Weekly Checks are Essential on Auto Levels
The Auto Level, while not great in cost, is a precision piece of measuring equipment. Being able to see and provide levels, to within a couple of millimetres, at a distance of 50m from the auto level is incredible. But having confidence that your auto level is giving you the correct results is also vitally important. This is why it is recommended that auto levels are checked weekly or before any crucial levelling works being undertaken.
An example of the implications of not checking an auto level.
Take the case of a site engineer working on a new proposed surface water drainage network for a new proposed access road. After setting out the new manhole positions along the new proposed road, the site engineer proceeded to work out profiles for the drain runs.
Each time he set up his auto level he was next to the downstream manhole and took readings to the upstream manhole. But because he had not checked his auto level, he did not realise that it was falling by 20mm over 30m.
He also did not close his levelling run so had no other clues as to the accuracy of his results. As the drainage works progressed, the pipe runs were installed lower than the design levels.
The mistake was only noticed when the cover and frames were being installed and the drainage crew were using more raising pieces than expected, and in the worst case an extra manhole ring was needed to be installed.
This mistake has some safety and commercial impacts. Laying the drains deeper than they need to be, brings with it, extra implications with regard to safety and the speed the work can be done. The commercial implications are the extra raising pieces or rings required and the longer time it takes to complete the works.
Why do auto levels need calibrating?
With any piece of precision engineering equipment, it will need to be adjusted to give accurate readings. We need to calibrate it.
Before any auto level can be calibrated, it needs to be in a serviceable condition. It needs to be checked that nothing else can affect the accuracy of the auto level.
There are many parts to an auto level and these all need to be working correctly to give the accurate readings desired.
The main parts of the auto level that enable accurate levels to be taken are the compensator and the crosshairs on the adjustable lens.
What is the standard way of checking an auto level?
The best way to check an auto level is to do a two-peg test. A thorough explanation of a two-peg test can be found on this website. Use this link to find out how to do a two peg test on auto levels. Before carrying out a two-peg test it is advisable to check the auto level for serviceability.
How are auto levels checked for serviceability?
I have forgotten how many two-peg test I have carried out over the years. But having done so many, one of the first things I do is to check for any play, or looseness, in the joints and moving parts. Any movement withing the auto level will mean the levels seen will not be consistent.
Once I have checked the auto level, I then set it up on a good sturdy survey tripod. It is then levelled up using the circular vial. Once the bubble is in the centre, the auto level is rotated through 90 degrees and the bubble checked to see if it is still in the centre. This is repeated at 90 degree intervals until a complete rotation is done. If the bubble strays from the centre, then the circular vial will need adjusting.
If the bubble stays central in the vial in the above test then the next serviceability check is to the compensator within the auto level.
How to check the compensator in auto levels.
With the auto level setup with the bubble in the circular vial in the centre, turn the telescope of the auto level to be above one of the foot screws. Sight a distant object, as far away as possible. While looking through the auto level, adjust the foot screw one way and watch for a level change through the telescope. A correctly working compensator will allow significant turning of the foot screw without observing a change in level through the telescope. This will be about half the bubble out of the centre of the circular vial before noticing any level change through the telescope.
Return the bubble into the centre of the circular vial and repeat, turning the foot screw in the opposite direction. The same effect should be observed when looking through the telescope of the auto level.
Why do auto levels go out of calibration?
There are a few reasons that an auto level will go out of calibration. This is usually dependant on time and use. From the very moment the auto level leaves the calibration station it can be susceptible to becoming inaccurate. The main reasons, as I understand it for auto levels to become inaccurate are down to shocks and vibrations to the auto level and changes in temperature over time that enable thermal creep.
Why do shocks and vibrations affect the accuracy of auto levels?
Shocks and vibrations can be seen through an auto level when viewing a staff. They predominantly affect the compensator. But over time they can also allow the lens with the crosshair on move. It is this lens that needs to be adjusted to calibrate the auto level.
Why are auto levels affected by thermal creep?
Changes in temperature affect accuracy in precision instruments. When materials warm up or cool down, they either expand or contract. Different materials will expand and contract at different rates. Over time this can lead to a loosening or tightening of certain components.
Over a long period of time, the lens with the crosshairs on could move. Hence leading to an inaccuracy in the auto level. As this is a very gradual inaccuracy, but need to be aware of, this leads to the requirement for weekly checks.
How are auto levels adjusted?
As long as the auto level is in a serviceable condition it can be adjusted. The adjustment is done to the lens with the crosshairs on. This lens is held in place by adjustment screws on the top and bottom of the lens.
By loosening the top screw and tightening the bottom, the lens and the crosshairs are raised within the auto level. This raises the value seen on the E-Grad Staff.
By loosening the bottom screw and tightening the top, the lens and the crosshairs are lowered within the auto level. This lowers the value seen on the E-Grad Staff.
A word of warning. Very little adjustment is needed of this lens to see a big difference in levels. Correctly setting the level of this lens can be tricky. If you are not sure what you are doing, then seek professional help. Having an auto level that is not accurate can lead to some very costly mistakes.
This is why auto levels need to be checked weekly.
Checking an auto level should be done weekly. Ideally straight after a calibration has been done, and before any critical levels are taken. By checking, and recording the results, auto levels on a regular basis you should be able to know how the inaccuracy changes over time.
There is no basis for knowing when an auto level might become inaccurate. But from my experience it is the cheaper auto levels that become more inaccurate quicker.
If you need to carry out checks on an auto level or you need to buy a new one then check out these related articles.
Top Tip for Getting the Best Results When Checking Levels.
Whenever you are checking an Automatic (Dumpy) Level, or a Rotating Laser Level, or a Total Station, make sure that the tripod head is as close to being level as possible. Whenever I am carrying out these checks I like to use a bullseye spirit level like this one I bought from Amazon. Its 65mm diameter so it spans the tripod access hole and is large enough to be easy to set.