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  1. #1
    Join Date
    Jun 2003
    Location
    Tampa Bay, FL.
    Posts
    119

    Charging by weight or superheat?

    Here's the scenario: On a True GDM-12 with 9 oz charge of 134a for example, I will weigh in the proper charge on the nameplate but often find it necessary to bump in a little more(1-2 oz) or so due to different drier, comp, hose length,etc. to end up around 15*f vsat at around 34*f box temp give or take. Now that I have switched to digital with the SMAN3 I decided to call True and ask for recommended superheat numbers. This is where it gets interesting.
    The first tech I spoke with immediately said 45-55* superheat at operating temp measured at compressor. So I began charging a few units that way but they seemed a bit overcharged to me so I called back and got a different tech. He said " a capillary system can't be charged by superheat or subcooling only by weight or pressure." He suggested 14-17psig suction and 135-150 head, but couldn't commit to a target box temp for those numbers. As luck would have it, I had one brand new GDM-12 still crated so I committed the cardinal sin and opened the system. I used stubby analog gauges and the pipe clamps on my sman3 to get my numbers. I checked it in an 86* warehouse and in the 75* office and found at 33* box temp low side 19/17 and high side 160/140 Superheat 65/61 measured at compressor.
    So it seems both techs were wrong and my old way was in line with factory specs. Just want to know anyone else's experience with these type units and what is the proper way to charge them?

  2. #2
    Join Date
    Oct 2008
    Location
    Ohio, USA
    Posts
    159
    I'm assuming this is a cap tube system.

    I will weigh it in but will usually add additional refrigerant if needed, to get my subcoooling ~12º. No more than 15.
    You also of course verify that your superheat at compressor is not below 20F.

    I think the techs you talked to are clueless. You cannot charge these systems on pressures alone, or superheat alone, etc.
    And this is on BRAND NEW units... imagine what would happen if this is an old unit with a cap tube changed out and you tried to follow these procedures! Disaster.

  3. #3
    Join Date
    Mar 2007
    Location
    wedged in freezer shelf
    Posts
    6,151
    Hydra if you are looking for more consistency check temps @ the evap not the compressor. SH will be bound to be much different between units and is greatly affected by heat exchanger length.
    If things are running right you will have low SC @ the cond outlet and low SH @ the evap outlet when the box is at cut out temp.
    I would still be aware of the name plate charge.
    Jared 12˚- 15˚ SC on a unit that was down to temp would indicate an overcharge or cap tube that was too restrictive from most that I've seen
    “If You Can Dodge A Wrench You Can Dodge A Ball”

  4. #4
    Join Date
    Jun 2003
    Location
    Tampa Bay, FL.
    Posts
    119
    Iceman, I agree it would be best to measure superheat at the evap outlet but unfortunately that's not practical in store with a box full of product and not particularly simple in the shop either. Not sure if pipe clamps will fit with the fan shroud installed and the perma-gum would have to be removed, but I guess theres no easy answer to it if you wanna check superheat accurately.

  5. #5
    Join Date
    Mar 2007
    Location
    wedged in freezer shelf
    Posts
    6,151
    Its not that hard or inconvenient at all. You can get to the outlet tubing from the back on most of these and if you've done it a few times you could too. If I could show you I would. A lot can be measure with a simple pocket thermometer and the fan blade(s) removed. I agree it would be a complete PITA with your SMAN sensors.
    “If You Can Dodge A Wrench You Can Dodge A Ball”

  6. #6
    Join Date
    Jun 2003
    Location
    Tampa Bay, FL.
    Posts
    119
    I think we may be doing the same thing except I don't remove the fan blades. Instead I fashioned a little hook for my pocket thermometer that gets the probe right against the back of the evaporator giving me what I think is a fairly accurate evap outlet temp. Does that seem reasonable? Of course it's a mixture of air and surface temp but so would a K-type probe unless it were securely taped down. Also I take my measurements at or near cut out temp.

  7. #7
    Join Date
    Mar 2007
    Location
    wedged in freezer shelf
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    If the cut out temp is 16˚ for the unit you are looking at (I don't know the GDM12 by heart) and you are reading 16˚ at cut out we are doing the same thing. Your thermometer contacting the fins would be better at showing what the t-stat is seeing. With a full charge the entire coil more or less will be the same temp fan on or off at cut out temp. With a fast reacting thermometer connected to the outlet tubing and fans off it might overshoot the cut out a little. On the units that work well I see 1˚- 4˚ SH at the evap outlet at cut out. FWIW a Cooper 4011 works well getting to the evap. outlet tubing.
    “If You Can Dodge A Wrench You Can Dodge A Ball”

  8. #8
    Join Date
    Jun 2003
    Location
    Tampa Bay, FL.
    Posts
    119
    Factory cut out is 25* but I have to adjust it up to 32* due to sugar free drinks freezing and bursting even at the warmest setting. Thanks for the tip on the Cooper 4011. I'll check into that.

  9. #9
    Join Date
    Mar 2007
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    wedged in freezer shelf
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    6,151
    25 can't be the coil temp. If that is a LAE setting you can view evap outlet temp with the i button
    “If You Can Dodge A Wrench You Can Dodge A Ball”

  10. #10
    Join Date
    Jun 2003
    Location
    Tampa Bay, FL.
    Posts
    119
    Sorry, meant 25* box temp

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