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Discseven Discseven is offline
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Join Date: Oct 2012
Location: Miami, FL USA
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AC pressure & temp results with 23 oz R134a


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.


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

Karl ~

Current: '80 Silver Targa w /'85 3.2
Prior: '77 Copper 924. '73 Black 914. '74 White Carrera. '79 Silver 930. '79 Black 930. '79 Anthracite 930.

Last edited by Discseven; 09-30-2018 at 05:49 AM.. Reason: Simplified
Old 09-29-2018, 07:19 AM
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