser-to-rear-lid require sealing.) I have stock front & rear condensers + fender unit with 12" pusher Spal and air-server system you know of. Filter in that system... your inspiration. Evap... brand new for this charge. Is Griffith's Kuehl serpentine. I gather you're happy with your AC---THAT's what matters. Below 30 dF from vent is interesting number to look at but is akin to having feeding piranha in cabin---temp bites.

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[GH85Carrera]

Karl, I am coming to Boca Raton next year for Porsche Parade. We have to get together and have a beer or two. I will head down to the keys and Key West just to say my 911 has been driven to the furthest south east possible. I have been to Freedom Park in San Diego so that will be both southern parts. Maine has been done, and lots of hot places in my 911 all over America.

All possible, or bearable because of good AC in my 168,000 mile 1985 911. See ya next year!

[KelogGes]

Originally Posted by Dr J

Coldest temp should be right after the expansion valve. Warmest near the exit. Congratulations Karl in completing this project. Looks fantastic!

Dr J, I completely agree with your statement, you are correct! on an OEM early model 911


I would like to add the coldest POINT of ANY A/C System; is at the point of first refrigerant expansion where the Liquid refrigerant phase changes and boils inside/after expansion valve & turns into a refrigerant Gas inside the evaporator inlet due to liquid refrigerant pressure drop
(depending on expansion valve design and type)

[Discseven]

Originally Posted by GH85Carrera

Karl, I am coming to Boca Raton next year for Porsche Parade. We have to get together and have a beer or two. I will head down to the keys and Key West just to say my 911 has been driven to the furthest south east possible. I have been to Freedom Park in San Diego so that will be both southern parts. Maine has been done, and lots of hot places in my 911 all over America.

All possible, or bearable because of good AC in my 168,000 mile 1985 911. See ya next year!

Next year it is Glen. Hoist to our Porsche Brothers! PM me more than minutes ahead of arrival. Driving USA in 911 is great stuff. Have many fond memories of such adventures as you likely do. 74 Carrera. Back & fourth. Up and Down. Favorite is through Rockies... in winter. No ST.

[T77911S]

Have you taken it on a long drive yet