What is a Field Calibration, and how do I conduct one for the PLT 400?

What is a field calibration?

A field calibration keeps a tool accurate. It's a check to re-align itself after shipping/handling, storage, temperature changes, wear and tear, and field set up inconsistencies. Without a field calibration, the sensors within the tool would risk being out of line, and therefore measurements would be inconsistent.

Field calibrations are a large part of how total stations maintain their reliability in measurements. Older total stations like the PLT 300 and POS 150/180 required users to manually perform field calibrations, which could lead to errors. The PLT 400 automates this process, ensuring more reliable and consistent results by eliminating human error.

By use of the mirrors on the total station (see below), as well as the collimator at the base of the tool, the tool is aligning its axes to the laser and prism tracker within the tool, making sure it is centered. You will notice that during a calibration, the tool spins several times, checking these axes in all ways possible to make sure 360° horizontal and vertical rotations will keep the prism tracker and EDM (laser) centered as measurements occur.

What parts of the PLT 400 are used in calibration?

The total station has four key locations on the telescope that is used to perform a field calibration: the lens (1), the collimator (2), and the two calibration mirrors (3). These locations run your calibrations and measurements as the tool is being used, and if anything (usually water or dust) is inhibiting them, it could lead to calibration and measurement failures

How do I conduct my own field calibrations?

To run your own field calibrations on your own time, simply open the field calibration app on the tablet:

Once opened, know that it will take approximately 3 minutes to complete once the process begins. You will have two options available to you:

1. Automatic Field Calibration - this aligns the optical path of the lens to make sure the EDM (laser) and the prism tracker are reading accurately at all angles the tool will turn.

2. Compensator Calibration - ensures the dual-axis mechanism within the tilt sensor is reading accurately, keeping the tool plumb.

Typically, it is wise to run both parts of a field calibration when you are manually triggering this process.

Simply press the green check mark and the tool will perform its calibration process. It is recommended to perform this manually whenever accuracy concerns appear, such as backsight checks after stationing are poor, a sudden change in temperature, or worry that the tool may have shifted because of jobsite vibrations.

What if the calibration fails?

If calibrations fail, it is often because the mirrors or lenses pictured above are not able to be read properly. The first step would be to clean these lens, as mentioned in this article. Another reason for failure could be that the tool is beyond 5° out of level, and needs to be adjusted.

In the images below, you can see how water can affect the collimator, especially, to be blocked by water, leading to calibration errors. Clearing this with a dry cloth should help.

The gap between the bottom of the telescope and the collimator is very small (less than 3mm). If parts of the telescope has water, it easily could drip to the bottom of the tool and pool around the collimator. Therefore, before a calibration, even if the collimator is dry, make sure the telescope itself is dry as well.

If a calibration continues to fail after cleaning the mirrors, ensure the tool is level and attempt again. If failure continues to occur, reach out to your local Hilti representatives for assistance on next steps.

When is a field calibration triggered automatically?

A field calibration is triggered based on both internal temperature changes, as well as when the tilt sensor recognizes it is 5° out of level.

Regarding internal temperature changes that trigger a calibration, the tool is reading both the telescope temperature and the alidade temperature. The telescope area is where the mechanics that run EDM, prism tracker, and other elements for calculating measurements are housed. The alidade area is where the tilt sensors are located toward the bottom of the head unit. Changes in internal temperature at both of these locations could cause expansion or contraction of components, leading to errors in distance or angle measurements unless a calibration is performed.

Therefore, on tool start up, the PLT 400 will calibrate for every 10° C of internal temperature change as it acclimates to the area it is working. Then, every 60 minutes of use, a calibration will occur again automatically to make any further adjustments because of temperature.

Can I increase or decrease how often field calibrations occur automatically?

Yes - this is handled in the settings screen:

You can adjust the frequency to "low" (low frequency mode) or keep it at "normal." If the frequency is changed to "low," automatic calibrations will decrease during startup, meaning that a field calibration shouldn't occur until 60 minutes after the initial calibration when the tool connects to the tablet for the first time.

In "normal" mode, however, the tool will automatically trigger a calibration once the internal temperature increases 10° after startup, regardless of how much time has passed. After this first "temperature" calibration during the first hour of work, 60 minute intervals of calibration should occur thereafter.

Note, this does not account for the tilt sensor adjusting to stay level. If the tool goes beyond 5° of being out of level, the tool must readjust to maintain accuracy, regardless of internal temperature.