adjusting pressure?
I'm wanting to up the pressure on my organ a notch to get to the desired spec. Someone who should know has told me that I need to do this by increasing the blower speed. Physics 101 suggests that I need to adjust the spring on the pressure release valve on the reservoir. It seems as if I can blow a lot more air into the reservoir and the pressure isn't going to go up unless I close down the spill valve. I can understand that I might need more air to maintain flow at higher pressure (E=IR, to a first approximation), but if I'm already spilling lots of air even while playing many notes it seems that more air isn't the solution. Am I missing something? Cheers, Roger

Hi there Roger The first thing is to increase the spring pressure on the reservoir. The release valve should only be opened by the reservoir top lifting enough for the end of the release valve to push against the stop, thus opening the other end of the valve. This is assuming the normal valve on the top of the reservoir. The springing on the reservoir against the size of the reservoir is the only thing that sets the pressure correctly. As the size of the reservoir is a constant, then the springing is the only adjustment available to you. Having got the pressure to the appropriate figure, run some books through the organ and see if you have enough volume of wind at that pressure. Choose some books with some real heavy chords or failing that, with a lot of notes in them. If the organ doesn't cope, only then will you have to increase the speed of the blower. If you are only increasing the pressure slightly, then it sounds as if you have a big enough blower already. You may find that you will need to adjust the freins on some of your violin pipes, and you will need to run through the tuning, as altering the pressure will surely alter the tuning on a lot of the pipes. Cheers Peter Griffiths

Adjusting Pressure Peter As a start would you suggest that the reservoir pressure can be increased by placing weights on it's top and if this indicates that the pressure should be increased then look at increasing the spring effort. Arthur Nichols

First of all, what is the wind pressure now? Before you add springs or weight to the regulator that should be determined. Generally speaking most fair organs that I've worked on the WP is between 8-10 inches water column. This is what the pipes are voiced at. Going higher will result in a screeching monster. Does the regulator stay up most of the time now? Or does it seem to be struggling? If it is that's not a pressure issue, it's a lack of volume. There can be several causes for that, ranging from leaks in the chests to an undersized blower.

It runs 8 to 8.5" now and spills air constantly. I was told to target 9.5"-10 as that's what it was when it was voiced. Probably the first thing to do (now that I think about it) is hook up a manometer and check the calibration of the gauge I'm using to measure the pressure. While I've been messing with the blower to switch it from a direct drive DC to a belt and pulley with a US standard motor it's not clear how a pressure drop might have happened over the last nine months. Thanks for the suggestion of adding a weight-- that's a quick and easily reversible way to try an experiment. Roger

Hi there All Yes, the weights are an excellent way to try upping the pressure, however, this needs to be purely a temporary measure. Keeping the weights on the reservoir tends to give a 'shock' of extra air now and then owing to the kinetic energy in the weights when they move quickly. I must admit, it does seem odd that the pressure has altered. There are one or two possibilities. One of the springs on the reservoir may be denaturing and loosing it's spring. More likely though is the fact that the old DC motor was running so fast that the amount of wind it was producing was more than the escape valve could cope with. Have you made new hooks for the spring ends, and made them slightly longer, or are the fastenings for the springs pulling out of the wood? Has the piece of wood that the release valve works on been replaced with a piece slightly thicker? If the release valve works on the bottom of the windchest as some Gaviolis do, is the bottom of the windchest bowing down under pressure and thus opening the release valve early. This can happen when the spacing bars that hold the back of the windchest together give way in the wood of the chest. Cheers Peter Griffiths

I don't know what kind of "gauge" you are using to check the pressure, Roger, but the simplest and most reliable is a simple "U" tube. Measuring the difference between the water levels in either side of the tube can't be easier, or more accurate. I must admit, as Peter says, I'm very puzzled the pressure has altered that much on its own, unless one or two of the springs have one awol. I've never known springs of that type to lose tension in a big way. The pressure developed with the reservoir at its lowest is not that much lower than when it is at its highest. As the top lifts, there is greater push downwards from the springs, but that is somewhat compensated by the opening of the ribbing, effectively trying to push the top upwards. Regards, John

Hasn't your Gavioli just been professionally restored throughout, Roger? How come you need to alter the wind pressure? It's not something that owners normally do.

I think what happened was that it was being driven by a variable speed DC motor when it came over and that motor could drive the fan considerably faster than the speed specified by the maker. If you turned the blower up high enough the air couldn't escape fast enough though the spill valve and the pressure rose. I replaced the worn motor with a standard American AC motor and shaft and pulley arrangement and set the speed to approximate that spec'd by the maker, probably slower than it had been. At the lower speed the spill valve works as designed and the pressure does not change with small changes in motor speed. What I've done at this point is changed out the motor pulley so the fan speed is on the high side of spec rather than the low side, producing 9" of pressure. It's sounding and playing fine, so I'm not going to mess with it more. John-- FWIW the Magnahelic pressure gauge I bought matches a water manometer perfectly. It doesn't freeze, the water doesn't evaporate, and it looks old-fashioned enough to fit in, so it's going to stay as the official pressure measure. I don't know what owner's normally do, if I want a real professional repair person familiar with this kind of instrument I'd need to fly them in either from England or almost as far away across the US. Fortunately I've done enough building and rebuilding of mechanical instruments that most repairs are within my grasp, especially with a good coach on the phone from the UK. I think I really need a hands-on lesson in how to tune this big and complicated an instrument though. :) Roger