APP-01Application
Compressed Air Monitoring: Pressure, Dew Point and Flow Measurement
Compressed air is an energy-intensive utility that is often managed by walking the plant. Measuring pressure, dew point, filter condition and flow at the right points shows whether the system delivers dry, clean air at the pressure users need — and where energy is being lost.
| Tag | Service |
|---|---|
| PT-101 | System pressure at the receiver |
| TT-102 | Dryer inlet temperature |
| MT-103 | Pressure dew point after the dryer |
| PDT-104 | Differential pressure across the filter |
| FT-105 | Header flow |
01Measurement map
Measurement points on a typical compressed air system
Each instrument sits where it is typically installed. Select one to see why it is measured, the typical technology and how its signal reaches the control system.
- Process line
- Electrical signal
- Instrument (ISA tag)
- Selected signal path
- 1PLC / SCADA
- 2Compressor
- 3Air receiver
- 4Dryer
- 5Filter
- 6Points of use
- 7Distribution header
System pressure at the receiver
- Why it matters
- Receiver pressure is the reference for compressor load/unload and sequencing, and shows how much stored air is available to absorb demand peaks. Compared with pressure further downstream, it also reveals the pressure drop across drying and filtration.
- Parameter
- Gauge pressure, bar(g)
- Typical technology
- Gauge pressure transmitter with piezoresistive or capacitive sensing
- Local pressure gauge as a field reference
- Pressure switch where a hard-wired limit is required
- Integration
- 4–20 mA into the PLC for sequencing logic, alarms and SCADA trends. HART adds diagnostics and remote configuration where the I/O supports it.
- Why it matters
- Receiver pressure is the reference for compressor load/unload and sequencing, and shows how much stored air is available to absorb demand peaks. Compared with pressure further downstream, it also reveals the pressure drop across drying and filtration.
- Parameter
- Gauge pressure, bar(g)
- Typical technology
- Gauge pressure transmitter with piezoresistive or capacitive sensing
- Local pressure gauge as a field reference
- Pressure switch where a hard-wired limit is required
- Integration
- 4–20 mA into the PLC for sequencing logic, alarms and SCADA trends. HART adds diagnostics and remote configuration where the I/O supports it.
- Why it matters
- Warmer air holds more water vapour, so every degree at the dryer inlet adds load to the dryer. A rising inlet temperature points to a fouled aftercooler, a failing fan or a hot compressor room — and usually shows up in the dew point next.
- Parameter
- Air temperature after the aftercooler, °C
- Typical technology
- Pt100 RTD in a thermowell
- Head-mounted or rail-mounted temperature transmitter
- 4–20 mA or HART output
- Integration
- Analog input to the PLC with a high-temperature alarm, trended against ambient temperature and compressor load.
- Why it matters
- Dew point is the direct measure of whether the dryer is doing its job. If it rises, condensate can form in the network — corroding pipework, washing lubricant out of tools and contaminating product where air touches it. Measured at line pressure, it compares directly with the required air-quality class.
- Parameter
- Pressure dew point, °C td
- Typical technology
- Capacitive dew-point transmitter (polymer or metal-oxide sensor) selected for the expected dew-point range
- Sampling block with a controlled bleed flow
- Periodic calibration or sensor exchange in the maintenance plan
- Integration
- 4–20 mA to the PLC with a high dew-point alarm. The trend is commonly used to verify desiccant regeneration or refrigerant dryer performance.
- Why it matters
- As a filter element loads, its pressure drop rises — and the compressors must run at a higher discharge pressure to make it up. A loaded filter costs energy long before it causes a supply problem. Monitoring the drop lets elements be changed on condition instead of on a fixed schedule.
- Parameter
- Differential pressure, mbar
- Typical technology
- Low-range differential pressure transmitter
- Differential pressure gauge or indicator on the filter housing
- DP switch for a simple change-element alarm
- Integration
- Analog DP to the PLC for alarm and trend, or a potential-free DP-switch contact to a digital input where a change alarm is enough.
- Why it matters
- Flow shows what the plant actually consumes. Trended against production, it separates demand from leakage — significant flow during non-production periods is the clearest indicator of leaks. It is also the basis for allocating compressed-air cost to areas and for compressor sequencing.
- Parameter
- Flow rate, Nm³/h, and totalised volume
- Typical technology
- Thermal mass flowmeter, insertion or inline — reads mass flow directly
- Vortex flowmeter with pressure and temperature compensation
- DP flow element for larger lines
- Integration
- 4–20 mA or pulse output for totalisation, or Modbus / fieldbus to carry flow, totals and diagnostics into SCADA energy reports.
02Engineering context
Why measurement matters
Most compressed-air problems stay invisible until they reach the point of use: moisture that condenses in distribution pipework, a filter element that has quietly loaded up, a header pressure that sags when demand peaks. Each one has a measurable signature further upstream.
Pressure dew point after the dryer confirms that treatment is working. Differential pressure across a filter shows when the element is adding pressure drop that the compressors must make up. Pressure and flow show how the system responds to demand, and give the basis for leak assessment and compressor sequencing.
Brought into a PLC or SCADA system, these signals turn a utility that is checked occasionally into one that is monitored, trended and alarmed — so maintenance can act on condition rather than on complaints.
Operating conditions
- Medium
- Compressed air carrying water vapour, and oil aerosol and particles depending on compressor type and treatment stage.
- Pressure
- Set by the most demanding point of use plus distribution losses; general plant-air networks commonly run at 6–8 bar(g).
- Temperature
- Hot at the compressor, cooled by the aftercooler and dryer, close to ambient in distribution.
- Moisture
- Saturated after compression and aftercooling. The dryer type sets the achievable pressure dew point — desiccant dryers reach far lower dew points than refrigerant dryers.
- Air quality
- Commonly specified as purity classes for particles, water and oil under ISO 8573-1, which sets the measurement and treatment needed.
- Installation
- Receiver and pipework connections, dryer outlets and filter housings; dew-point sensors usually sit in a sampling block with a controlled bleed flow.
03Measurement parameters
What is measured, where, and why
The parameters typically measured in this application, with the location, the reason and the technologies commonly used. The right selection always depends on the process conditions.
| Tag | Parameter | Location | Why it matters | Typical technology |
|---|---|---|---|---|
| PT | Pressure | Receiver, distribution header, critical points of use | Compressor control, available storage, pressure drop across treatment | Gauge pressure transmitter; gauge or switch for local indication and limits |
| TT | Temperature | Aftercooler outlet / dryer inlet | Cooling health and dryer load | RTD in a thermowell with a temperature transmitter |
| MT | Pressure dew point | Dryer outlet | Confirms drying and protects the network from condensate | Capacitive dew-point transmitter in a sampling block |
| PDT | Differential pressure | Across each filter stage | Element condition and the energy cost of pressure drop | Low-range DP transmitter, DP gauge or DP switch |
| FT | Flow | Main header and area branches | Consumption, leak assessment and cost allocation | Thermal mass or compensated vortex flowmeter |
PTPressure
- Location
- Receiver, distribution header, critical points of use
- Why it matters
- Compressor control, available storage, pressure drop across treatment
- Typical technology
- Gauge pressure transmitter; gauge or switch for local indication and limits
TTTemperature
- Location
- Aftercooler outlet / dryer inlet
- Why it matters
- Cooling health and dryer load
- Typical technology
- RTD in a thermowell with a temperature transmitter
MTPressure dew point
- Location
- Dryer outlet
- Why it matters
- Confirms drying and protects the network from condensate
- Typical technology
- Capacitive dew-point transmitter in a sampling block
PDTDifferential pressure
- Location
- Across each filter stage
- Why it matters
- Element condition and the energy cost of pressure drop
- Typical technology
- Low-range DP transmitter, DP gauge or DP switch
FTFlow
- Location
- Main header and area branches
- Why it matters
- Consumption, leak assessment and cost allocation
- Typical technology
- Thermal mass or compensated vortex flowmeter
04Field → Control → Supervision
From the field to the control room
The same application seen as layers of one system — and the Spaaronn capability that engineers each layer.
Field
Measure and analyse at the process
Instrumentation
Pressure, differential-pressure, dew-point, flow and temperature instruments selected for the air-quality class, pipe sizes and installation points.
- Transmitter and sensor selection
- Sampling arrangement for dew point
- Installation, calibration and loop checks
Analytical
Dew point is the analytical measurement here; oil-vapour or particle monitoring can be added where the required air-quality class demands it.
- Dew-point range and sensor type
- Additional air-quality measurements where specified
Control
Decide and act on the signals
Automation & monitoring
PLC logic that turns the signals into alarms, condition-based maintenance triggers and inputs for compressor sequencing.
- High dew-point and low-pressure alarms
- Filter-change alarms from DP
- Local HMI where operators need it
Supervision
See, record and report across the plant
Integration
Signals brought into SCADA or the plant historian for trending, energy reporting and alarm management across compressor rooms and areas.
- Trends and energy reports
- Leak-assessment views
- Alarm routing to maintenance
05Project sequence
Typical project sequence
How this application is typically taken from requirement to a supported, running system.
- 01
Define the air requirement
The pressure and air-quality class each point of use needs, and where air contacts product.
- 02
Survey generation and treatment
Compressors, receivers, dryers, filters, existing instruments and how the system is controlled today.
- 03
Select points and technologies
Where to measure pressure, dew point, DP, flow and temperature, and which technology suits each point.
- 04
Integrate with PLC / SCADA
Signals, alarms, trends and energy reports engineered into the existing or a new control system.
- 05
Commission and maintain
Loop checks, dew-point sensor calibration intervals and a maintenance plan for the instruments.
Next steps
Where to go next
- 01IndustryManufacturingMachine automation, plant utilities and line-level monitoring.
- 02ResourcePressure vs Differential Pressure MeasurementGauge, absolute and differential pressure — what each measures and where each is used.
- 03SolutionAMC & Lifecycle SupportPreventive maintenance, breakdown support, calibration support, troubleshooting and upgrades for installed systems.
Engineering enquiry
Monitoring a compressed air system?
Share the compressor and dryer arrangement, the air quality your points of use need and what is measured today. We can help define the measurement points and how they reach your control system.