MasterCool 99903 Operating Instructions Manual page 5

NOTE: The unit will shut down after 15 minutes automatically. Press the POWER button again. Allow
30-45 seconds for initializing and the correct vacuum reading to appear. Once the target vacuum is
reached, close the vacuum valve and disconnect the vacuum gauge port from the system. Press the
POWER button to turn the unit off. (If pressure, temperature or deep vacuum is being used for an
extended period of time turn the unit off and turn back on again when reading is required.)
WARNING!! Never stop the vacuum pump unless the vacuum gauge is disconnected.
Failure to do so may cause oil to enter the sensor chamber.
IMPORTANT: When checking a system for leaks under high vacuum (less than 1000 microns),
connect the vacuum gauge directly to the system. If additional connections are required use
copper tubing (do not use rubber hoses) and high vacuum shut-off valves. Standard hoses and
manifold gauge set shut-off valves may have a small amount of leakage under high vacuum.
When initiating a high vacuum test, the vacuum gauge reading may "drift" higher until the sys-
tem has equalized. After this short stabilization period (5 minutes) the vacuum reading should
hold steady. An upward "drift" of the vacuum gauge reading may indicate a leaking system.
MEASURING ACTUAL SUPERHEAT AND SUBCOOLING (SEE FIG. C)
Superheat is the difference between the actual temperature (sensor temperature) of the refrigerant
(gas) as it leaves the evaporator and the boiling point temperature of the refrigerant in the evaporator
coil (saturated temperature). After boiling, the refrigerant continues to warm up. The number of
degrees it "warmed up" after boiling is called the superheat. Under worst-case conditions (low load
for fixed orifice systems), the refrigerant in the evaporator boils off near the end of the evaporator
coil. To make sure liquid doesn't enter the compressor under the worst case condition, the AC
manufacturers publish charts. The charts indicate what the superheat should be at a given indoor wet
bulb measurement and outdoor air temperature. Measuring superheat is your best indication on a fixed
orifice system of the proper refrigerant charge and operating conditions. If everything else is working
properly and the actual superheat is too high, add refrigerant. If it's too low, remove refrigerant.
Subcooling is the difference between the boiling point of the refrigerant in the condenser (saturated
temperature) and the actual temperature (sensor temperature) of the refrigerant as it leaves the
condenser. The degrees that the refrigerant "cools down" below the boiling point is the sub-cooling.
Under worst case scenario low load for thermostatic expansion valve systems (TXV) the subcooling will
continue to rise. If the subcooling rises too high, liquid may be backed into the compressor causing
damage and failure.
FIG. C
LOW HIGH
REF
VAC
Liquid
Vapor and Liquid
Vapor
On TXV systems, the subcooling is the best indication of the state of the charge in the refrigerant system
since these systems are designed to maintain constant superheat. Properly charging a system ensures
maximum efficiency and longer equipment life.
Use caution whenever working with electricity and high-pressure liquid or gas.
Always wear safety glasses.
Follow all instructions provided with equipment being serviced or installed.
TARGET SUPERHEAT AND SUBCOOLING
Follow all equipment manufacturer's specifications, warnings and suggestions. To determine the target
superheat (fixed orifice system) or subcooling (charts vary dramatically from one system to another),
you will typically need three things. Outdoor dry bulb (outdoor air temperature), indoor wet bulb, and the
manufacturers target superheat chart or subcooling chart.
You can use Mastercool part number 52232 Humidity Temperature Meter for both indoor wet bulb and
outdoor dry bulb. (Refer to diagram.)
Throttle Valve (TXV, Cap Tube, Fixed Orifice)
Condenser
www.mastercool.com
Compressor
Liquid
Evaporator Vapor and Liquid
Vapor
LOW HIGH
REF
VAC
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