The TE connection to my varios has an interesting feature. Instead of a straight tubing run to the varios, the TE connects to an uninsulated bottle of about .5 liter capacity. A second tube runs out of this bottle to the varios. A fellow pilot suggested that the bottle might be a "Schuman box" installation. Can anyone enlighten me with details?
Hi vmessina, I am sorry to say I have never heard of a "Schuman box" installation
A simple variometer can be constructed by adding a large reservoir (a thermos bottle) to augment the storage capacity of a common aircraft rate-of-climb instrument. In its simplest electronic form, the instrument consists of an air bottle connected to the external atmosphere through a sensitive air flow meter. As the aircraft changes altitude, the atmospheric pressure outside the aircraft changes and air flows into or out of the air bottle to equalise the pressure inside the bottle and outside the aircraft. The rate and direction of flowing air is measured by the cooling of one of two self-heating thermistors and the difference between the thermistor resistances will cause a voltage difference; this is amplified and displayed to the pilot. The faster the aircraft is ascending (or descending), the faster the air flows. Air flowing out of the bottle indicates that the altitude of the aircraft is increasing. Air flowing into the bottle indicates that the aircraft is descending.
So, the "capacity" - the bottle you see - is increasing the sensitivity of the vario. It is common with older mechanical varios, which can work very well. For best performance, they should be insulated - either a thermos bottle, covered in foam, and I recall that some recommended filling the inside with copper wool, to reduce heating/cooling effects (larger thermal mass). If you take the bottle out, the sensitivity of the vario will decrease (it will become useless). Newer, electronic varios use thermistors, which replace the capacity, using ones and zeros to differentiate wildly in all directions (a techno-approximation), rather than a simple bottle.
A more detailed explanation is on the eglider site, http://www.eglider.org/newsarticles/ totalenergycompensationinpractice.htm.
I should mention that the connections are prone to leaking, and a leak affects your compensation. Redoing the connections should be done periodically (there are good how-tos on some sites online), and in the major soaring books.
Thank-you for your comments, however, I don't think I was clear enough on my original question. My vario has the standard reference flask connected to "one side" of the vario, but unlike all other installations I have seen it has a second non-insulated reservoir on the total energy side of the variometer. Does anyone have an idea what the purpose of this second resevoir is?
I also should add that the mechanical vario seems to work fine on this set-up but that the electric vario has never worked. The previous owner sent the unit back to the manufacturer who checked it out and found nothing wrong with it. This electric vario will initially indicate a climb on tow, but after a short while, just wanders all over the place. Anyone have a suggestion?
Could be a gust filter, or a condensation reservoir... alternately, it could be a diaphragm total energy compensator. I have one that connects to the pitot and static, that has a capillary tube in parallel with the diaphragm; this then goes to past the capacity. On the other side of the vario is static pressure. Faster you go, the diaphragm adjusts the the volume of the capacity. If you have "new soaring pilot" by ann welch, there's a diagram on p299, fig 18.8. I may someday pull my panel, and take a picture of the vintage/legacy/old vario, which works surprisingly well.
When sharing TE amongst instruments, you should try to tee the two some distance from the instruments. From my Borgelt B-40 installation instructions:
"The most common mistake in vario installations is to connect two vario systems with a t-piece at the panel. The only time this is permissible is when both are of the pressure transducer type. That is, no flasks, hence no flow. Flow sensor type instruments cause significant flows in the line to the te probe and these flows cause the instruments to interact with each other causing weird behaviour or a general slowing of both instruments." It goes on to say t-ing under or behind the pilot's seat should suffice.
I really appreciate your response. The varios are indeed tee'd together just behind the panel with only a few inches of tubing run to the varios. I will relocate the tee as you suggest.
Let us know how it works! I guess if it doesn't, you could disconnect each in turn, to see which is correct (if you only have one source, you know your rate of climb; if you have two, you never quite know...).