The short answer. A compressor log book is a few minutes of readings a day that turn breakdowns into planned repairs. Record pressure, discharge temperature, oil level, hour meters, separator pressure drop, dryer dew point, motor current and room temperature every day; review the week every Monday; and keep a separate history of every service and fault. The three printable formats on this page cover screw and piston compressors, and the worked example shows how a filled week exposed two faults before either caused a stoppage.
Who this page is for: maintenance supervisors, compressor operators and plant engineers who want a simple, reliable record for one compressor or a whole compressor room.
Key facts
- Most compressor failures announce themselves days or weeks in advance as a slow drift in temperature, pressure drop, current or loaded hours. A log catches the drift; a single reading does not.
- Readings are only comparable if they are taken at the same time of day, at similar load and with the machine warm.
- The hour meters are the compressor’s diary: loaded hours rising for the same production usually mean leaks, new users or a weakening machine.
- All normal bands on this page are typical values. Your machine’s operating manual and data sheet always take precedence.
- A good log also protects you commercially: it supports warranty claims, service planning and decisions to repair, overhaul or replace.
What to record, and the typical normal band
Write your machine’s own normal values into the header of each sheet on the day it is commissioned or freshly serviced. That baseline is more useful than any general figure.
| Reading | Where to read it | Typical normal band (confirm with the maker) | What a drift usually means |
|---|---|---|---|
| Discharge (airend outlet) temperature, oil-injected screw | Controller display | Typically about 75–95 °C; trip usually set near 105–110 °C | Rising: fouled cooler, low oil, thermostatic valve, hot room |
| Working pressure band | Controller or panel gauge | Your load/unload settings, for example 7.0 to 8.0 bar(g) (101.5 to 116.0 psig) | Not reaching unload: demand above capacity or a fault |
| Oil level, screw | Sight glass, as the maker instructs (running or stopped) | Between the marks | Falling: carryover or leak. Rising: water in the oil |
| Separator pressure drop | Controller or differential gauge | Typically about 0.2–0.4 bar when new; change near the maker’s limit, often 0.7–1.0 bar | Rising steadily: element loading; sudden fall: element damage |
| Refrigerated dryer pressure dew point | Dryer display or indicator | Typically about +3 to +7 °C | Rising: condenser dirty, hot room, overload, refrigerant fault |
| Hour meters, total and loaded | Controller | Your own daily pattern | Loaded share rising for the same output: leaks or wear |
| Motor current (loaded) | Panel ammeter or clamp meter | At or below nameplate full-load current, unless the maker’s data sheet allows service-factor loading | Rising: high pressure, airend drag, low voltage |
| Room temperature | Thermometer at the compressor intake | Ideally within about 10 °C of outside air | High: hot-air recirculation or poor ventilation |
| Piston compressor: pump-up time and head temperature | Stopwatch from cut-in to cut-out; touch or infrared | Your baseline time; head hot but steady | Longer pump-up: valves, rings or belts. Hotter head: leaking discharge valve |
| Condensate drains | Receiver, filters, dryer | Discharging normally | No discharge: blocked drain. Constant discharge: stuck open, wasting air |
How to keep the log so it stays useful
- Same time, same condition. Take daily readings at a fixed time, ideally mid-shift, with the compressor warm and loaded.
- One sheet per machine per month. Keep sheets in a file beside the compressor room, and photograph each completed sheet so a copy is never lost.
- Write numbers, not ticks. “86 °C” can be trended; a tick cannot.
- Review weekly. Every Monday, compare the week’s readings with the baseline in the sheet header. Mark anything that has moved by more than the warning margins below.
- Act and record. Every action, from a cooler clean to a separator change, goes into the service and fault history with the hour-meter reading.
Warning margins worth acting on
| Change from your baseline | Likely meaning | Action |
|---|---|---|
| Discharge temperature up 5 °C or more at similar room temperature and load | Cooling chain losing capacity | Clean cooler, check oil level and thermostatic valve. See overheating |
| Separator pressure drop doubled, or near the maker’s limit | Element loaded | Plan separator change with the next service kit |
| Loaded hours per day up 10 % or more for the same production | New leaks, new users or falling compressor output | Leak walk; pump-up test if leaks are ruled out |
| Oil top-ups more frequent than usual | Carryover or leak | Check scavenge line, separator, oil level setting |
| Dew point above the band for more than a day | Dryer struggling | Clean condenser, check room temperature and drains |
| Motor current rising at the same pressure | Mechanical drag or electrical fault | Call a service engineer before it trips |
Worked example (illustrative): a week that caught two faults
A 30 kW oil-injected screw compressor in a fabrication plant has a baseline, set after its last service, of 84 °C discharge temperature at a room temperature of 36 °C, separator pressure drop 0.35 bar, and about 14.2 loaded hours a day. Production during the week is steady.
| Day | Room °C | Discharge °C | Separator ΔP (bar) | Loaded hours that day | Dew point °C |
|---|---|---|---|---|---|
| Mon | 36 | 85 | 0.36 | 14.3 | 5 |
| Tue | 37 | 86 | 0.36 | 14.6 | 5 |
| Wed | 36 | 88 | 0.37 | 14.9 | 5 |
| Thu | 36 | 89 | 0.37 | 15.2 | 6 |
| Fri | 37 | 91 | 0.38 | 15.4 | 6 |
| Sat | 36 | 92 | 0.38 | 15.6 | 6 |
Reading the week.
- Discharge temperature rose from 85 to 92 °C, a rise of 7 °C, while room temperature stayed at 36–37 °C. That is more than the 5 °C warning margin and is not explained by weather. The cooler was found heavily coated with oily dust from a nearby grinding bay; cleaning it brought the reading back to 85 °C.
- Loaded hours rose from 14.3 to 15.6 a day with unchanged production: (15.6 − 14.2) ÷ 14.2 = about 9.9 % above baseline by Saturday: just under the 10 % warning margin but rising every day, so the trend alone justified a check. A walk with the machines stopped found a split hose on a new press and two leaking quick couplings. After repair, loaded hours returned to about 14.3.
- Separator pressure drop moved only slightly (0.35 to 0.38 bar), which is normal ageing, and the dew point stayed in band, so neither needed action.
Without the log, the cooler would probably have been found only after a high-temperature trip on the next hot afternoon, and the leaks would have stayed hidden in the electricity bill. Figures are illustrative.
Sheet 1: Daily checks, screw and piston compressors
| Machine and serial | Month | Load / unload set (bar g) | Baseline discharge °C | Baseline separator ΔP (bar) | Baseline loaded h/day |
|---|---|---|---|---|---|
| Date | Time | Total hours | Loaded hours | Pressure (bar g) | Discharge / piston head °C | Room °C | Oil level OK | Separator ΔP (bar) | Dryer dew point °C | Drains OK | Motor A | Piston: pump-up s | Initials |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Sheet 2: Weekly and monthly record
| Week / month | Loaded hours this period | Loaded share of total % | Average discharge °C | Max room °C | Oil added (litres) | Cooler condition | Belts / couplings | Leak walk done | Safety valve and receiver drain checked | Remarks |
|---|---|---|---|---|---|---|---|---|---|---|
Sheet 3: Service and fault history
| Date | Hour meter | Service or fault | Parts replaced (with part numbers) | Oil type and quantity | Readings after work | Done by | Next due (hours / date) |
|---|---|---|---|---|---|---|---|
Turning a year of logs into decisions
After twelve months, the log book holds the most honest record of the machine you will ever have. Read it three ways before any major decision.
- Repair, overhaul or replace. Plot loaded hours per month against production. If loaded hours have crept up year on year with leaks under control, the compressor is delivering less air per hour; a pump-up test confirms it, and the trend supports an airend overhaul or replacement decision with numbers rather than opinion.
- Energy. Multiply monthly loaded and unloaded hours by the machine’s power to see what the air costs. The running cost calculator does this from the same hour-meter readings you already log.
- Service planning. Separator pressure drop, oil top-ups and filter changes recorded against hours show your real consumption of service parts, so kits can be ordered before they are needed and the next shutdown planned around production.
Where several compressors share a header, keep one daily sheet per machine but review them together. If one machine’s loaded share keeps rising while another’s falls, the control settings or sequencing need attention rather than the machines themselves.
Paper, spreadsheet or remote monitoring?
Paper beside the machine is the most reliable start, because it is always there and needs no power or login. A simple spreadsheet, filled from photographs of the sheets each week, makes trends easy to chart. Many modern controllers can also log readings or send them to a remote monitoring system; that is valuable, but it does not replace a person walking past the machine, listening to it and looking at the drains. The best plants use both: automatic data for the numbers, and a daily walk-round for the things no sensor sees.
Exceptions and cautions
- Safety first. Daily checks are made from outside the machine with covers on. Never open panels, remove filler caps or touch drains on a pressurised or running machine; oil-injected sumps hold hot oil under pressure.
- Variable speed machines show speed or percentage load instead of simple loaded hours. Record the average load or energy reading the controller provides.
- Oil-free machines have no separator; record the stage temperatures and pressures the maker specifies instead.
- Portable diesel compressors also need engine oil pressure, coolant temperature, fuel added and engine hours.
- Bands given here are typical. If your maker’s values differ, the maker’s values apply.
Common mistakes
- Filling in the sheet from memory at the end of the week.
- Recording ticks instead of numbers, so trends cannot be seen.
- Reading temperatures sometimes loaded and sometimes unloaded, which makes them impossible to compare.
- Never resetting the baseline after a service, so the log keeps comparing against an old condition.
- Keeping the log but never reviewing it.
How Reddy Compressors helps
Since 2002, Reddy Compressors has supplied, rented, serviced and supported spare parts for compressors of all major makes. Our service engineers can set your baseline values after a service, check your logs during visits and turn the trends into a service plan before a fault becomes a breakdown. We work across Telangana, Andhra Pradesh and every district within about 300 km of Hyderabad, with supply across India.
What to send us
- Photographs of the last two to four weeks of log sheets.
- The compressor nameplate and the controller’s total and loaded hours.
- Any alarm messages and when they occurred.
Send them through our repair and service enquiry, or Call/WhatsApp 99590 60718 or 92468 88812.
Frequently asked questions
What should be recorded daily for an air compressor?
Total and loaded hours, working pressure, discharge temperature, room temperature, oil level, separator pressure drop, dryer dew point, drain function, motor current and, for piston machines, pump-up time. Write numbers, not ticks.
What is a normal discharge temperature for a screw compressor?
Typically about 75–95 °C for an oil-injected screw compressor, with a trip usually set near 105–110 °C. Your maker’s manual gives the exact values for your machine.
When should the separator element be changed?
When its pressure drop reaches the maker’s limit, often around 0.7–1.0 bar, or at the hours or calendar interval in the service schedule, whichever comes first.
Why are my loaded hours increasing?
If production has not changed, the usual causes are new leaks, new air users or a compressor delivering less air than before. A leak walk followed by a pump-up test separates them.
How often should the log be reviewed?
Weekly, against the baseline recorded after the last service. Compare like with like: same time of day, similar load and room temperature.
Can the same log be used for piston and screw compressors?
Yes. The daily sheet includes columns for both; piston machines use pump-up time and head temperature, while screw machines use separator pressure drop and discharge temperature.
Related pages
- Compressor service kit finder
- Screw compressor service in Hyderabad
- Reciprocating compressor maintenance
- Compressed air troubleshooting
- Compressor pump-up test and capacity check
- Air compressor running cost calculator
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 5 November 2026.