Point of Beginning

GIAA Mailbag

May 1, 2005
Tool tips on steady poles and Lithium-ion batteries.

Q. I find that I am able to hold some prism poles (antenna poles) much steadier than others. What causes them to be different? I've been told that part of the issue is that the bubbles on some are more sensitive than others.

A. Some prism and antenna poles can appear to be easier to hold than others. Unless there is something attached to the pole that makes it harder to hold steady, or the pole lacks rigidity, the actual difference is the sensitivity of the circular vial. All level vials, whether tubular (as in the plate level of a total station or theodolite) or circular (as in prism and antenna poles, and tribrachs) work on the principle of aligning their horizontal axis to be perpendicular to gravity, the force used by surveyors to define the vertical. Level vials have a curved surface of a known radius: circular for a tube vial, spherical for a circular vial. As the radius increases, the sharpness of the curvature decreases, increasing the sensitivity of the vial. Sensitivity is indicated by the angular change in the horizontal axis of the vial per two millimeters of bubble movement. In tubular vials typically used for plate vials of total stations, it is common to see sensitivities of 20 arc-seconds to 40 arc-seconds per 2 mm. Circular vials on tribrachs, to facilitate rapid leveling, generally have sensitivities in the range of 10 arc-minutes per 2 mm. Because they are intended to be handheld during measurements, the vials of prism and antenna poles have a much lower sensitivity; they range from approximately 20 arc-minutes to 60 arc-minutes (one degree) per 2 mm. As the sensitivity of the vial decreases, the pole will appear to be easier to hold steady, as a greater angular change is required for small bubble movements. For example, if the bubble is rated at 20'/2 mm, and the prism is located 6' above the point, when the bubble has moved 2 mm the prism will have moved a little more than 0.03' away from the vertical. But if the sensitivity of the bubble is 60'/2 mm, the antenna phase center will have to move 0.1' away from the vertical before the bubble moves 2 mm. Thus, if the user holds both poles with equal stability, tiny movements of the bubble position observed on the 60' pole will appear to be much larger with the 20' pole, giving the appearance that the 60' pole is steadier.

Q. In the last year I have acquired products that have Lithim-ion (Li-ion) cells. I've been told they are more efficient and have fewer "memory" problems than the older Ni-Cd batteries. However, I noticed that battery life had declined by the time we moved into winter (roughly six months after purchase). Do I have defective batteries?

A. More than likely, no. All batteries have a variety of characteristics regarding discharge and charging cycles. For Li-ion batteries, it is true that their resistance to loss of capacity due to charge/discharge cycles of partially discharged cells is high. However, they aren't perfect in this regard. Also, the temperatures under which they are charged and discharged can also affect performance. While the loss of capacity of Li-ion batteries is not huge (10%) when the temperature drops from room temperature (68°F) to freezing, the loss becomes 30% when the temperature drops to -4°F. So if you were used to getting six hours of operating time from a battery in the summer, you'd likely get about four hours at -4°F. This could explain part of the apparent loss you have observed. Low temperatures have more of an effect when batteries are charged at temperatures outside their rated charging temperatures. For a Li-ion battery, the lowest permissible charging temperature is generally 32°F. Charging at lower ambient temperatures, particularly if the battery is at ambient temperature, can cause battery capacity loss. An important difference in this case is that the loss of capacity may be permanent. Ni-Cd and Ni-MH batteries also have charging temperature specifications. It is advisable to strictly follow these specifications.