tank (thermos or capacity flask) as a reference chamber. The  tubing is plumbed from the reference chamber through the  variometer instrument to an outside static port.  [Figure 4-20] proper local altimeter setting (QHN), indicated and  true altitude are synonymous. A variometer with tubing is connected to an outside static  port is  uncompensated. The internal hairspring mechanism  determines  sensitivity of the variometer. The variometer has  a very rapid  response due to the small mass and lightweight  construction of the moving parts.  [Figure 4-21] Pressure  differences between the air inside the variometer/ reference chamber system and the air outside of the system  tend to equalize as air flows from high pressure areas to low  pressure areas.  When pressure inside the reference chamber is  greater than the  pressure outside, air flows out of the reference  chamber through  the mechanical variometer to the outside  environment. When air pressure outside the reference chamber  is greater than  pressure inside, air flows through the variometer  and into the reference chamber until pressure is equalized. The  variometer needle indicates a vertical descending air mass or  sink which is falling air that forces the glider to lose height.  Figures 4-22  and  4-23  illustrate how the variometer works  in level flight and while the glider is ascending. In addition,  Figure 4-24  illustrates certain flight maneuvers that cause the  variometer to display changes in altitude.  Electric-powered variometers offer several advantages over  the non-electric variety. These advantages include more  rapid response rates and separate audible signals for climb  and descent. When conditions are standard, pressure altitude and density  altitude are the same. Consequently, if the temperature is above  standard, the density altitude is higher than pressure altitude.  If the temperature is below standard, the density altitude is  lower than pressure altitude. The density altitude determines  the gliders performance and greatly affects the performance of  towplanes and powerplants in self-launching gliders. Some electric variometers operate by the cooling effect of  airflow on an element called a thermistor, a heat-sensitive  electrical resistor. The electrical resistance of the thermistor  changes when temperature changes. As air flows into or out  of the reference chamber, it flows across two thermistors in  a bridge circuit. An electrical meter measures the imbalance  across the bridge circuit and calculates the rate of climb or  descent. It then displays the information on the variometer. Variometer instruments measure the vertical ascent or descent  of the local air mass and glider combined and displays that  information as speed. The principles of operation are similar  to the altimeter. The variometer is viewed as a critical part of  the glider’s instrument display, giving the pilot information on  performance of the glider while flying through the atmosphere.  The variometer depends upon the pressure lapse rate in the  atmosphere to derive information about rate of climb or rate  of descent. A non-electric variometer uses a separate insulated  Newer electric variometers operate on the transducer  principle. A tiny vacuum cavity on a circuit board is  sealed with a flexible membrane. Variable resistors are  embedded in the membrane. When pressure outside the  4-11 Variometer •  Density altitude—pressure altitude corrected for  nonstandard temperature variations. Density altitude  is a yardstick by which we can reference the density of  air. The wings of the glider are affected by the density  of air because they use air molecules to generate lift.  Thinner air restricts fuel burn and lowers compression  in the engine thereby reducing the power output.  Thinner air necessitates an increased AOA to generate  the same amount of lift. High density altitudes decrease  towplane (and self-launching combustion powerplant)  power, so expect much longer takeoff runs during tows,  slower climbs, and the higher TAS results in longer  landing rollouts due to higher ground speeds. •  Pressure altitude—altitude indicated at which the  altimeter setting window (barometric scale) is adjusted  to 29.92 is the standard datum plane, a theoretical  plane where air pressure (corrected to 15 °C or 59 °F)  is equal to 29.92 "Hg. Pressure altitude is used for  computer solutions to determine density altitude, true  altitude, TAS, etc. •  Absolute altitude—vertical distance above the terrain,  above ground level (AGL). An altimeter set to the  proper pressure reading (QFE setting) indicates zero  feet at touchdown. It is referred to as QFE. The  METAR weather report in the remarks section lists  this setting as sea level pressure (SLP) and is expressed  in millibars (mb). Absolute altitude is very important  when flying low and determining when an out landing  must be accomplished. An out landing is a glider  landing not at the intended airfield but required when  lift is insufficient to remain aloft. 40 50 60 70 10 0 90 80 Capacity flask Static vents Static pressure inlet Variometer Static port Figure 4-20.  Uncompensated variometer plumbing.