Clock Gauges

Overfill Prevention and Clock Gauges

Most states require two methods that prevent aboveground storage tanks that store combustible hazardous liquids (such as oil and fuel) from being overfilled resulting in a spill - even a small one. The first method is a mechanical overfill prevention device mounted inside the pipe where the liquid is loaded into the tank. When the fuel gets to a certain level it is supposed to automatically close, preventing any more liquid from going in. I say "supposed to" because mechanical devices do fail from time to time. Thus the reason for the second method, which is a visual way to see if the tank is almost full or is full. A "clock gauge" is normally used for that purpose.

What is a Clock Gauge?

A clock gauge is a device that is mounted on top of an aboveground storage tank (AST) and its purpose is to determine and indicate the amount of fuel in the tank. A clock gauge by itself actually only reads the level of the liquid in the tank measured in feet and inches (that is how deep it is from the top of the liquid to the bottom of the tank), so making the determination is actually a two step process. First you read the clock gauge to get the level in feet and inches and second you read a conversion chart to get the number of gallons based on the feet and inches. We will talk more on how to read clock gauges and conversion charts below.

Types of Clock Gauges

There are two standard types of clock gauges. The differences are primarily in the face as can be seen below. The internal workings are generally the same. More on how a clock gauge works below. One displays feet and inches with a typical dial indicator face with a small hand and a big hand (like a clock that tells time). The other type displays feet and inches digitally. As an option, clock gauges can be purchased with an alarm that can be set to go off when the liquid reaches a level you set. Most state regulations require it to be set to go off just before the overfill prevention device closes (seals off) the fill pipe, which is somewhere between 90% and 95% full.

Clock Gauge: Dial Face Style - from
Dial Face
Clock Gauge: Digital Style - from
Digital Face
Clock Gauge: Dial Style with Alarm - from
Dial Face with Alarm

How a Clock Gauge Works

A standard clock gauge's internal parts consist of a coil-spring loaded pulley with a cable wrapped around it. The pulley's shaft has a gear on it that turns the gears that move the large and small hands or digits on the face of the clock. The clock gauge also has an adjusting mechanism. It's shaft with a groove in the end on the back side of the clock with a gear tied to the clock hands or digits. It allows their position to be adjusted with a screw driver independent of the pulley for calibrating purposes. More on calibrating in a bit.

Externally, the cable drops down through the bottom of the clock gauge where the clock gauge connects to the tank. The cable has a connecting mechanism on the end of it that provide a way to easily connect the float. During installation the float is carefully lowered into the tank until it touches and floats on the liquid.

The coil-spring in the pulley is very sensitive. It provides just enough tension on the pulley to turn it and the clock gears, yet not pull the float out of the liquid. With that operation the hands or digits on the clock move as the liquid in the tank raises and lowers.

Calibrating a basic clock gauge is simple. Before installation a tape measure is lowered into the tank until it touches the liquid. One way to know it touched is to see ripples on the surface. Sometimes it takes a few tries before getting it just right. The tape measure is then read for the distance in feet and inches to the point where the clock gauge connects to the tank. However the exact point to be measured could vary depending on the manufacturer's instructions. After the clock gauge is installed, the adjustment mechanism described above is used to set the clock hands or digits to the feet and inches measured. From that point on the movement of the clock gauge hands or digits is relative to that setting. More on that below.

Clock Gauge: Installation - from

Installing a Clock Gauge

Not all clock gauges are the same, so it's important to follow the manufacturer's installation instructions. That being said, you can get a good idea by watching this video at Morrison Bros. Co. website.

How to Read a Clock Gauge

As described above, standard clock gauges actually only show you the level/depth of the liquid in a tank (in feet-inches). To determine how many gallons are in the tank based on what the clock gauge shows you must use a chart that converts the feet-inches into gallons. See Clock Gauge Conversion Charts below.

The only difference between reading a dial clock gauge and dial clock you read time from is that on a clock gauge the small hand tells feet and the big hand tell inches. Otherwise they look very similar.

How to Convert Clock Gauge Feet-Inches to Gallons

It doesn't matter what type of clock gauge it is, to convert clock gauge feet-inches to gallons simply go to the clock gauge conversion chart associated with the tank's specific shape and size and find the feet-inches you read on the clock gauge. Then go across to find the corresponding gallons. It's that simple.

WARNING! Only use clock gauge conversion charts that are prepared for the specific shape and size of the tank the clock gauge is on. Always verify! See Clock Gauge Conversion Charts below.

Clock Gauge: Conversion Chart Sample - from

Clock Gauge Conversion Charts

As warned above, it's important to understand that the conversions indicted on a clock gauge conversion chart are based on the associated tank's shape and size. The symbols under Downloads below show the different shapes. In other words clock gauge conversion charts must be customized for each tank, unless of course all of your tanks are the same. See Tank Shape Legend below.

Tank Shape Legend
Tank Shape - Cylindrical with Flat Ends - from
Horizontal-Cylindrical with Flat Ends
Tank Shape - Cylindrical with Dished Ends - from
Horizontal-Cylindrical with Dished Ends
Tank Shape - Cylindrical with Eliptical Ends - from
Horizontal-Cylindrical with Elliptical Ends)
Tank Shape - Cylindrical with Hemispherical Ends - from
Horizontal-Cylindrical with Hemispherical Ends)
Tank Shape - Vertical Cylinder - from
Tank Shape - Box or Rectangle - from
Box / Rectangle

For structural reasons most tanks are cylindrical in shape. The most important thing to know when preparing a clock gauge conversion chart for a cylindrical tank is whether it sits in a vertical or horizontal position. The reason is because the calculations are different.

Compared to horizontal cylindrical tanks, vertical cylindrical tanks are relatively simple to calculate because the bottoms are always flat. Horizontal tanks are more difficult to calculate because the calculation must account for partial diameters when the tank is only partially filled. If the ends are dished, elliptical, or atmospherical in shape the calculation becomes even more complex. We mention calculations here mostly to stress the importance of using the correct clock gauge conversion chart.

Clock Gauge Conversion Chart Downloads

The Clock Gauge Conversion Charts listed below are available for download here or by clicking on them. They are Excel spreadsheets and customizable to the limits indicated on them. If you try to input a diameter or width that is greater than the "Max" allowed a message box will alert you. Should you need a chart for a tank with a diameter or width greater than what we provide, just let us know via our Contact Us page and we should be able to prepare one for you.

The max limits exist for formatting reasons only. For example, "Max10ftDia" means that only calculations up to a 10' diameter or width tank will fit on an 8 1/2" x 11" chart. Any larger than that and larger paper would have to be used (such as legal 8 1/2" x 14" or ledger 11" x 17").

After downloading and opening, be sure to adjust the figures in blue so that the volume in gallons (gal) will recalculate according to the size of your tank.

File name legend:
  • Cyl = Cylindrical
  • Dia = Diameter
  • Max = Highest the chart will go
  • Hori = Horizontal
  • Vert = Vertical
  • Rec = Rectangle

Tank Shape - Cylindrical with Flat Ends - from
Clock Gauge Conversion Chart (Hori-Cyl-Max10ftDia-Flat Ends)
Tank Shape - Cylindrical with Dished Ends - from
Clock Gauge Conversion Chart (Hori-Cyl-Max10ftDia-Dish Ends)
Tank Shape - Cylindrical with Eliptical Ends - from
Clock Gauge Conversion Chart (Hori-Cyl-Max10ftDia-Elliptical Ends)
Tank Shape - Cylindrical with Hemispherical Ends - from
Clock Gauge Conversion Chart (Hori-Cyl-Max10ftDia-Hemispherical Ends)
Tank Shape - Vertical Cylinder - from
Clock Gauge Conversion Chart (Vert-Cyl-Max10ftDeep)
Tank Shape - Box or Rectangle - from
Clock Gauge Conversion Chart (BoxRec-Max10ftDeep)

Purchasing a Clock Gauge

There are three things that must be determined before purchasing a clock gauge:
  1. Diameter of cylindrical tank or height of box/rectangular tank.
  2. Diameter of available connection to tank.
  3. Type of clock gauge you want: dial face or digital face.
Clock Gauge: Dial Style (cutaway) - from

Clock Gauge Manufacturers

You can find them on the internet. Morrison Bros. Company is a good place to start.

Wrapping It Up

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