Basic care of instruments at the office so they operate properly in the field.



Q: I am an LSIT at a small engineering and surveying company. I have been trying to determine the basic steps to make sure our instruments are properly maintained and calibrated. What about cleaning? Are there certain chemicals or materials I should avoid? My desire is to perform the basic care of our instruments here at the office so they are operating properly in the field. I know a great deal of this type of work must be done at a repair shop, but I would like to set up a regular schedule of maintenance and checks for our field crews.

A: You do not say which types of instruments you are trying to maintain. We will assume here that they are automatic levels and total stations. The first thing you should do is carefully review the manufacturers' instruction manuals to make sure that you and your crews are following the guidelines for proper use. Also in those manuals will be methods for checking the calibration of the instruments and, where it is possible for you to do them, adjustments. With respect to levels, the basic tests are the crosshairs, to make sure the horizontal hair is perpendicular to the (mechanical) vertical axis of the instrument, and to ensure that when leveled, the horizontal crosshair represents a horizontal plane. These are relatively simple checks and adjustments. The latter check is done using the two-peg test, which is also found in most good surveying texts.

With respect to total stations, the particular technology used may make certain methods for testing them more suitable than others. While surveying texts are useful for understanding the principles of calibration and adjustment of instruments, the manufacturer can often suggest the most meaningful ways of testing them. Some things that should be tested include the tubular and circular vials (if they have them), the electronic tilt sensor (if they have it), and horizontal and vertical crosshair adjustment. The so-called height of standards or trunnion axis tests should be done, though if an error is found, the ad-justment will have to be done by a qualified service shop. Easily forgotten are all your tribrachs, not just the one on the instrument, especially the optical plummets. These are also covered in most good surveying texts. You can also check with your instrument dealer for suggestions on methods and accessories you may use to make the testing and calibration easier.

EDM technology generally requires sophisticated processes to test for errors. The National Geodetic Survey and cooperating agencies have set up EDM base lines throughout the country. Locate one near you and verify that it is being maintained. Refer to the NOAA Technical Memorandum NOS NGS-10 "Use of Calibration Baselines" by Charles J. Fronczek for tips on how to use the base lines. The procedure is not trivial and requires good preparation and quality control to get meaningful results. NGS also distributes free software for analysis of the data.

For cleaning, use a good camel's hair brush to gently remove accumulated loose dust and dirt. For hard-to-reach places, use a gentle squirt from an air-bulb sold at camera or optical supply stores. Also purchase lens cleaning fluid and tissue to clean all glass surfaces. A lightly dampened lint-free cloth may be used to clean the outside of the telescope barrel, side covers, etc. Do not use solvents or detergents of any type unless the manufacturer specifically recommends any.