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newSchoolWorks.com › VoTechWorks › Business & Cross-Cutting › Appliance Repair › Sealed Systems: Refrigerant Handling, Diagnosis, and Repair › Lesson 5
BUS-APPL73-06 · Lesson 5 · 12 clock hours · lab

Evacuation, Dehydration, and System Charging

Objective. Students will be able to evacuate a sealed system to 500 microns or lower using a two-stage vacuum pump and micron gauge, hold the vacuum to verify dryness, and recharge the system to the manufacturer's specified refrigerant weight using a refrigerant scale, verifying charge by suction-line temperature differential.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a credentialed professional in the field or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.

Why Evacuation and Dehydration Matter

Before you charge a sealed refrigeration system, you must remove two enemies: non-condensable gases (mainly air) and moisture. Air raises head pressure and reduces efficiency. Moisture reacts with refrigerant and compressor oil to form acids that destroy the compressor and plug the metering device with ice or sludge. A proper evacuation pulls the system pressure so low that any remaining water boils off as vapor and is removed by the pump. This process is called dehydration. Skipping or rushing it causes premature system failure — often within months.

Equipment You Will Use

Two-stage vacuum pump: A two-stage pump can pull system pressure down to the range needed for proper dehydration (500 microns or lower). Single-stage pumps typically cannot reach this depth reliably. Change the pump oil regularly — contaminated oil drastically reduces achievable vacuum depth.

Micron gauge: Standard manifold gauges cannot read pressures this low. A micron gauge (electronic thermocouple or Pirani type) attaches directly to the system — never downstream of the manifold gauge valves, which can restrict flow and give false readings. It reads in microns of mercury (µmHg), sometimes called microns. One millimeter of mercury equals 1,000 microns. Atmospheric pressure is roughly 760,000 microns.

Manifold gauge set and hoses: Use only hoses rated and dedicated for the refrigerant you are handling (R-134a in today's lab). Purge hose air before connecting to the system.

Refrigerant scale: Measures refrigerant weight added to the system in ounces or grams. Zeroing (taring) the scale with the cylinder in place before opening any valves ensures accurate weight measurement.

The Triple-Evacuation Procedure

A single long pull is less effective at removing moisture than three sequential evacuations broken by dry nitrogen.

1. Connect the vacuum pump and micron gauge to the system. Pull the system down to 1,000 microns.

2. Isolate the pump and break the vacuum with dry nitrogen to approximately atmospheric pressure. Dry nitrogen dilutes and carries residual moisture out of the system.

3. Repeat steps 1 and 2 a second time.

4. On the third and final pull, evacuate the system to 500 microns or below and hold.

Record micron readings every five minutes on your data log. Calculate the pull-down rate to confirm the pump is working and the system has no large leaks.

The Standing Vacuum (Hold) Test

Once you reach 500 microns or below, isolate the pump by closing the valve between the pump and the system. Watch the micron gauge for at least five minutes:

Do not charge the system until the instructor verifies the micron reading. This is a required checkpoint.

Charging by Weight

The manufacturer's specified refrigerant charge is printed on the appliance data plate, usually located inside the cabinet or on the compressor compartment. This weight is the only correct reference — do not estimate.

Procedure:

1. Place the refrigerant cylinder on the scale and zero (tare) the scale with the cylinder and any hose weight already on it.

2. Connect the charging hose from the cylinder to the low-side service port. Purge the hose of air by briefly cracking the cylinder valve to push refrigerant through the hose, then tighten.

3. With the system off (liquid charging) or running (vapor charging per the manufacturer's method), slowly open the cylinder valve. Monitor scale weight loss.

4. When the scale shows you have added the specified charge weight (within ±0.25 oz), close the cylinder valve and then the system valve.

5. The instructor must be present for all valve operations and must verify the final scale reading.

Verifying Charge: Superheat Method

After charging, verify the charge by measuring suction-line superheat:

1. Clamp a thermocouple or digital thermometer probe to the suction line, close to the compressor, and insulate the probe from ambient air.

2. Read the running suction pressure on the manifold low-side gauge.

3. Use the P-T (pressure-temperature) chart for R-134a to find the saturation temperature that corresponds to the suction pressure.

4. Superheat = suction-line temperature − saturation temperature

A properly charged domestic refrigerator typically shows superheat in a moderate positive range. If superheat is very high, the system may be undercharged or restricted. If superheat is very low or zero, the system may be overcharged. Compare your calculated value to the expected range your instructor provides for your specific trainer appliance.

Safety

Key terms10

Micron (µmHg)A unit of pressure equal to one-thousandth of a millimeter of mercury, used to measure deep vacuum levels in refrigeration systems; target for evacuation is 500 microns or below.
Two-stage vacuum pumpA vacuum pump with two compression stages that can pull system pressure low enough (typically below 500 microns) for proper dehydration of a sealed refrigeration system.
Triple evacuationA dehydration method in which a system is evacuated to approximately 1,000 microns, broken with dry nitrogen, and repeated three times, with the final pull reaching 500 microns or below.
Standing vacuum testA procedure in which the vacuum pump is isolated from the system after reaching target vacuum and the micron gauge is monitored to verify the system is leak-free and dry.
SuperheatThe temperature of refrigerant vapor above its saturation (boiling) temperature at a given pressure; calculated by subtracting the saturation temperature (from a P-T chart) from the measured suction-line temperature.
Saturation temperatureThe temperature at which a refrigerant changes phase (boils or condenses) at a specific pressure; found by looking up the suction pressure on a pressure-temperature (P-T) chart.
Tare (zeroing the scale)Resetting a refrigerant scale to zero with the cylinder and hose already in place so that only the weight of refrigerant leaving the cylinder is measured.
Non-condensable gasesGases such as air or nitrogen that do not condense at normal refrigeration operating pressures and temperatures; their presence raises head pressure and reduces system efficiency.
P-T chart (pressure-temperature chart)A reference table that lists the saturation temperature of a refrigerant at various pressures, used to determine saturation conditions and calculate superheat or subcooling.
DehydrationThe removal of moisture from a sealed refrigeration system by pulling a deep vacuum, causing residual water to boil off and be removed by the vacuum pump.

Worked examples4

1. Your micron gauge reads 480 microns after the final evacuation pull. You isolate the pump and watch the gauge for 5 minutes. The reading rises to 520 microns and then holds steady. Should you proceed to charging?
  1. Note the final vacuum depth: 480 microns, which is at or below the 500-micron target — this part passes.
  2. Evaluate the standing vacuum behavior: the reading rose slightly from 480 to 520 microns, but then stopped rising and held steady.
  3. A slow rise that plateaus is consistent with residual moisture boiling off, not an active leak — active leaks cause a continuous rise that does not level off.
  4. However, the held reading of 520 microns is above the 500-micron pass threshold. Continue pulling until the reading stabilizes at or below 500 microns.
  5. Once the gauge holds at 500 microns or below, get instructor verification before breaking the vacuum.
Answer: Do not charge yet. The held vacuum is above 500 microns. Continue evacuating, perform additional pull-down, and confirm the reading stabilizes at or below 500 microns before proceeding.
2. The appliance data plate specifies a charge of 3.50 oz of R-134a. After charging, your scale shows a total weight loss of 3.20 oz from the cylinder. Is this within the acceptable tolerance, and what should you do?
  1. Identify the target charge: 3.50 oz.
  2. Identify the actual charge added: 3.20 oz.
  3. Calculate the difference: 3.50 oz − 3.20 oz = 0.30 oz short.
  4. Compare to the tolerance: the acceptable range is ±0.25 oz of the specified charge, meaning the system must receive between 3.25 oz and 3.75 oz.
  5. 0.30 oz short places the charge outside the lower tolerance limit (3.25 oz minimum).
Answer: The charge is not within tolerance — the system is 0.30 oz undercharged, which exceeds the ±0.25 oz limit. Notify the instructor; additional refrigerant must be carefully added to bring the total to within 0.25 oz of 3.50 oz.
3. After charging, your manifold low-side gauge reads 2 psig on an R-134a system. The P-T chart shows the saturation temperature for R-134a at 2 psig is −10°F. Your clamp-on thermocouple reads the suction line at 18°F. What is the superheat?
  1. Identify the suction-line temperature measured: 18°F.
  2. Identify the saturation temperature from the P-T chart at the measured suction pressure (2 psig): −10°F.
  3. Apply the superheat formula: Superheat = suction-line temperature − saturation temperature.
  4. Superheat = 18°F − (−10°F) = 18 + 10 = 28°F.
Answer: The superheat is 28°F. Compare this value to the expected superheat range provided by your instructor for the specific appliance to determine whether the charge is correct.
4. During your evacuation data log, the micron gauge reads 3,200 microns at minute 0, 1,800 at minute 5, 1,100 at minute 10, and 700 at minute 15. What is the average pull-down rate in microns per minute over the first 15 minutes?
  1. Identify starting reading: 3,200 microns at minute 0.
  2. Identify ending reading: 700 microns at minute 15.
  3. Calculate total pressure drop: 3,200 − 700 = 2,500 microns.
  4. Calculate elapsed time: 15 minutes.
  5. Average pull-down rate = total pressure drop ÷ elapsed time = 2,500 ÷ 15 ≈ 167 microns per minute.
Answer: The average pull-down rate is approximately 167 microns per minute over the first 15 minutes. This rate should continue to slow as the system approaches deep vacuum; a very sudden flattening of the pull-down rate may indicate a leak or saturated pump oil.

The math in this lesson1

Learners read micron gauge values against a 500-micron target and interpret a 480-to-520 rise.
N-Q.3
Brush up in the math halls at newSchoolWorks.com:
In class: Instructor demonstrates two-stage pump setup and micron gauge connection on a trainer refrigerator: triple-evacuation procedure (evacuate to 1,000 microns, break with dry nitrogen, repeat twice, final pull to 500 microns or below); students record micron readings at 5-minute intervals on a data log sheet and calculate pull-down rate. · Instructor demonstrates refrigerant charging by weight using a refrigerant scale: zero the scale with the refrigerant cylinder, connect the charging hose, purge hose air, open system valves, monitor scale weight loss and stop at the manufacturer-specified charge weight (found on the appliance data plate); students each perform the weighing-in procedure on their assigned trainer with instructor present for all valve operations. · After recharging, students measure suction-line temperature with a clamp-on thermocouple or digital thermometer and compare to expected saturation temperature from the P-T chart; students record ambient temperature, running suction pressure, suction-line temperature, and calculated superheat on a post-charge data sheet, then write a one-paragraph assessment of whether the system is properly charged.
Materials: Two-stage vacuum pumps, micron gauges (electronic thermocouple or Pirani type), manifold gauge sets, refrigerant cylinders (R-134a), refrigerant scale, digital clamp-on thermometers or thermocouple probes, trainer refrigerator units (freshly brazed or recovered), P-T reference charts, appliance data plates, evacuation data log sheets, post-charge data sheets, PPE
Check: Micron gauge must read 500 microns or below before student is authorized to break vacuum and charge; instructor verifies reading. Charge weight must be within ±0.25 oz of the manufacturer specification (verified by instructor on the scale). Calculated superheat on the post-charge data sheet must be within the expected range for the appliance type; instructor reviews data sheet and must award 80% or better to pass.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Sealed System Service
1. What is the maximum micron reading that a sealed system must reach before a technician is authorized to break vacuum and charge refrigerant?
Sealed System Diagnosis
2. During a standing vacuum test, a technician isolates the vacuum pump and observes that the micron gauge reading rises continuously without leveling off over several minutes. This most likely indicates:
Safety and Environmental Practices
3. Why must a technician use only dry nitrogen — not compressed air — to break vacuum between evacuation pulls?
Refrigerant Handling and Charging
4. A technician places a refrigerant cylinder on a charging scale and zeros the scale before opening any valves. The purpose of this step is to:
Refrigerant Handling and Charging
5. The appliance data plate specifies 4.00 oz of R-134a. Which of the following charged amounts is within the acceptable tolerance of ±0.25 oz?
System Performance Verification
6. Superheat on a suction line is calculated as:
EPA 608 Regulations
7. Under EPA Section 608, venting R-134a from a refrigeration system to the atmosphere is:
System Performance Verification
8. A technician measures a suction-line temperature of 35°F and reads 10 psig on the low-side gauge. The P-T chart for R-134a shows a saturation temperature of 20°F at 10 psig. The technician calculates a superheat of 15°F. After checking the calculation, the instructor says it is wrong. What is the correct superheat?
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