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Measuring isopropyl alcohol levels in confined ATEX zones
Resolving IPA level to within ±1mm in a compact, intrinsically safe installation
Introduction: Measuring IPA Levels in Confined ATEX Environments
Application overview: Automated IPA dosing in a confined space
An automated machine was designed to dip a rubber seal into IPA every 15 seconds to apply a consistent coating. The IPA was contained in a small cylindrical cup — Ø64 × 80mm, filled to 70mm — mounted within an ATEX-classified zone due to the flammable nature of isopropyl alcohol.
Challenges of measuring liquid level in ATEX zones
Maintaining the IPA level within ±1mm was essential. Too low, and the seal wouldn’t receive full coverage; too high, and the process would be inconsistent. The machine needed a way to monitor that level continuously and trigger an alert before it drifted out of tolerance.
A further complication: the cup had a cycling lid that closed for 14 seconds between each dip and opened for roughly one second to allow the robot to enter. That cycling created pressure transients in the headspace that would corrupt any single-port pressure measurement.
Why standard level sensors were not suitable
Ultrasonic and float-based level sensors were considered and ruled out. Float switches lack the resolution for ±1mm monitoring. Ultrasonic sensors can work in some IPA applications, but the confined geometry made external mounting impractical without line-of-sight issues, and achieving reliable sub-millimetre resolution in a 64mm diameter cup at short range introduces significant uncertainty.
Submersible level sensors were physically too large to fit inside the cup without displacing a meaningful volume of liquid or interfering with the dipping mechanism.
Why differential pressure is ideal for confined liquid level measurement
The correct approach for a small, confined liquid column — particularly one with a controlled headspace — is hydrostatic head pressure measurement using a differential pressure transmitter.
Hydrostatic level measurement: principles and accuracy considerations
Hydrostatic level measurement calculates liquid depth from the pressure exerted by the fluid column above a fixed point. The relationship is straightforward: pressure increases linearly with depth, so a calibrated pressure sensor at the base of the tank gives a direct, continuous depth reading.
For IPA at 70mm depth, the hydrostatic pressure is approximately 5.4 mbar. A ±1mm change in level produces a pressure change of roughly 0.077 mbar — a small signal that demands a high-accuracy sensor to resolve reliably.
Eliminating headspace pressure errors with differential measurement
The differential configuration is key here. Rather than measuring absolute pressure at the base (which would include any headspace pressure), the transmitter’s reference port is connected to the headspace above the liquid. Pressure changes from the cycling lid appear equally on both ports and cancel out, leaving only the hydrostatic signal from the liquid column itself.
Sensor selection for ATEX-compliant level measurement
Interface recommended the 236Ai — a 0–1 PSI differential unit with ±0.05% FSO BFSL static accuracy, 0–5V output, and ATEX/IECEx intrinsically safe approval.
The 0–1 PSI range wasn’t selected because it was the ideal fit for the application — in an unconstrained sensor selection, a much lower range would improve output resolution considerably. It was selected because it is the lowest differential pressure range available in an ATEX intrinsically safe approved unit. Standard low-range differential sensors can achieve ranges well below 1 PSI, but intrinsic safety certification places strict limits on stored energy within the sensor circuit, which tends to push the lower boundary of available ranges upward. The practical consequence here is that at 70mm IPA depth, the sensor is operating at roughly 8% of its full scale. The 236Ai’s ±0.05% FSO BFSL accuracy specification is referenced to full scale output rather than reading, so the absolute error remains fixed at ±0.034 mbar regardless of where in the range the measurement falls — which is what makes it viable despite the narrow working span.
The 236Ai resolved IPA level to within ±1mm in a compact, intrinsically safe installation
Performance calculations: achieving ±1 mm measurement accuracy
At 70mm IPA depth, the output sits at approximately 0.391V. The sensor’s ±0.05% FSO accuracy equates to ±0.034 mbar — equivalent to around ±0.5mm of IPA — giving a useful margin below the ±1mm application target.
A ±1mm level change produces a voltage change of approximately ±0.006V, so downstream instrumentation needs to resolve to at least 5mV. This is achievable with standard industrial instrumentation but is worth confirming at system design stage.
Installation and signal conditioning in confined systems
The high-pressure port connects to the base of the IPA cup via a 1/4″ NPT fitting. The reference port connects via tubing to the headspace, positioned well above the maximum liquid level to avoid splash ingress.
A short filter delay is applied in the control software after each lid opening event, allowing headspace pressure to equalise before the level reading is sampled. Without this, the brief pressure spike as the lid opens would register as a false level drop.
Results: Reliable ±1 mm IPA level measurement in an ATEX zone
The 236Ai resolved IPA level to within ±1mm in a compact, intrinsically safe installation. The differential configuration handled the headspace pressure cycling without issue, and the sensor’s accuracy provided enough margin to meet the application target while accounting for minor downstream instrumentation errors.
A differential pressure approach proved to be the most practical intrinsically safe method for continuous liquid level monitoring in a compact, confined geometry where conventional sensors could not meet the physical or accuracy requirements.
System components and materials
- 236Ai Compact High Accuracy Differential Pressure Transmitter
- 1/4″ NPT fittings and reference port tubing
- High-resolution voltage instrumentation (customer-supplied; 5mV resolution minimum)
- IPA cup assembly (Ø64 × 80mm, customer-designed)
How the differential pressure measurement system works
- The transmitter mounts externally, with the high-pressure port plumbed into the base of the IPA cup via 1/4″ NPT
- The reference port connects to the headspace above the liquid, clear of splash
- As IPA level drops, hydrostatic head at the base decreases proportionally, reducing the differential pressure and the 0–5V output signal
- Headspace pressure changes from the cycling lid appear on both ports simultaneously and cancel out in the differential measurement
- A software filter delay after each lid opening event prevents pressure transients from producing false readings
- The 0–5V output feeds directly to the customer’s control system for continuous monitoring and threshold alerting.
Frequently Asked Questions
What is isopropyl alcohol (IPA)?
Isopropyl alcohol (IPA) is a flammable liquid commonly used as a solvent, cleaning agent, and coating medium in industrial processes.
What is an ATEX zone?
An ATEX zone is a hazardous area where explosive atmospheres may be present. Equipment used in these environments must meet strict ATEX certification requirements to ensure safety.
What is differential pressure level measurement?
Differential pressure (DP) level measurement calculates liquid level by measuring the pressure difference between the bottom of a vessel and the space above the liquid.
Why is differential pressure suitable for confined spaces?
DP sensors can be mounted externally and do not require internal space, making them ideal for small or mechanically constrained vessels.
How does hydrostatic pressure relate to liquid level?
Hydrostatic pressure increases linearly with liquid depth. By measuring this pressure, the liquid level can be calculated accurately.
Why is headspace pressure a problem in level measurement?
Changes in headspace pressure (e.g. from lids or gas flow) can distort readings in single-port sensors, leading to inaccurate level measurement.
How does differential pressure eliminate headspace effects?
By measuring both the bottom pressure and headspace pressure simultaneously, a DP sensor cancels out common pressure changes, isolating only the liquid level signal.
What level of accuracy can differential pressure achieve?
With appropriate sensor selection and calibration, differential pressure systems can achieve sub-millimetre accuracy — as demonstrated in this application (±1 mm).
Why is ATEX certification important for sensors?
ATEX certification ensures that the sensor is safe to use in explosive environments and will not ignite flammable vapours.
Isopropyl alcohol level measurement in ATEX zones – find out more
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About Interface Force Measurement Solutions
Interface Force Measurements Ltd is a UK-based engineering specialist in force, torque, and pressure measurement systems. As the master distributor and technical centre for Interface load cells across the UK, Ireland, the Middle East, and North Africa, we do far more than simply supply products — we design, build, and support complete measurement solutions tailored to customer applications.
Our newly established UK calibration laboratory represents a major investment in UK capability. It enables us to provide comprehensive in-house calibration, testing, and verification services, to our customers. While ensuring every solution we deliver meets the highest international standards. This facility reinforces our commitment to supporting UK industry with precision, reliability, and fast turnaround times.
Working with world-class partners such as Interface, DDM Sensor Solutions, AMTI Force Measurement Systems, GP:50 Pressure Sensors, and XSENSOR Intelligent Dynamic Sensing, we integrate cutting-edge transducers, sensors, and instrumentation into fully engineered systems.
All of our partners share our values of quality, reliability, and customer focus. Whether you need a standard transducer, a custom-designed force measurement system, or complete system calibration and support, Interface Force Measurements provides manufacturer-grade solutions with UK-based technical expertise and service.