Bubbles in the sight glass

Causes, most likely first

The order is approximate and based on common faults in the field. Always measure before replacing anything.

1. 1
   Flowchart D

   Too little refrigerant, usually because of a leak.

   Remedy: Leak test and repair the leak before adding refrigerant.

   🛠 How do I do it? Step by step

   Only qualified personnel may work on refrigeration and electrical systems. Follow the manufacturer's manual and local rules; the steps are guidance, not a replacement for training.

   Leak test a refrigeration system

   Find leaks systematically, confirm the repair and record the work. Work that opens the refrigerant circuit requires F-gas certification (Regulation (EU) 2024/573 or national equivalent, EPA 608 in the USA).

   1. Prepare the system and the detector. The system must be under pressure (running, or at standstill with warm ambient). Stop fans in the area to get calm air and ventilate low or enclosed spaces. With A2L/A3 refrigerants use no ignition sources and only detectors rated for them; with CO2 (R744) remember the very high pressure. Check the electronic detector (EN 14624) against its reference leak or as the manufacturer describes.

      ✓ Check: The detector reacts to the reference leak, and the air around the system is calm.

   2. Search with the electronic detector. Move the probe slowly, a few centimetres per second, close to every brazed joint, flare, valve stem, Schrader core, gauge and coil bend. Refrigerants are heavier than air, so hold the probe at and below each joint. Follow the circuit in a fixed order so that nothing is missed.

      ✓ Check: Every joint along the circuit has been passed, and each alarm has been repeated at the same spot.

   3. Confirm with bubble solution. Spray bubble leak solution on each suspected point and watch it for at least a minute; small leaks form bubbles or foam slowly.

      ✓ Check: Bubbles or foam pinpoint the leak, or the point is confirmed tight.

   4. Use UV dye only where it is approved. If the system already contains a UV dye approved by the compressor manufacturer, light the joints with a UV lamp and wear UV glasses.

      ✓ Check: A fluorescent trace marks the leak, or no trace is visible.

   5. Repair correctly. Recover the refrigerant (or isolate the section), repair with nitrogen flowing when brazing, pressure test with nitrogen, evacuate and charge by weight. Never top up refrigerant 'to test' without finding and repairing the leak.

      ✓ Check: The repaired joint holds the nitrogen test pressure and the system is charged with the specified amount.

   6. Record and re-check. Write the leak location, cause, repair, refrigerant amount and your certificate number in the equipment logbook. Check the repair again within one month, as the F-gas regulation requires.

      ✓ Check: The logbook entry is complete and a follow-up check is booked.

   Leak check of refrigeration systems: F-gas intervals and steps →

2. 2
   Flowchart B

   The filter drier is blocked, so the refrigerant partly flashes because of the pressure drop.

   Remedy: Replace the filter drier and find out why it blocked: dirt, moisture or acid after a compressor burnout.

   🛠 How do I do it? Step by step

   Only qualified personnel may work on refrigeration and electrical systems. Follow the manufacturer's manual and local rules; the steps are guidance, not a replacement for training.

   Replace a filter drier

   Replace the filter drier after moisture, a compressor burnout, a restriction across the drier or whenever the circuit has been opened. Work on the refrigerant circuit requires F-gas certification; brazing is hot work.

   1. Isolate or recover the refrigerant. Pump down or close valves to isolate the drier section where the system allows, otherwise recover the refrigerant with a recovery unit into a marked recovery cylinder. Break the vacuum with dry nitrogen to just above atmospheric pressure before opening. With A2L/A3 refrigerants purge with nitrogen and check with a gas detector.

      ✓ Check: The section reads slightly above 0 bar(g) on nitrogen and the pressure does not rise (no refrigerant left).

   2. Choose the right drier. Use a drier of the same size or larger, with connections and capacity for the refrigerant. After a burnout or acid in the oil use a burnout (acid-removing) type and plan to replace it again after the clean-up; after moisture choose one with ample water capacity.

      ✓ Check: The new drier matches the refrigerant, size and situation, and its arrow follows the flow direction.

   3. Cut out the old drier. Cut the pipes with a pipe cutter (never a hacksaw, which leaves filings) and deburr with the pipe end held downward. Keep the new drier capped until just before fitting.

      ✓ Check: Pipe ends are round, clean and burr-free.

   4. Braze with nitrogen flowing. Get a fire permit, have an extinguisher at hand and remove flammable material. Purge a low flow of dry nitrogen through a regulator while brazing, and cool the drier body with a wet cloth so the desiccant is not overheated. Never use oxygen or compressed air. Flare driers are fitted with torque spanners instead.

      ✓ Check: Clean joints with full penetration, and no black oxide scale inside the pipe ends.

   5. Pressure test with nitrogen. Pressurise with dry nitrogen through a regulator to the test pressure for the system, never above PS, and leak test the new joints.

      ✓ Check: The pressure holds and no leak is found at the joints.

   6. Evacuate, charge and record. Evacuate with a micron gauge to the manufacturer's level (commonly below 500 micron/0.67 mbar), close off the pump and check that the vacuum holds. Charge the specified amount by weight and record the work and refrigerant amounts in the logbook.

      ✓ Check: The vacuum holds, the charge is weighed in and the logbook entry is complete.

   Test the oil for acid

   Acid and moisture in the oil follow a motor burnout, overheating or a wet system, and they destroy a new compressor quickly. An acid test tells you whether the oil and drier must be changed and when the clean-up is finished.

   1. Choose the right kit. Use an acid test kit made for the oil in the compressor (POE, PAG, mineral or PAO; see the compressor nameplate). Check the expiry date. Wear goggles and gloves: burnt oil is acidic and the reagent is a chemical; follow its safety data sheet.

      ✓ Check: The kit matches the oil type and is within its expiry date.

   2. Take the oil sample. Take the sample from the compressor's oil drain or sampling valve, opening it slowly with low pressure in the crankcase and the sample bottle held firmly. On a hermetic compressor without a valve, the refrigerant must be recovered first and the oil drained from a removed compressor; that work requires F-gas certification (EPA 608 in the USA). Let the first few drops run to waste.

      ✓ Check: The clean bottle holds the amount the kit asks for, without refrigerant foaming over.

   3. Look at and smell the oil. Before testing, judge the oil by eye: clean oil is clear and light. Dark or black oil, a sharp burnt smell, sludge or metal particles point to a burnout or mechanical wear.

      ✓ Check: Colour, smell and any particles are noted.

   4. Run the test. Mix the oil and the reagent exactly as the kit instructions say (amounts, shaking time, waiting time).

      ✓ Check: The mixture has stood for the time stated in the kit instructions.

   5. Read the result against the kit chart. Compare the colour with the chart supplied with the kit. The colours differ between makes, so use only that kit's chart, never colours remembered from another brand.

      ✓ Check: The result is read as 'acceptable' or 'acid' according to the kit's own chart.

   6. Decide and record. If the oil is acidic: change the oil, fit a burnout filter drier (suction and liquid line) and repeat the test after some operating time until it passes. Record the result, the action and the date in the logbook.

      ✓ Check: The logbook shows the test result, the action taken and when the next test is due.

3. 3
   Flowchart C

   Unstable or too low condensing pressure, for example when the condenser fans start and stop.

   Remedy: Check the head pressure control.

   🛠 How do I do it? Step by step

   Only qualified personnel may work on refrigeration and electrical systems. Follow the manufacturer's manual and local rules; the steps are guidance, not a replacement for training.

   Check head pressure control

   Too low condensing pressure in winter starves the expansion valve; too high in summer costs energy and trips the HP switch. Check fan control, head pressure valves and water regulating valves against the setpoint.

   1. Read the condensing pressure and setpoint. Connect a calibrated gauge to the discharge or liquid line port and note the condensing pressure setpoint and fan stages in the controller or speed controller. Never exceed the system PS.

      ✓ Check: You know the actual condensing pressure and the setpoint.

   2. Check fan staging and speed control. Watch the fans as the pressure rises and falls: stages cut in and out, or the speed changes, around the setpoint. Measure the speed controller output if a fan does not respond.

      ✓ Check: Fans start, stop or change speed at the set pressures.

   3. Check the fans with power off. If a fan does not run, isolate and lock off, check the fan motor and blades and that the coil is clean before blaming the control. Fans can start automatically.

      ✓ Check: Fans turn freely and the condenser is clean.

   4. Check head pressure valves in cold weather. On systems with head pressure valves (e.g. Sporlan ORI/ORD or Danfoss KVR/NRD), check during low ambient that the bypass to the receiver gets warm when the valve regulates, and that there is enough refrigerant for the flooded condenser (winter charge, sight glass full).

      ✓ Check: The condensing pressure is held at the valve setting and the sight glass stays full in cold weather.

   5. Check the water regulating valve. On water-cooled condensers, check that the water regulating valve opens more as the condensing pressure rises and closes at standstill. Compare water in and out temperatures.

      ✓ Check: The valve modulates and the condensing pressure is steady; no water flows at standstill.

   6. CO2 gas coolers. On transcritical CO2 (R744) the high pressure is set by the high pressure valve and the gas cooler fans together; check it as described in the CO2 high pressure valve guide.

      ✓ Check: Gas cooler outlet temperature and high pressure follow the controller setpoints.

4. 4
   Flowchart A

   The charge is enough; the liquid flashes on its way to the sight glass: pressure drop in the filter drier, a valve or a riser, heat gain in the liquid line, or a short-term change in operation.

   Remedy: Measure the temperature before and after the filter drier and the shut-off valves, check that the liquid line is not running through a warm space or uninsulated, and reassess when operation is stable.

   🛠 How do I do it? Step by step

   Only qualified personnel may work on refrigeration and electrical systems. Follow the manufacturer's manual and local rules; the steps are guidance, not a replacement for training.

   Find a restriction in the liquid line

   A partly blocked filter drier, strainer or valve makes the liquid flash and starves the evaporator. Pressure drop shows up as a temperature drop at the restriction.

   1. Confirm the symptoms. In stable operation, measure superheat and subcooling. A restriction typically gives high superheat and low suction pressure while subcooling before the restriction is normal or high.

      ✓ Check: The measurements fit a restriction rather than a simple shortage of refrigerant.

   2. Look and feel along the line. Follow the liquid line from the condenser or receiver. Look for sweating or frost and feel for a cold spot right after a component. Wear gloves against cold burns.

      ✓ Check: You have found any spot that is clearly colder than the line before it.

   3. Measure across each component. Clamp thermometers before and after the filter drier, solenoid valve, strainer, sight glass and shut-off valves, one component at a time.

      ✓ Check: The temperature is the same on both sides of a free component; a noticeable drop of more than a degree or two marks the restriction.

   4. Measure the pressure drop. Where there are service ports before and after the filter drier or valve, measure the pressure on both sides and compare with the maker's permitted pressure drop.

      ✓ Check: The pressure drop is within the maker's data, or the component is confirmed as restricted.

   5. Check the sight glass and valves. Bubbles after a component but clear liquid before it point to that component. Check that hand and shut-off valves are fully open and that the solenoid valve opens fully when energised.

      ✓ Check: All valves are fully open and you know where flashing starts.

   6. Repair. Replacement opens the circuit and needs F-gas certified personnel: pump down or recover, never trap liquid between closed valves (especially CO2), braze with nitrogen flowing, pressure test, evacuate and recharge by weight.

      ✓ Check: After repair there is no temperature drop across the component and superheat is back to normal.

   Read the sight glass and moisture indicator

   The sight glass shows whether the liquid line is full of liquid, and the moisture indicator whether the refrigerant is dry. Both must be read in stable operation and together with other measurements.

   1. Check marking and position. Check that the sight glass is marked for the refrigerant in the system and its pressure, and that it sits in the liquid line after the filter drier.

      ✓ Check: Marking and position are correct; otherwise the colour reading cannot be trusted.

   2. Run until stable. Let the system run at steady load for at least 15 minutes, not just after start, a defrost or a capacity step. Wipe the glass and light it with a torch.

      ✓ Check: Pressures and temperatures are steady and you can see clearly into the glass.

   3. Read the moisture indicator. Compare the indicator colour with the colour scale printed on the sight glass ring. Colours differ between makers (often green for dry and yellow for wet). The indicator reacts slowly: after a drier change it can take many hours to show the true value.

      ✓ Check: The colour matches the 'dry' field on the scale; a 'wet' colour means moisture in the system.

   4. Read the liquid. Look for a full, clear liquid column, foam or streams of bubbles. Note that a completely clear glass can also mean it is full of vapour in a nearly empty system; confirm with subcooling.

      ✓ Check: You can say whether the glass is full of liquid, and subcooling confirms it.

   5. Judge the bubbles. Bubbles are normal at start, after defrost, during capacity changes and when the liquid flashes because of pressure drop, a vertical lift or warm surroundings. Persistent bubbles in stable operation call for a subcooling measurement and a check for restriction before you suspect undercharge.

      ✓ Check: You know whether the bubbles come from the operating state, a restriction or too little refrigerant.

   6. Act on the result. Moisture: find the cause (leak, poor evacuation), replace the filter drier and evacuate. Too little refrigerant: never add refrigerant to clear bubbles; leak test, repair, recover and weigh in the correct charge. Work that opens the circuit needs F-gas certified personnel.

      ✓ Check: After the action the glass is full of liquid in stable operation and the indicator returns to 'dry'.

5. 5

   The receiver shut-off valve or a solenoid valve does not open fully and throttles the flow.

   Remedy: Open the shut-off valve fully and check the solenoid coil and voltage.

   🛠 How do I do it? Step by step

   Only qualified personnel may work on refrigeration and electrical systems. Follow the manufacturer's manual and local rules; the steps are guidance, not a replacement for training.

   Check valve positions and trapped liquid

   A service or shut-off valve left closed or half open gives symptoms like a restriction or a low charge, and liquid shut in between closed valves can burst the pipe when it warms up. Wear goggles and gloves and never exceed the system PS.

   1. Go through the circuit with the diagram. Follow the flow diagram and list every service valve, ball valve and king valve from the compressor through condenser, receiver, liquid line, evaporators and suction line.

      ✓ Check: You have a list of all valves and their required position in normal operation.

   2. Check the service valves. With a valve key, check that suction, discharge and liquid service valves are fully open (back-seated). Turn slightly off the back seat only while a gauge is connected. Refit the caps afterwards.

      ✓ Check: Spindles are fully open and caps are tight.

   3. Look for partly closed valves. With the system running, measure pipe temperature before and after each valve on the liquid line with a clamp thermometer. A clear temperature drop, frost or sweating after a valve means it is partly closed or restricted.

      ✓ Check: There is no noticeable temperature drop across any valve in the liquid line.

   4. Check the solenoid valves. Check that solenoid valves open on a call: the coil is energised (magnet tester or voltage on the coil) and there is flow (temperature change after the valve).

      ✓ Check: Each solenoid valve opens when it is called and closes when it is not.

   5. Never trap liquid. Before closing two valves on a liquid-filled section, pump the liquid out or make sure the section has a pressure relief path (relief valve or open path to the receiver). Never leave a liquid line closed at both ends.

      ✓ Check: Every section that can be closed in is empty or has a relief path.

   6. CO2: check standstill protection. On R744 systems, confirm that the standstill unit, the relief valves and any pressure-holding function are in service before valves are closed, because the pressure of trapped CO2 rises quickly as it warms (see the CO2 standstill guide).

      ✓ Check: The standstill pressure stays well below PS and the relief valves are open to their outlet.

   Check a solenoid valve (liquid line or hot gas)

   A solenoid valve that does not open starves the evaporator (low pressure, warm room); one that does not close lets refrigerant through when it should not (pump-down fails, flooding, or hot gas leaking into the evaporator). Check the coil first, then the valve itself.

   1. Find out whether the valve should be open. Read in the controller or the wiring diagram when the valve is energised (for example with cooling demand, during pump-down or during hot gas defrost). Compare with what the system does right now.

      ✓ Check: You know whether the valve should be open or closed at this moment.

   2. Check that the coil gets voltage. Measure the voltage across the coil terminals when the valve should be open. Hold a steel screwdriver blade near (not touching) the top of the coil: a live coil pulls it slightly. Check that the coil voltage on its label matches the supply.

      ✓ Check: The coil has the rated voltage (label) when the valve should be open; if not, the fault is upstream: thermostat, controller output, contactor, fuse or wiring.

   3. Check the coil itself. Isolate and lock off, prove dead, and disconnect the coil. Measure its resistance and compare with the manufacturer's value; look for a burnt smell, discolouration or a cracked coil. Make sure the coil is the right type for the valve body and sits fully down on the armature tube with its nut tight.

      ✓ Check: Coil resistance matches the data (an open circuit or a very low value means a faulty coil), and the coil is the right one and correctly fitted.

   4. Check that the valve opens. With the coil energised and the system running, measure the temperature on both sides of the valve with clamp thermometers. An open valve has (almost) the same liquid temperature on both sides; a clear temperature drop or frost after the valve means it is not opening fully or is restricted. Check the flow direction arrow on the valve body.

      ✓ Check: Little or no temperature difference across the valve when it should be open, and it is fitted in the direction of the arrow.

   5. Check that the valve closes. De-energise the coil (or let the controller close it) and watch what happens: on a pump-down system the suction pressure must fall and the compressor must stop on the low pressure switch and stay off; a hot gas valve must leave the line after it cool during the freeze or cooling cycle.

      ✓ Check: The system pumps down and stays down, or the line after a hot gas valve stays cool; a valve that leaks through must be repaired or replaced.

   6. Repair or replace. A faulty coil can be changed without opening the circuit. If the valve body leaks or sticks, opening the circuit is work for certified personnel: recover the refrigerant, replace the valve (or its internal parts) with nitrogen flowing while brazing, pressure test with dry nitrogen, evacuate and charge by weight. Protect the valve from heat while brazing as the manufacturer describes.

      ✓ Check: The valve opens and closes as it should, there is no leak at the new joints, and the work is recorded in the logbook where the refrigerant rules require it.