Details for Calibration of Capacitive Diaphragm Gauges

Australian Pressure Laboratory has a unique capability in vacuum among commercial NATA accredited laboratories, enabling us to offer calibration of capacitive diaphragm gauges (CDGs) down to 0.2 Pa absolute (0.002 mbar, 0.0015 Torr) and zeroing at base pressures down to 2x10-4 Pa. 

CDGs are accurate sensors for measuring vacuum in the rough to medium vacuum range, utilising a sensitive diaphraphm element referenced to an internal vacuum capsule. 

This sensing method has a variety of benefits, such as:

  • Good linearity, repeatability and stability in span, which supports relatively low uncertainty calibration (compared with Pirani/thermal sensing)

  • Relative insensitivity to test medium/gas type

However, there are also some drawbacks, such as:

  • Prone to zero drift

  • Sensitive to orientation

Accredited vacuum calibrations with direct traceability to Australia's National Measurement Institiute

Sensor Orientation

As mentioned above, zero drift and orientation sensitivity are drawbacks of CDGs.  Due to this, we will routinely perform a zero adjustment of your CDGs prior to calibration.  We will report the zero reading from your sensors before and after the zero adjustment so that you can track the zero performance.  For orientation, we use the below definitions. To ensure our zero calibration is well suited to your application, please let us know the installed orientation of your CDGs.  If you don't inform us, we will adopt the vertical-down orientation, which is recommended for most applications as it is less sensitive to orientation and less vulnerable to contamination in this configuration.

Sensor Output

Most commonly CDGs have a linear 0-10 V DC analogue output and/or a digital RS-232 data output, although it can vary across makes, models and specific applications. 

Where an analogue output signal is used, it is important that the CDG is either paired to a specific vacuum controller (or digital display) and calibrated as a system, or the output is measured with a calibrated electrical meter to ensure traceabilty of the calibration is maintained.  If you are using the analogue output signal from your CDGs, please be sure to inform APL prior to calibration and we can discuss options for calibration.  This may require you to send your vacuum controller/display along with the sensor for calibration, or we can calibrate your sensor in applied pressure/vacuum vs analogue output, but this would require you to also seperately calibrate your display/reading device for the corresponding analogue input.  We are capable and accredited in electrical calibration also, so we can offer calibration of your analogue mesaurement equipment if required.

Where digital output signal is used, the sensor instead outputs a datastream which should not be impacted by the particular display device used and allows your sensor to be portable between displays.  APL is capable of communicating with many sensor types via RS-232, RS-485 and HART, so please contact us to discuss the calibration approach prior to sending in your CDG.  We may be able to receive your sensor alone without a corresponding display.

Compliance Rules

Calibration of vacuum sensors below 1400 Pa is performed using APL's Vacuum Comparison System (VCS) following ISO 3567 Vacuum gauges — Calibration by direct comparison with a reference gauge.  Unlike other areas of pressure where APL is always focussed on confirm compliance to accuracy specifications, it is not generally possible or feasible to confirm manufacturer's accuracy specifications for CDGs, even with our direct traceability and best uncertainty available from the National Measurement Institute of Australia.  Therefore we will instead perform calibration to the best of our ability, including adjustment to optimise performance where possible, but we will not assess your CDGs for compliance to an accuracy specification unless you tell us your particular accuracy requirements.  Please review our best capability in our NATA Scope of Accreditation before accepting our offer for calibration.

As these are highly specialised devices, we encourage you to contact us directly to discuss the above factors and determine appropriate test points for your application prior to calibration.

If you require high quality accredited calibration of your vacuum instruments, request a quote online, or call us on +61 (0)3 9431 3658