Company Profile

Thorpe Tube Flowmeters and Regulators: Calibrated Medical Gas and Oxygen Dispensing Systems

Quick Answer: Thorpe tube flowmeters deliver direct readings in liters per minute for oxygen, air, and other medical gases. They use a calibrated tapered tube and a float at a fixed inlet pressure, typically 50 psi or 345 kPa. Silver Automation Instruments helps buyers source these units with the right flow range, gas calibration, and connection standard.

How a Thorpe Tube Flowmeter Works

A Thorpe tube is a variable area meter. The body has a tapered tube and a small float. Gas enters at the bottom. The float rises until the area around the float matches the flow. You read the flow rate at the top of the float or the center of the ball, depending on the scale design.

Most hospital units are open to atmosphere at the outlet. That means back pressure changes can affect the reading. For standard oxygen therapy and dispensing, the tube is calibrated at a supply pressure of 50 psi and near atmospheric outlet pressure. If you connect a long cannula or a humidifier, the flow shift is usually small. But for test rigs and mixing panels, you should confirm the back pressure with our engineers.

Here is the thing. The tube scale is gas specific. Oxygen, air, nitrous oxide, and carbon dioxide have different densities and viscosities. A float height that means 10 L/min for oxygen does not mean 10 L/min for air. Always match the tube label to the gas in the line.

Regulator Integration and Supply Pressure

A Thorpe tube flowmeter often sits on a regulator or a manifold outlet. The regulator reduces cylinder pressure to a fixed working pressure. In many gas systems, the set point is 50 psi or 345 kPa. Some markets and hospital networks use 4 bar or 400 kPa as the standard. The flow scale is only valid at the calibration pressure. If a clinic in Peru runs oxygen at 4 bar and the tube was calibrated at 50 psi, the display will drift.

We ask buyers for the supply pressure before quoting. The same flow tube can be matched to a different regulator spring and scale. For central pipeline systems with 4 bar pressure, a compensated Thorpe tube is used. For cylinder regulators, the inlet pressure may be stepped down inside the regulator body. This is why the regulator model and the flowmeter model are often ordered together.

Common Flow Ranges and Gas Types

Standard flow ranges include 0 to 1 L/min, 0 to 2 L/min, 0 to 3 L/min, 0 to 5 L/min, 0 to 8 L/min, 0 to 15 L/min, 0 to 25 L/min, and 0 to 30 L/min. Neonatal units use the low ranges. Adult oxygen therapy and ambulance systems commonly use 0 to 15 L/min or 0 to 25 L/min. Dental and veterinary clinics often use 0 to 5 L/min or 0 to 8 L/min for oxygen and nitrous oxide.

Medical air is used for nebulizers and ventilators in some wards. Carbon dioxide is used in laparoscopic surgery and cell culture labs. The gas name must be marked on the scale. Silver Automation Instruments can supply tubes with oxygen, air, CO2, nitrous oxide, and other gas calibrations.

Connection Standards and Wetted Materials

Connection types vary by country and outlet brand. Common interfaces are CGA DISS, Chemetron, Ohmeda, Schrader, AFNOR, JIS, and 1/4 inch or 3/16 inch hose barbs. Do not guess the connection. A hospital group in Vietnam ordered 50 oxygen flowmeter units with DISS outlets and later found two wall outlets were Chemetron. We now recommend photographing the outlet before finalizing an order.

Wetted materials are usually brass or aluminum with chrome plating. The tube shield is often polycarbonate or borosilicate glass. Seals can be EPDM or Viton. For oxygen service, cleanliness is critical. All parts must be free of oil and grease. Ask for oxygen cleaned components if the line runs above ambient pressure or feeds a compressor system.

Accuracy and Maintenance

Typical therapy grade Thorpe tube accu