Multiple Sonic Nozzle Calibration System

RITTER Data sheet
Multiple Sonic Nozzle Calibration System

Table of Contents

Operating Principle

Sonic Nozzles and Critical Flow Venturis are constant volumetric flow measurement meters that can be used as a calibration standard for gas ?ow meters. Due to their high repeatability they are extremly precise. Sonic devices vary in size with an internal design of a nozzle consisting of a circular arc which leads to a minimum throat area with a smooth circular inlet. This design operates on the principle that as gas ?ows through the throat of the nozzle, the gas accelerates (Venturi effect) as it approaches the throat. As the differential pressure increases, the velocity at the throat, the minimum area, increases and the density decreases. When the velocity reaches the speed of sound (Mach 1), it is considered choked, sonic or critical. The Sonic Nozzle is operated by applying inlet pressure or using a vacuum to evacuating the exit to achieve an Inlet/Outlet pressure ratio of approx. 1.3 or greater. This ratio maintains the nozzle in a »choked« or »sonic« state. In this state, only the upstream pressure and temperature can in?uence the ?owrate through the Nozzle. Nozzle ?owrate becomes close to a linear function of the inlet pressure. Increasing the inlet pressure can increase the ?ow rate. One of the greatest advantages of Sonic Nozzles when compared to sub-sonic ?owmeters is that downstream pressure disturbances cannot move upstream past the throat or sonic point of the Nozzle. Since throat velocity is higher and in the opposite direction of downstream disturbances, these pressure disturbances cannot move upstream past the throat or affect the velocity or the density of the ?ow through the Nozzle. Uncertainty levels of ±0.25% of reading or better can routinely be achieved since there are no moving parts.


  • Sonic Nozzle measurement principle
  • Calibration of gas volume ?ow and gas mass ?ow
  • Calibration Certifcates for nozzles by German National Primary Standard »PTB«
  • Calibration uncertainty of nozzles 0.15% ~ 0.65%
  • System calibration uncertainty 0.3% ~ 0.8%
  • Long-term repeatability and accuracy from extreme sonic nozzle stability
  • No retro-action from unit under test because of high pressure and »sonic« state
  • Single & multi-point calibrations fully automated by PC & multi-lingual software
  • Data storage to hard disc, export to MS-Excel, paper print of diagram and reports
  • Computerized individual pneumatic valve control for each nozzle

Specifcations (for stated example below)

  • 13 sonic nozzles
  • Smallest nozzle: 1 ltr/h
  • Largest nozzle: 15,000 ltr/h
  • Flow rate range:1 to 34,000 ltr/h
  • Number of possible calibration ?ow rates with 13 nozzles: 8,191 (= 213 – 1)

Installation Confgurations

There are two possible confgurations for Sonic Nozzles:

  1. Pressurized System
  2. Vacuum System

The RITTER system is designed to operate using the most common Pressurized System. It was decided against the alternate vacuum system, because it would draw from the meter being calibrated. This could introduce water or oil vapour of the Unit Under Test (UUT) into the calibration system causing disruption of the Sonic Nozzle design. A pressurized gas source like filtered lab air is used with a pressure regulator upstream of the Sonic Nozzle system. The measured process downstream can be at atmospheric pressure or greater as long as a minimum of 1.4 pressure ratio is maintained across the Nozzle. Using the pressure system approach, the Sonic Nozzle System will operate properly as designed, regardless of ?ow pulsations or ?uctuations from the process downstream. The air?ow range is directly related to the number and size of the installed nozzles as well as from the maximum pressure of the available air supply.

RITTER Multiple Sonic Nozzle Calibration System

Calibration service facilities that must maintain multiple types and sizes of meters, require the ability to calibrate a wide range of ?ow rates. RITTER wet test meters measure from 1 to 18,000 litres/hr. Dry gas meters can measure from 40 to 160,000 litres/hr. In order to extend the ?ow range beyond that of a single nozzle, the Sonic Calibration System can include multiple nozzles which can be installed in parallel. In the example arrangement shown in the schematic above, the installed 13 nozzles can be operated individually, in combination, or all together, providing the possibility of generating more than 8,000 different ?ow rates. With the number and sizes of those nozzles a total ?ow rate range of 1 litre/hour to approx. 35 m³/hr can be achieved. Sonic nozzles offered are ASME and ISO certifed. As with all RITTER products, the RITTER Multiple Sonic Nozzle Calibration System can be custom made to the requirements of the customer.

V 1.0 / Rev. 2024-02-07 / Subject to alterations.