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Understanding Alicat Calibration and Accuracy Specifications

Understanding Alicat Calibration and Accuracy Specifications

Each Alicat device comes with a NIST-traceable Certificate of Calibration, guaranteeing that its calibration can be traced back to the primary SI standards, such as mass and pressure. As a result, an Alicat device will output mass flow rates that fall within the same accuracy range as measurements obtained by any other similar NIST-traceable unit with an identical accuracy specification. During calibration with a NIST-traceable transfer standard, each step of the calibration process must be carefully controlled to ensure reliable readings. Some factors that can lead to errors during calibration include variations in temperature and pressure, leaks, setpoint changes, and control stability. During a NIST-traceable calibration of an Alicat device, all of these variables are carefully monitored and controlled, ensuring your device is calibrated to output precise, high accuracy mass flow readings every time.

How your device is calibrated

Unless a custom calibration is requested, Alicat calibrations evaluate readings at five points within the full scale range of your device: 0%, 25%, 50%, 75% and 100% of full scale.  These readings are compared with readings taken from one of our in-house flow or pressure standards.

What the calibration report tells you

Every report contains three sections: Device under test, equipment used, and calibration.

Device Under Test

This section contains information about your device and the conditions of the calibration.

  • Range: The full scale range of the unit. Since we can configure devices with custom ranges, this may not match the body size found in the part number.
  • Process Gas: The pre-selected gas from the gas menu. This can be changed through the front panel or during recalibration.
  • Calibration Gas: The gas used to calibrate the unit. Unless a customer specifically asks for an actual gas calibration, we typically use air.
  • Temperature: The temperature of the gas that was used to calibrate the device at our facility.
  • Humidity: The ambient humidity of our facility at the time of calibration.
  • Control Algorithm (P/D/I values): These are the default tuning settings on the device. Note: Most units will only have P and D settings, and only controllers will have this part.
  • Calibration Procedure/Rev. #: The standard procedure and revision used to calibrate the device.

Equipment Used

This section gives information about the tools Alicat uses to calibrate your device.

  • Type: The type of calibration performed.
  • Tool Name: The names of the tools used for the calibration.
  • Manufacturer/Model: The manufacturer and the model of the calibration tools. Note: some Alicat devices are used.
  • Uncertainty: The uncertainty of the calibration tools.
  • Due Date: The due dates for the tool’s NIST-traceable calibrations.

Calibration

This section contains information about the results of the calibration and has helpful tools for interpreting these readings. First, there is some information about the conditions and parameters of the calibration.

  • Accuracy: The uncertainty of the measurements. See below for a more thorough explanation.
  • Full Scale Range: The full scale range of the device being calibrated.
  • Calibration Pressure: The pressure at which the calibration was performed.
  • Valve Adjustment: P1 is the inlet pressure and P2 is outlet pressure used to tune the valve performance.  This table is only shown for controllers.

Then, there is information about your device’s readings before calibration (As-Found) and after calibration (As-Left). This lets you know if the unit was in spec when it arrived for calibration, and if not, which points were out of tolerance.  Only devices returned for recalibration will have As-Found data.

  • Indicated: This value is the reading from your Alicat device. Note: Units are taken from full scale range, in this case NLPM.
  • Actual: This value is the reading from the device performing the calibration.  Note: Units are taken from full scale range, in this case NLPM.
  • In Tolerance: This lets you know if any measured points were out of tolerance.
  • Output: These are your device’s analog outputs.

More recent reports also include error and control response charts.

  • Error Chart: This plots As-Found and As-Left data with reference to tolerance values.
  • Control Response Chart: This plots the valve response relative to a step change in setpoint using the specified P1 and P2 in the Valve Adjustment Table.  This chart is only present for controllers.

Accuracy: What it means and how it is calculated

Consider the below example, a 5 SLPM device with accuracy specification of ± 0.8% of reading or 0.2% of full scale.  The 0.8% of reading portion of the spec is a variable which scales down with decreasing flowWhen this meter is flowing at 100% of full scale, 0.8% of the reading is 40 SCCM.  But at 0.5% of full scale, 0.6% of the reading is only 0.2 SCCM0.2% of full scale is the set proportion of the instrument’s full scale. In this case, 0.2% of five liters is 10 SCCM.

Helpful video for understanding mass flow meter accuracy

Special calibration options

  • Bidirectional Calibration:  Bidirectional calibration is available for flow devices, gauge pressure devices, and differential pressure devices.
  • FP-25 Calibration:  Alicat FP-25 devices receive NIST-traceable calibrations for mass flow, temperature, and barometric pressure. Flow calibration accuracy is 1% of reading.
  • 3rd-Party 17025 Calibration Services:  Alicat conducts 17025 calibrations with the help of 3rd-party calibration houses. Upon calibration of the Alicat devices, the customer receives the standard Alicat NIST-traceable calibration documentation as well as a 17025 certificate of conformity provided by the 3rd-party. 17025 calibrations are available with L.A.B. or A2LA accreditation. For more information, see Calibration Types.
  • High Accuracy Calibration: High accuracy calibration is available on most mass flow and pressure devices. See the Specification Sheets for availability.

Recalibrating your device

Yearly recalibration is recommended for most Alicat devices. The most recent calibration date is located on the back of the device and is also stored in MENU > ABOUT > DEVICE INFO. To schedule recalibration service through Alicat, complete the Service Request Form. On receipt of your request, Alicat will issue you an RMA (Return of Materials Authorization) and send you shipping instructions.

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