Company Profile

Thorpe Tube Flowmeters: Specialized Regulation for Medical and Gas Applications

Quick Answer: Thorpe tube flowmeters are variable area flowmeters built for low pressure gas service. They show flow rate on a direct reading scale and need no external power. Silver Automation Instruments supplies these flowmeters with brass or 316 stainless steel bodies for oxygen, air, CO2 and nitrous oxide lines.

Hospitals use Thorpe tube flowmeters on oxygen therapy lines. Industrial gas users install them on purging and blanketing panels. The flowmeter has a vertical glass tube, a float and a needle valve. It is simple, rugged and does not need wiring or 4-20 mA signals for basic use.

What Is a Thorpe Tube Flowmeter?

A Thorpe tube is a specific type of variable area flowmeter. Gas enters at the bottom, lifts a float inside a tapered glass tube, and flow rate is read at the top of the float. This flowmeter works only in vertical mounting. Medical gas systems use it because the reading stays visible even when power is lost.

Most engineers skip the material question on the first enquiry. Do not do that. Body material and seat material change service life in dry oxygen and humid gas lines. Silver Automation Instruments stocks brass body units for general service and 316 stainless steel units for corrosive or high humidity areas.

Medical Applications and Gas Regulation

Medical gas systems operate at stable low pressure. A Thorpe tube flowmeter connects to a wall outlet or regulator outlet. It controls oxygen flow to a patient or to an anesthesia circuit. Common scales include 0 to 1 L/min, 0 to 3.5 L/min and 0 to 15 L/min for oxygen. Gas specific scales matter. A flowmeter calibrated for oxygen should not be used for CO2 without recalibration.

Hospitals in Southeast Asia and the Middle East often ask for oxygen flowmeters with 1/8 NPT or Chemetron type connections. Industrial gas users ask for nitrogen purge rotameters with 1/4 NPT or G1/4 connections. We keep both styles available for quick shipment.

Why Specialized Regulation Matters

Thorpe tube flowmeters combine a needle valve and a direct reading tube in one body. Some models have the valve upstream. Some have the valve downstream. The difference changes accuracy under back pressure. For medical oxygen delivery, the valve is usually downstream from the tube to keep flow stable. For purge gas, an upstream valve is more common.

Here is the thing. Many cheap Thorpe tubes use a soft brass valve seat that wears fast on dry oxygen. We use PTFE or PEEK seats for oxygen service because dry gas tears through soft brass over time. In practice, our engineers ask for gas type, inlet pressure, outlet pressure and required flow range. A flowmeter without this data may end up with the wrong scale or the wrong valve trim. We have seen this on customer sites. A hospital buys a cheap flowmeter for oxygen, then complains that the float sticks after six months. The cause is usually a soft valve seat or contaminated gas.

Materials and Specifications

Standard Thorpe tube bodies are brass with a clear polycarbonate shield. For corrosive gas or humid environments, we recommend 316 stainless steel with PTFE seals. The float is usually stainless steel or sapphire. Accuracy for Thorpe tube flowmeters is typically plus or minus 5 percent of full scale. This is enough for medical therapy and purge monitoring. For custody transfer, you need a different device such as a Coriolis mass flowmeter.

Typical specifications for the Silver Instruments STF series:

Flow range: 0 to 1 L/min up to 0 to 100 L/min air equivalent

Pressure: 4 bar standard, 10 bar optional

Temperature: up to 60 degrees Celsius

Connections: 1/8 NPT, 1/4 NPT, G1/4, Chemetron, DISS

Body material: brass or 316 stainless steel

Va