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Condition monitoring (colloquially, CM) is the process of monitoring a parameter of condition in machinery (vibration, temperature etc. ), in order to identify a significant change which is indicative of a developing fault. It is a major component of predictive maintenance. The use of condition monitoring allows maintenance to be scheduled, or other actions to be taken to prevent consequential damages and avoid its consequences. Condition monitoring has a unique benefit in that conditions that would shorten normal lifespan can be addressed before they develop into a major failure. Condition monitoring techniques are normally used on rotating equipment, auxiliary systems and other machinery like belt driven equipment, (compressors, pumps, electric motors, internal combustion engines, presses), while periodic inspection using non destructive testing (NDT) techniques and fit for service (FFS) evaluation are used for static plant equipment such as steam boilers, piping and heat exchangers.

Often visual inspections are considered to form an underlying component of condition monitoring, however this is only true if the inspection results can be measured or critiqued against a documented set of guidelines. For these inspections to be considered condition monitoring, the results and the conditions at the time of observation must be collated to allow for comparative analysis against the previous and future measurements. The act of simply visually inspecting a section of pipework for the presence of cracks or leaks cannot be considered condition monitoring unless quantifiable parameters exist to support the inspection and a relative comparison is made against previous inspections. An act performed in isolation to previous inspections is considered a Condition Assessment, Condition Monitoring activities require that analysis is made comparative to previous data and reports the trending of that comparison. Slight temperature variations across a surface can be discovered with visual inspection and non destructive testing with thermography. Heat is indicative of failing components, especially degrading electrical contacts and terminations. Thermography can also be successfully applied to high speed bearings, fluid couplings, conveyor rollers, and storage tank internal build up. A scanning electron microscope can take an image of a carefully taken sample of debris suspended in lubricating oil (taken from filters or magnetic chip detectors). Instruments then reveal the elements contained, their proportions, size and morphology. Using this method, the site, the mechanical failure mechanism and the time to eventual failure may be determined. This is called WDA – Wear Debris Analysis. Spectrographic oil analysis that tests the chemical composition of the oil can be used to predict failure modes. For example, a high silicon and aluminium content indicates contamination of dirt or grit (aluminium silicates) etc., and high iron levels indicate wearing components. Individually, elements give fair indications, but when used together they can very accurately determine failure modes e. g. for internal combustion engines, the presence of iron (liner), aluminium (piston) and chrome (rings) would indicate upper cylinder wear. Ultrasound can be used for high speed and slow speed mechanical applications and for high pressure fluid situations. Digital ultrasonic meters measure high frequency signals from bearings and display the result as a dBuV (decibels per microvolt) value. This value is trended over time and used to predict increases in friction, rubbing, impacting, and other bearing defects. The dBuV value is also used to predict proper intervals for re lubrication. Ultrasound monitoring, if done properly, proves out to be a great companion technology for vibration analysis. Headphones allow humans to listen to ultrasound as well. A high pitched 'buzzing sound' in bearings indicates flaws in the contact surfaces, and when partial blockages occur in high pressure fluids the orifice will cause a large amount of ultrasonic noise. Ultrasound is used in the Shock Pulse Method of condition monitoring. Performance analysis, where the physical efficiency, performance, or condition is found by comparing actual parameters against an ideal model. Deterioration is typically the cause of difference in the readings. After motors, centrifugal pumps are arguably the most common machines. Condition monitoring by a simple head flow test near duty point using repeatable measurements has long been used but could be more widely adopted. An extension of this method can be used to calculate the best time to overhaul a pump based on balancing the cost of overhaul against the increasing energy consumption that occurs as a pump wears. Aviation gas turbines are also commonly monitored using performance analysis techniques with the original equipment manufacturers such as Rolls Royce plc routinely monitoring whole fleets of aircraft engines under Long Term Service Agreements (LTSAs) or Total Care packages. Wear Debris Detection Sensors are capable of detecting ferrous and non ferrous wear particles within the lubrication oil giving considerable information about the condition of the measured machinery. By creating and monitoring a trend of what debris is being generated it is possible to detect faults prior to catastrophic failure of rotating equipment such as gearbox's, turbines, etc.

The Criticality Index is often used to determine the degree on condition monitoring on a given machine taking into account the machines purpose, redundancy (i. e. if the machine fails, is there a standby machine which can take over), cost of repair, downtime impacts, health, safety and environment issues and a number of other key factors. The criticality index puts all machines into one of three categories:
Critical machinery – Machines that are vital to the plant or process and without which the plant or process cannot function. Machines in this category include the steam or gas turbines in a power plant, crude oil export pumps on an oil rig or the cracker in an oil refinery. With critical machinery being at the heart of the process it is seen to require full on line condition monitoring to continually record as much data from the machine as possible regardless of cost and is often specified by the plant insurance. Measurements such as loads, pressures, temperatures, casing vibration and displacement, shaft axial and radial displacement, speed and differential expansion are taken where possible. These values are often fed back into a machinery management software package which is capable of trending the historical data and providing the operators with information such as performance data and even predict faults and provide diagnosis of failures before they happen. Essential machinery – Units that are a key part of the process, but if there is a failure, the process still continues. Redundant units (if available) fall into this realm. Testing and control of these units is also essential to maintain alternative plans should critical machinery fail. General purpose or balance of plant machines – These are the machines that make up the remainder of the plant and normally monitored using a handheld data collector as mentioned previously to periodically create a picture of the health of the machine.