Gas bubble interference is one of the most common causes of false high readings in optical oil-in-water monitors used in produced water applications. It occurs when dissolved gas in the produced water stream comes out of solution as gas bubbles as pressure drops through the sample conditioning system, and those bubbles enter the monitor measurement cell alongside — or instead of — oil droplets.
The consequence is a monitor reading that is significantly higher than the actual oil concentration in the produced water — potentially triggering a false alarm, stopping discharge unnecessarily or, in some cases, masking the real reading pattern and undermining the reliability of the compliance record. Understanding why gas bubble interference occurs, which monitors are most affected and how to prevent it is essential for instrumentation engineers specifying produced water monitoring systems. For a broader overview of the causes of high oil readings in produced water monitors, see What Causes High Oil Readings in Produced Water Monitors?.
| Key thing to remember Gas bubbles scatter light in the same way that oil droplets do. A simple light scattering oil-in-water monitor cannot tell the difference between a gas bubble and an oil droplet — it measures both as oil. In a produced water stream where gas bubbles are present, this produces readings that are higher than the actual oil content of the water. The reading is not wrong because of equipment failure — it is wrong because the measurement technology cannot distinguish the two particle types. |
What Is Gas Bubble Interference in an Oil-in-Water Monitor?
Gas bubble interference occurs when gas bubbles are present in the water sample inside the oil-in-water monitor measurement cell at the time of measurement. In optical measurement systems — which use a light source and detector to identify and quantify oil in the water — gas bubbles scatter or absorb light in a way that is indistinguishable from oil droplets at the measurement technology level.
The result is that the monitor counts gas bubbles as if they were oil droplets and adds their apparent volume to the oil concentration reading. If a significant number of gas bubbles are present in the sample, the reported oil concentration can be substantially higher than the actual oil content of the water.
This is a measurement error caused by the presence of an interfering substance in the sample — not by any malfunction of the monitor itself. A monitor that produces false high readings due to gas bubble interference may be functioning exactly as designed; the problem is that it is measuring something other than what it is intended to measure.
Where Does Gas in Produced Water Come From?
Gas in produced water originates from several sources, each of which creates a different gas bubble profile in the water stream:
Dissolved gas from the reservoir
Produced water from reservoir formations contains dissolved gases — primarily methane, carbon dioxide and hydrogen sulphide — held in solution under the elevated pressure of the reservoir. As produced water passes through the topsides processing system and pressure progressively reduces, dissolved gases come out of solution in a process known as flashing. The released gas forms fine bubbles in the produced water stream.
Gas entrainment from the flotation unit
Gas flotation units — both IGF and DGF — intentionally introduce gas bubbles into the produced water stream to float oil droplets to the surface for skimming. After the flotation unit, the produced water may carry a proportion of residual gas bubbles that were not separated in the flotation vessel. These residual bubbles can persist into the downstream sample conditioning system and measurement cell.
Gas from pump cavitation and turbulence
Pumps, control valves and restrictions in the produced water pipework can cause local pressure reduction below the bubble point of the water, releasing dissolved gas as fine bubbles. This is particularly relevant in the sample conditioning system itself — a back pressure regulator or flow control valve that creates a large pressure drop can generate gas bubbles immediately upstream of the measurement cell.
Gas ingress from the production system
In some operating conditions — particularly at low production rates or following process upsets — gas from the production system can break through into the produced water stream above the normal dissolved gas level, creating a slug of gas-contaminated produced water that produces a transient spike in monitor readings.
Which Oil-in-Water Monitor Technologies Are Most Affected by Gas Bubble Interference?
Different oil-in-water measurement technologies respond differently to gas bubble interference. The table below summarises the susceptibility of the main optical measurement technologies used in produced water monitoring applications.
| Measurement technology | How it works | Susceptibility to gas bubble interference |
| Simple light scattering | Measures scattered light from all particles in the sample, including gas bubbles and solids as well as oil droplets | High susceptibility to gas bubble interference — cannot distinguish between oil droplets and gas bubbles; false high readings are common in produced water streams with gas |
| Optical microscopy | Captures images of individual particles and classifies each one by size, shape and optical properties | Low susceptibility — classifies gas bubbles separately from oil droplets; significantly more robust against false high readings from gas interference |
| Ultraviolet fluorescence | Measures fluorescence of aromatic hydrocarbon compounds under UV light | Moderate susceptibility — gas bubbles can cause scattering that affects the baseline signal, but the fluorescence specificity to hydrocarbons provides some discrimination |
| Infrared absorption | Measures infrared light absorption by oil and water at specific wavelengths | Lower susceptibility than simple light scattering — infrared absorption is more specific to hydrocarbon compounds than simple scattering, but large gas bubbles can still cause baseline disturbance |
The SMART PFM 107 uses optical microscopy — capturing images of individual particles in the measurement cell and classifying each one by size, shape and optical characteristics. This particle-by-particle classification allows gas bubbles to be identified and excluded from the oil concentration calculation, making the SMART PFM 107 significantly more robust against gas bubble interference than simple light scattering monitors. Full details of the SMART PFM 107 and its application to FPSO produced water monitoring are on the SMART PFM 107 product page.
How Does Gas Bubble Interference Produce a False High Reading?
The mechanism by which gas bubbles produce false high readings in a light scattering monitor can be understood by following what happens to a gas bubble as it passes through the measurement cell:
- A gas bubble enters the measurement cell with the water sample
- The light beam from the monitor light source hits the gas bubble
- The gas-water interface of the bubble has a different refractive index from the surrounding water, causing light to scatter in all directions from the bubble surface
- The detector records the scattered light signal
- The monitor’s algorithm interprets the scattered signal as coming from an oil droplet of corresponding size
- The bubble’s apparent volume is added to the oil concentration calculation
- The reported oil concentration is higher than the actual concentration of oil in the water
The magnitude of the false reading depends on the size and number of gas bubbles in the sample. A single large bubble can produce a dramatic spike in the reading — temporarily suggesting very high oil content. A persistent presence of fine micro-bubbles produces a sustained upward bias in readings that is more difficult to identify as a measurement error because there is no obvious spike pattern.
| Persistent micro-bubble interference is the hardest to identify A large gas bubble produces an obvious spike in the monitor reading — operators and engineers will typically recognise this as an interference event. A persistent population of fine micro-bubbles from dissolved gas release produces a smaller but sustained upward bias in readings — readings that are consistently 5 to 15ppm higher than the actual oil content. This pattern is more likely to be mistaken for real high oil content, potentially triggering unnecessary treatment changes or an incorrect conclusion that the treatment system is underperforming. |
How Can Gas Bubble Interference Be Identified?
Identifying gas bubble interference as the cause of elevated readings requires a systematic diagnostic approach. The following observations and tests help confirm whether gas is the source of a high reading:
Pattern of the reading
Gas bubble interference from dissolved gas release typically produces readings that rise sharply when the produced water stream undergoes a pressure change — for example, when the produced water pump starts, when a control valve opens, or when flow rate changes. A reading that correlates with pressure events in the process is more likely to be caused by gas than by actual high oil content.
Grab sample correlation
Taking a grab sample from the same point as the monitor sample tap and having it analysed by a laboratory provides an independent measurement of the actual oil content. If the grab sample result shows oil concentration significantly below the monitor reading, gas bubble interference is a likely cause. Grab samples settle during collection and transport, allowing gas bubbles to escape — so the laboratory result reflects oil content without gas interference.
Sample conditioning pressure check
Checking the pressure in the sample line at the monitor inlet confirms whether dissolved gas could be coming out of solution at that point. If the sample pressure is below the bubble point of the produced water at the operating temperature, gas will be released. Increasing the back pressure in the sample conditioning system — by adjusting the back pressure regulator — and observing whether the monitor reading falls confirms that gas release was the cause.
Visual inspection of sample discharge
Inspecting the sample discharge from the monitor measurement cell — if accessible — can reveal the presence of visible gas bubbles in the sample stream. Milky, cloudy or visibly bubbly sample discharge confirms gas entrainment in the sample.
How Can Gas Bubble Interference Be Prevented?
Gas bubble interference can be prevented or significantly reduced through a combination of sample conditioning design and measurement technology selection. The table below summarises the available prevention measures.
| Prevention measure | How it works | When it is effective |
| Back pressure regulator in sample conditioning | Maintains elevated pressure in the sample line above the bubble point, preventing dissolved gas from coming out of solution before it reaches the measurement cell | Effective where gas interference is caused by pressure reduction across the sample conditioning system; must be correctly sized for the sample conditions |
| Degassing vessel or separator | A small vessel in the sample line allows gas bubbles to coalesce and separate from the sample before it reaches the analyser | Effective for removing larger gas bubbles; less effective for very fine micro-bubbles that may remain entrained in the sample |
| Sample flow rate control | Controlling sample flow rate reduces turbulence in the sample line and measurement cell, reducing bubble generation and entrainment | Useful as a complementary measure; should be specified alongside pressure control rather than as an alternative |
| Optical microscopy measurement technology | Selecting a monitor that differentiates gas bubbles from oil droplets by particle characterisation rather than simple light scattering | The most fundamental solution — eliminates the measurement error rather than managing the sample conditions; most effective for applications where gas interference cannot be reliably controlled by sample conditioning alone |
| Temperature control in sample conditioning | Maintaining sample temperature prevents gas from coming out of solution due to temperature changes between the process and the sample conditioning system | Particularly relevant where the process operates at elevated temperature and the sample conditioning system is exposed to ambient conditions |
The most effective long-term solution for produced water monitoring applications where gas interference is a persistent risk is to select a monitor with optical microscopy measurement technology — such as the SMART PFM 107 — that classifies particles individually and excludes gas bubbles from the oil concentration calculation. This eliminates the measurement error at source rather than managing it through sample conditioning. For FPSO applications where multiple crude oil types are involved, the OCD Xtra provides calibration flexibility across up to six oil types while also being designed for the variable conditions of produced water streams.
What Are the Implications for Monitor Selection at the Specification Stage?
Gas bubble interference is a factor that must be considered at the monitor selection stage — not discovered during commissioning or service. The Produced Water Discharge Monitoring for FPSOs explainer page sets out the full selection framework, including the role of gas bubble interference in technology choice.
For EPC instrumentation engineers specifying produced water monitoring for an FPSO project, the key questions to address at specification stage are:
- Is dissolved gas expected in the produced water stream at the monitoring point? This depends on the reservoir gas-to-oil ratio and the pressure at the monitoring point relative to the bubble point of the produced water
- Does the sample conditioning system design adequately prevent gas release before the sample reaches the measurement cell? This requires confirming that the sample pressure at the monitor inlet is above the bubble point across all operating conditions
- If gas bubble interference cannot be reliably prevented by sample conditioning, is a monitor with particle classification technology specified?
For a full discussion of how sample conditioning requirements interact with monitor technology selection in FPSO produced water applications, see How to Specify an Oil-in-Water Monitor for FPSO Produced Water Discharge.
Frequently Asked Questions
What is gas bubble interference in an oil-in-water monitor?
Gas bubble interference occurs when gas bubbles are present in the water sample inside the monitor measurement cell. In optical light scattering monitors, gas bubbles scatter light in the same way as oil droplets — the monitor cannot distinguish between the two and counts gas bubbles as oil. This produces a reported oil concentration that is higher than the actual oil content of the water. The monitor is not malfunctioning — it is measuring accurately but measuring the wrong thing.
Which types of oil-in-water monitors are most affected by gas bubble interference?
Simple light scattering monitors are most susceptible to gas bubble interference because they measure scattered light from all particles in the sample without distinguishing between oil droplets, gas bubbles and solids. Optical microscopy monitors are significantly more robust because they classify individual particles by size, shape and optical characteristics — identifying and excluding gas bubbles from the oil concentration calculation.
How do you tell if a high oil reading is caused by gas bubble interference?
Key indicators include readings that spike when process pressure changes, a consistent discrepancy between the monitor reading and grab sample laboratory results, visible gas bubbles in the sample line or discharge from the measurement cell, and readings that fall when back pressure in the sample conditioning system is increased. Grab sample correlation is the most reliable diagnostic method — if the laboratory result shows significantly lower oil content than the monitor reading, gas interference is the likely cause.
Can sample conditioning prevent gas bubble interference?
Yes, in many cases. Maintaining elevated back pressure in the sample line above the bubble point of the produced water prevents dissolved gas from coming out of solution before the sample reaches the measurement cell. A degassing vessel in the sample line can remove larger bubbles. However, where gas interference cannot be reliably controlled by sample conditioning — for example, where the bubble point of the produced water varies with operating conditions — selecting a monitor with optical microscopy technology is the more fundamental solution.
Is gas bubble interference more common in FPSO produced water applications than in bilge water applications?
Yes. FPSO produced water from topsides processing typically contains significantly more dissolved gas than bilge water from a vessel machinery space. Reservoir produced water is saturated with dissolved gases under high reservoir pressure, and as it passes through the treatment system and pressure reduces, dissolved gas is progressively released. This makes gas bubble interference a particularly important consideration in FPSO produced water monitor selection — and one of the key reasons why a standard bilge alarm is not suitable for produced water applications.