Moisture indicator in the sight glass shows wet

Causes, most likely first

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

1. 1
   Flowchart A

   The indicator has not settled: it reads wrong for a while after work on the circuit.

   Remedy: Wait until the system has run steadily and read it again.

   🛠 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.

   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'.

2. 2
   Flowchart B

   Moisture got in during service.

   Remedy: Replace the filter drier and evacuate properly, with a vacuum gauge, before charging.

   🛠 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.

   Evacuate a refrigeration system

   Evacuation removes air and moisture from the circuit before charging. A good vacuum and a decay test show that the system is dry and tight; moisture and air left in the circuit give acid, ice in the valve and high pressures.

   1. Check before you start. Evacuate only after the pressure test with nitrogen has passed. Opening the circuit requires F-gas certification (EPA 608 in the USA). For flammable refrigerants (A2L/A3) use a vacuum pump rated for them and remove ignition sources.

      ✓ Check: The pressure test is passed and documented, and the pump suits the refrigerant.

   2. Connect for a fast vacuum. Connect the vacuum pump to both the high and low side with short, large-bore hoses. Remove the valve cores with a core tool if you can. Fit the micron gauge on the system side, as far from the pump as practical, so that it can be isolated from the pump.

      ✓ Check: All connections are tight and the micron gauge reads the system, not the pump.

   3. Pull the vacuum. Check the pump oil, open the valves and start the pump. Never start the compressor or megger it while the system is in vacuum.

      ✓ Check: The micron gauge reading falls steadily.

   4. Reach the target vacuum. Continue until the gauge reaches the target in the equipment or compressor manufacturer's instructions; many use 500 microns (about 0.67 mbar). Large or wet systems take longer; warming the system helps moisture to boil off.

      ✓ Check: The gauge shows the target vacuum.

   5. Do the decay test. Isolate the pump from the system and watch the micron gauge, for example for 15 minutes or the time in the instructions. A reading that rises and levels off means moisture; one that keeps rising means a leak.

      ✓ Check: The reading rises only a little and levels off within the limit in the instructions.

   6. Charge or repeat. If the decay test fails: for moisture, evacuate longer; for a leak, find and repair it and pressure test again. When it passes, refit the valve cores and charge the refrigerant by weight directly from the vacuum.

      ✓ Check: The system is charged from vacuum, and the vacuum and decay results are recorded.

   Evacuating a refrigerating system step by step: dehydration and standing vacuum test →

3. 3
   Flowchart D

   The filter drier is saturated.

   Remedy: Replace the filter drier. Do an acid test of the oil and change oil and suction filter if it is acidic. Follow the indicator until it shows dry.

   🛠 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.

   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'.

4. 4
   Flowchart C

   Water leaks into the circuit through a broken heat exchanger or a leak in a part under vacuum.

   Remedy: Pressure test and leak test the heat exchanger and the suction side. Repair the leak, replace the filter drier and check the oil.

   🛠 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.

   Pressure test with nitrogen

   After repair or installation the circuit is strength and tightness tested with dry nitrogen before evacuation. The test pressures follow from the system's PS according to EN 378-2. Certified personnel only.

   1. Find the test pressure. Find the system's PS (nameplate or documentation) and the strength and tightness test pressures EN 378-2 or the plant documentation give for it. Never use oxygen or compressed air.

      ✓ Check: You have the test pressures in writing and they do not exceed what every component in the test section tolerates.

   2. Isolate sensitive components. Isolate or remove components with a lower PS or that must not see the test pressure: safety valves, transducers, sight glasses, some controls.

      ✓ Check: Every component in the test section is rated for the test pressure.

   3. Connect nitrogen through a regulator. Connect dry oxygen-free nitrogen through a pressure regulator and a calibrated gauge. Never connect a cylinder without a regulator. Ventilate small rooms.

      ✓ Check: The regulator is fitted and its outlet is set below the test pressure.

   4. Raise the pressure in steps. Raise the pressure in steps to the strength test pressure, keep it for the time given and keep people away from the circuit while it is at strength pressure.

      ✓ Check: The pressure holds and no joint gives way.

   5. Lower to tightness pressure and search for leaks. Lower the pressure to the tightness test pressure. Apply leak spray to all joints made or disturbed.

      ✓ Check: No bubbles form on any joint.

   6. Hold and check the pressure. Hold the pressure for the time required, note the pressure and the ambient temperature at start and end, and correct for temperature.

      ✓ Check: The temperature-corrected pressure is unchanged.

   7. Release and record. Release the nitrogen slowly to the outside, refit the isolated components and record test pressures, time and result.

      ✓ Check: The circuit is at 0 bar(g), the components are back and the test is recorded.

   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.

   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.

5. 5

   The indicator is damaged, got oil or water on it during installation, or is made for a different refrigerant (for example after a retrofit), and reads wrong.

   Remedy: Check the refrigerant marking on the sight glass. Replace the sight glass or its insert with one for the refrigerant in the system.

   🛠 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.

   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'.