There are multiple reasons for errors that may happen while using a caliper be it the vernier, dial, and digital model. The sources of error may be generated by the faults in the instrument, human error in the method of calculation, environmental factors, etc. These errors, further, can affect negatively the measurement results; though some of the errors are still within the tolerance.
Therefore, a knowledge of all errors is important for their ultimate control. A few of the errors in the caliper are discussed below:
1. Parallax Error
Parallax error occurs in vernier and dial caliper models. Perhaps, this is the most errors that happen in the caliper.
A parallax error, as the name suggests, occurs when the eyes are not parallel to the markings of the device in a straight line. In other words, it occurs when the object is viewed at an angle. Due to this reason, the measuring graduation appears at a different position than its actual position.
Parallax error can be rectified/controlled by improving the method/technique of taking measurements. Or simply, you can change your vernier or dial model with the digital one.
2. Abbe Error
Unlike the micrometers that are not susceptible to abbe errors, the caliper is prone to this error. Worsely, abbe errors can occur with any type of caliper. The reason behind this error happens is that the calipers do not conform to Abbe’s rule of alignment.
In comparison to parallax error, the Abbe error does not occur due to the alignment of the viewing angle like parallax error, instead, it is due to the alignment of the machine axis.
The distance between the axis of the instrument and the axis along which the object is measured is called the abbe offset. Now, if the distance along the scale does not get the distance along the object transferred in a perpendicular direction, then it will cause an error in measurement. This error can be measured by taking the tangent of the angular error multiplied by the abbe offset.
Abbe error is controllable by taking care while setting the object between the jaws of the caliper carefully. It is important to not use the tip of the caliper for hard objects. Besides, excessive force applied to solid hard objects may also break the jaws and cause misalignment. Moreover, any bend or irregularity in the vernier’s body may result in this error. Thus, it is an urge to not use a vernier caliper for any sturdy work like a hammer, etc.
3. Error Due to Low Power Battery
This error happens to the digital caliper. When the digital calipers have a weak battery, the digits on the screen start to blink and lose their contrast due to lower voltage. This may result in the taking of wrong measurements by the user due to an imperfect display. Aside from that, low power can also display a wrong number on the LCD display.
If you tend to use a digital caliper, always have the battery reserve. It is advised to change the batteries when the sign of low battery starts to appear to avoid undetected errors. There are various digital calipers on the market and they come with different battery models. So, you should know what type of battery you are using.
It is important to store the caliper by removing the batteries if you are sure to not use it for months. Not only does it save power but also helps prevent rust between the contacts of the battery and caliper as well as leakage.
4. Random Error
Random error addresses the precision issue of a certain measuring instrument. This error is the reading variability of the same object from the true reading.
It deviates from the true reading in terms of magnitude (small or large) and direction (positive or negative).
Random error may be due to parallax error that causes inconsistent readings. Environmental conditions may also register this error as different materials have different tendencies to react to temperature and other conditions. The extreme sensitivity of the measuring instrument may also address this random error issue since slight changes can alter the readings.
These error sources are hard to eliminate, but there is a solution.
Because of the data variability, random error can be eliminated by taking multiple measurements of the same quantity and then taking the mean value.
Simply put, you have to use a digital caliper with a transferrable data feature (either wireless or wired) to an external device such as a tablet, computer, etc. By using this digital caliper, you can take repeatable measurements, easily record those readings in a spreadsheet, and then calculate the mean value. The more data you take to find the mean value, the more accurate it’s to the true value.
Digital calipers with this feature are available at affordable prices such as iGaging 100-700-06-I and Vinca DCLA-0605.
Performing the measurements under controlled conditions is also advised to control this error. Moreover, maintaining a good method is also important to not experience this error.
5. Systematic Error
Unlike the random error that causes an issue to the precision, systematic error is about the accuracy issue.
It consistently deviates the readings from the true value. Though it contains variability, the mean value shows a great deviation from the true value.
The systematic error makes it impossible to take the correct reading as it affects all readings in the same magnitude and direction. Unfortunately, it cannot be eliminated by finding the mean value (averaging). It may be caused by some fault in the instrument or the wrong use of method to take the measurement.
Systematic error can only be eliminated if its cause is known. Zero error is just a version of a systematic error.
This error may also be caused due to poorly calibrated equipment, fixed changes to equipment due to environmental changes, or inexperienced users. Therefore, it is important to be certain about any of the above causes prior to correcting them for an accurate reading using the caliper.
6. Zero Error
Zero error happens because the zero reading of the vernier scale and the main scale don’t coincide forming an alignment. If the measurement continues, it will generate a misleading reading.
It has two types: positive and negative zero error. Positive zero error occurs when the zero reading of the vernier scale stops after the zero reading of the main scale. In contrast, negative zero error occurs when the zero reading of the vernier scale stops before the zero reading of the main scale.
Calculate the zero error and find the value. Every time you take measurements with that caliper, you have to add or subtract with that zero error value.
7. Excessive Force
Excessive force on the jaws of the caliper results in errors while taking measurements. It is always advised to not apply increased force on the jaws and always put the object between the jaws gently. It becomes even more important when one is dealing with soft objects which may change their dimensions upon application of increased force.
As said before, clamp the objects with the jaws gently. Clamping the jaws with decent force should be trainable. Another thing to solve this issue is simply by using a low-force caliper such as Mitutoyo 573-291-30.
8. Human Errors
Errors such as parallax error, mistakenly seeing the number, and improper calculations are considered human errors. This is mainly due to the inexperience of the users of the caliper. The excessive measuring force on the jaws of the caliper is also a source of error in the caliper.
Therefore, proper training of the user before using the caliper is highly important to eliminate this error.
9. The Accuracy of Graduation
A low-cost caliper may be manufactured at a cheap cost. The graduation may be marked with low precision laser etching. Surely, it generates a misleading reading as well. A plastic-made caliper may be subject to this issue. The chance of error is easy to spot if you inspect the reading with a set of gauge blocks with different lengths.
Word of Advice
It is necessary to always take care of calipers properly. You have to monitor the accuracy of the caliper through a regular calibration. Don’t forget to clean before and after use. Moreover, the measurements must be taken at a controlled temperature and conditions. Further, the user must be trained with the right methods to avoid unnecessary errors in measurements.