How to Calibrate A Ruler

When it comes to ruler calibration, we are talking about the accuracy of its graduation.

Ruler is a linear measuring hand tool we use in broad areas such as school, office, workshop, manufacturing, etc. It’s not only for measuring length but also for marking and scribing lines.

Depending on the purpose of using the ruler, the accuracy can matter. When it comes to the ruler’s accuracy, it refers to its graduation accuracy. And checking the graduation can be achieved through a ruler calibration or inspection.

If you want to check whether the ruler you are using is accurate or not, then this post can help you get the idea of what to do. Simply put, you will be comparing your ruler with a calibrated precision ruler. By using a magnifying loupe, you will notice how aligned the graduation is between them. Then, you can decide whether your ruler is good or not.

Calibrated Precision Rule

In British, the standard that governs the accuracy of steel ruler is BS 4372 “Specifications of Engineer’s Steel Measuring Rules’.

According to this standard, for the scale of 300 mm and shorter, the maximum permitted unalignment of the distance between any two graduation lines is 0.1 mm. And the maximum permitted unalignment for the position of the 10 mm graduation line from the flat ned datum is 0.08 mm.

The ruler must maintain a set level of tolerance at a standard temperature as per the BS 4372 standard. These set of tolerances are as follows:

  • The edge of the rule should be straight to within 0.1 mm on any rule segment up to 300 mm.
  • The edge of the rule should be parallel to within 0.1 mm on any rule segment up to 300 mm.
  • The flat ends of the rule should be square to the edges of the rule within 0.05 mm over rule width.
  • The tolerance of the graduation lines for ruler lengths from 300mm up to 500mm should be maintaining the following standard of tolerances:
    • A single scale distance between any two graduation lines should be 0.1-0.25 mm
    • Distance between any two adjacent graduation lines should be 0.05 mm
    • The position of the 10mm graduation line from the flat end datum should be 0.08 mm.

In some cases, a ruler uses a tolerance for the tape measure. According to EEC Class 1 standard, the tolerance is ±0.3 mm per 2 meters.

How about the standard that regulates the ruler accuracy in the USA?

Now, it’s clear what kind of ruler accuracy that can be used as a reference or standard. The next step is to discuss how to use it.

Steps On Checking Steel Rule Accuracy

  1. It is recommended to clean the ruler with oil and a soft cloth to remove any dust or dirt particles from its surface which may cause trouble in the successive stages of checking ruler accuracy.
  2. First of all, it is advised to check if the rule is not deformed or bent from anywhere by placing it on a uniform, dry, and clean flat surface. You can also use a rectangular straightedge as the replacement for the flat surface. If the ruler does not wobble, then it is a good sign.
  3. Take a quick check of the graduation finish of the rule to detect any surface issues with its graduation lines. If any such issue exists, it is important to fix it before validating the rule’s accuracy because it causes poor readability of the rule.
  4. Prepare the standard ruler. You will compare this standard ruler with your ruler. As we have discussed, it should be a calibrated ruler that is known for its accuracy tolerance.
  5. Now place the ruler and the standard ruler side by side on a flat surface and clamp them together. This way is to make both rules rigidly hold together.
  6. Take a good magnifying glass and before using it, clean it well.
  7. Take the magnifying glass. Don’t forget to ensure that the inspection performs in a well-lit environment or at least use a light-aided magnifying glass.
  8. Using the glass, check every gradation line of both the rules that are stacked in front of each other. Any inconsistency will be marked by the lines of both the rules not being in line with each other.
  9. It is advised to mark the graduations that are not consistent with the standard rule for record and further analysis.

What to Do with Your Ruler After the Checking Process

After validating the graduation accuracy of your ruler with a calibrated ruler, now you know whether it’s aligned or not. It is important for users to know what to do if the rulers deviate from the acceptable accuracy standards and how to do so.

  • If the ruler shows that the gradation lines show increased values on the whole throughout the rule, then it is most likely due to a certain degree of minimal expansion of the rule. Either due to excessive heating or due to extensive elongation force. Such rules should be kept in use if they are used for non-critical tasks that do not require a really high level of accuracy.
  • In the case of the ruler that shows deviations out of tolerance being used for precision purposes, then it is important to stop the use of that ruler and replace it with another brand new accurate ruler.

Factors that Help in Checking the Accuracy of Steel Rule

1. Good Material and Finish

Steel rules made from rust-resistant and anti-glare material are really useful for the validator to test the ruler’s accuracy. Rust and corrosion can damage the graduation lines, and make them unclear for vision. The anti-glare finish may help you see the lines well through the loupe and minimize the light reflection.

2. Font and Ruler Gradation

The various lines of the ruler’s graduation must be distinctly different for the user to realize the significance of each. It is also observed that the font used for the ruler and the boldness take a key role in helping the user read the graduation. Aside from that, how the graduations are put on the ruler can matter. Compared to the printed graduation, it’s easier to read the embossed, laser engraved, or etched graduation.

3. Color of the Rule and Font

The colors of the rule help greatly in improving the readability of the scale. Moreover, the combination of font colors should be considered carefully for the numbers and lines. Generally, a grey to silver color with black font is preferred in rules for a great visual experience.

Word of Advice

Various models of rulers are available on the market to suit your need. They come with different accuracy levels, from the lower accuracy to the higher one. In terms of material, some of them are made from plastic, while some are made from steel. These rulers require validation of their graduation accuracy through calibration or inspection, in which you can rely on a calibration service or do it yourself.

Interestingly, a ruler can be a feature of a certain tool. These are the tools that incorporate ruler as the feature: speed square, steel framing square, T ruler, combi square, contour surface profile gauge, multi-angle replicator ruler, and many more. That said, when you assess the accuracy of those tools, don’t forget to check the accuracy of the ruler’s graduation as well.

A ruler calibrator exists to purchase, but it’s expensive. You can start your own inspection at a lower cost by buying a calibrated precision ruler, then start comparing the graduation between these two rulers through a magnifying glass. If your rulers are showing great realignment, then it’s not accurate. Depending on your accuracy requirement, you can decide whether your ruler is usable or not. Make sure to stick to the standard that regulates in your country (or which one you prefer) when it comes to conforming to the tolerance standard.

Don’t Forget to Check the Ruler Edge

It’s worth mentioning that in some cases, your ruler can also perform as a straight edge. That means your ruler can be used to check the straightness of a certain surface. In this case, your ruler must perform two inspections in which checking the graduation accuracy and the ruler’s edge straightness. When it comes to validating the straightness, you require a higher grade straight edge made for calibration or inspection, not the workshop tool grade. And, make sure it should be the rectangular model as we will check the ruler.