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Why 420? A closer look at IUOE’s Steam Gauge

We are commonly asked why the needle on the International Union’s steam gauge points to 420 psi, so we wanted to dive a little deeper on it today, on 4/20. This is by no means official.

The IUOE steam gauge was officially changed from 75psi to 420psi at an IUOE Executive Board meeting between the years of 1926 and 1932. The decision to make the change is in the minutes of the meeting, but the reason for the change of the gauge is not listed.  Though there is no hard evidence to support or deny any of the stories or theories we have heard over the years, we wanted to lay some of those theories out for you.

We were between World Wars and the use of steam boilers for construction equipment had taken off.  After the national roadway system started in the 1920’s, steam-powered shovels and construction equipment were being used. Steam shovels of the day only required 125psi to operate, but 420psi boilers being used for marine application including naval ships and steam boats. As the United States entered the Industrial Era, much of our manufacturing was based on naval applications. They required a higher steam operating pressure, and that application may have caught someone’s attention. 

Additionally, boiler steam gauges are manufactured for specific steam applications.  A steam gauge, as any other analog device, is designed to operate with maximum efficiency and accuracy at about two-thirds of the maximum gauge pressure.  Our steam gauge is a 600psi gauge, so 400psi is right around 2/3 of the gauge pressure.  Since there were boilers operating at 420psi in 1926, this very well have been a reason to depict our logo at that pressure, but again this is speculation. 

Per ASME code, boiler steam gauges are designed to have a maximum operating pressure of one and a half times the operating pressure of the boiler. Therefore, if the operating pressure of the boiler was 400 psi, the steam gauge maximum pressure for the boiler steam gauge would be 600psi.  It would not be unusual to see a boiler like this operating at 420psi.

The exact reason for the change may never be known, but we welcome your take on these theories or your own.

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