| Author | Message | ||
     Brian Vogel Grand Master Username: guyslp Post Number: 2304 Registered: 6-2009 |
Hello All, I have just received a private e-mail message related to this and thought I'd turn to the cohort based upon my own experience to see whether there's a consensus or not. There are two different versions of a "build your own gauge" article that I've posted over the years: Building Your Own Hydraulic Pressure Gauge (W. Hunter) Building a Hydraulic Pressure Test Gauge (B.Vogel) The first shows a gauge with no bleed screw, while mine includes one based on the fact that several other designs sent to me as examples included them. That being said, I have to question whether the bleed screw is necessary. In practice I've gotten the same reading whether I used the bleed screw to bleed the short segment of brake line between the port it's screwed in to and the gauge head or not. Even if you do bleed it, there is still enough space that a bubble could exist past it. Since air is compressible (almost infinitely so) and would be very rapidly compressed to the pressure of the fluid flowing in behind it the reading on the pressure gauge should be unaffected by whether the line is bled or not, correct? I am thinking about modifying my design to delete the bleed screw if there is a consensus that it really serves no functional purpose. This is one of those cases where "close enough" is really close enough. We're not looking at a problem if the measurement is off by a few pounds plus or minus, as the system itself has a degree of slosh in its operation between examples to begin with, and this is documented in the workshop manuals. What say you? Brian | ||
| Geoff Wootton Grand Master Username: dounraey Post Number: 1706 Registered: 5-2012 |
Hi Brian I have not yet built a hydraulic pressure gauge so can only give my initial thoughts on this, rather than an opinion based on hard experience. I hope you decide to keep your design unchanged. One of the reasons for this is anyone building this piece of kit will almost certainly be reasonably "tech savvy". It's nice to see the various design options and decide for one's self what particular facets of each design to use. For instance, I much prefer the use of flexible hose in your design over the rigid brake pipe used in the other. The incorporation of a bleed screw has to be an advantage in all areas other than that of it's fabrication. It occurs to me in the event of the hydraulic pressure staying high even after pumping the brake pedal to depressurize the system, maybe due to a collapsed brake hose further up the line, the bleed screw offers a very controllable way of depressurizing that part of the system. As I mentioned, the only disadvantage of the bleed valve is a slightly more difficult fabrication. On looking at this design I first queried myself as to whether the bleed screw would seat properly on the raw drilled female seat in the plug. I assume this design has been tested, negating my concern about the bleed screw leaking. The use of a plug (rather than a cap) means the length of the thread that holds the bleed screw would keep things safe. I think when I build mine I will drill straight through the plug with a 1/8" drill and then set the depth gauge on my drill press  for the 21/64" hole. I will also look into gluing the plug in place to prevent it loosening in lieu of the bleed screw. A great design and I hope you leave it unchanged. Individual owners can decide which one best suits their capabilities and requirements. As a footnote, a forum member recently admitted his stupid mistake (viz: unable to open his passenger door which turned out to simply have been locked). Here's my admission. I once loosened a rear brake pipe, forgetting I had not depressurized the system. Nothing particularly spectacular happened other that a partial fan of brake fluid sprayed all over the underside of the car. Fortunately, it missed my face. In the event, I was wearing eye protection anyway but I just wanted to emphasize the point. Always always always wear eye protection when working on brake lines. In the heat of battle, it's easy to make a mistake. Geoff | ||
| Brian Vogel Grand Master Username: guyslp Post Number: 2305 Registered: 6-2009 |
Geoff, Perhaps I should revise my statement to, "Make a note in the instructions that the bleed screw is optional." You are correct that my concern is mostly with the fabrication end of things. I find that for the DIY community the easier it is to fabricate the more likely it is to be fabricated when it's needed. This particular tool gives you a lot more definitive information about how the accumulator system as a whole is actually performing, along with ancillary information about the pumps since if something's wrong there you won't build pressure. This is more about noting that there is a choice here if there is very little in the way of accuracy to be lost if one exercises that choice. My own memory (which could be incorrect four years later) is that I got the same results in terms of measuring the pressure (in all aspects, including cut-out, fall back, etc.) whether I bled that line or not. I agree that the flexible brake hose design is better as far as ease of fabrication and even getting the thing in place without needing to bend and flare brake line to make one end of the gauge setup. Brian | ||
| Geoff Wootton Grand Master Username: dounraey Post Number: 1707 Registered: 5-2012 |
Hi Brian I totally agree - just make a note that the bleed screw is optional. Thanks for putting all this information up Brian. We tend to forget just how valuable it is, particularly for new owners. Geoff | ||
| Patrick Francis Prolific User Username: jackpot Post Number: 127 Registered: 11-2016 |
Hi Brian I would leave the bleed screw in for the reasons that Geoff rightly mentions. As far as operation of the gauges go, I think the Bourdon tubes in pressure gauges are designed to work dry (air filled), so I would hesitate to bleed fluid up the tube before pressurising, since that means that the fluid may compress the air above enough for the fluid to reach the Bourdon tube. Does that make sense? | ||
| David Gore Moderator Username: david_gore Post Number: 2563 Registered: 4-2003 |
Patrick, From my experience with hydraulic systems on mining and construction machines, all the hydraulic system test/diagnostic equipment we used had both a shut-off valve and pressure relief valve for safety reasons to protect users from being sprayed with hydraulic fluid due to inadvertent disconnection while still pressurised. I strongly recommend inclusion of these essential safety items in the guides plus large print warnings about ensuring the test equipment is fully depressurised before disconnection after use plus the use of appropriate eye and face protection at all times. | ||
| Brian Vogel Grand Master Username: guyslp Post Number: 2307 Registered: 6-2009 |
Patrick and Geoff, As you have probably already deduced, this design has been field tested as I put the thing together for my own use. I came up with it because it can be put together by virtually anyone with a drill and a block of wood to hold the parts in if they don't have a drill press (and even though I have one I drilled the brass bits by hand). I have no leaks. I also "gorrila-cized" the plug when I screwed it in to the brass tee, while the bleed screw that threads into it is tightened just enough to keep fluid from bleeding unless I want it to do so. As to the bleed screw, while the reasons Geoff mentioned are, I guess, bonuses in the event of system malfunction, and I can note that too, I want people to know that this would really be the only reason one would use the bleed screw. When I was putting this thing together I was given the impression by some that one had to bleed from the bleed screw once the system was pressurized to insure an accurate pressure reading. My personal experience plus what's been said here so far indicates that this is not the case. As a result, I'd like people to know why they might, or might not, wish to include the bleed screw. I know I probably wouldn't have in retrospect given what I already knew about the system on which I was doing my testing. Brian | ||
| Brian Vogel Grand Master Username: guyslp Post Number: 2308 Registered: 6-2009 |
David Gore wrote, in part, ". . .to protect users from being sprayed with hydraulic fluid due to inadvertent disconnection while still pressurised." The problem being that the bleed screw does not perform this function, at least not if you depressurize the system, and if you don't depressurize the system then any sudden opening of the system, whether from the bleed screw or the port, is going to result in a fountain of brake fluid one way or another. Having once inadvertently opened the bleed screw on a pressure switch just a bit too much when trying to bleed that part of the system I know whereof I speak. Brian | ||
| Larry Kavanagh Frequent User Username: shadow_11 Post Number: 71 Registered: 5-2016 |
Even though the use of a bleed screw near the gauge may not influence the pressure readings the bleed screw is still a handy addition if one wants to flush clean the test equipment. It's more than a year ago since I built my pressure gauge and I haven't had to use it since then so I can imagine it has absorbed some water from the atmosphere in the meantime due to the hydroscopic nature of the brake fluid. I have a female brass plug on the end of the pipe while it is stored awaiting it's next assignment but no doubt there is some moisture contaminating the equipment in the meantime. Another benefit of the bleed screw is that it allows compressed air to be blown through the equipment without having to separate the gauge from the t-piece. Another advantage that I found with the bleed screw is that it's use can indicate if the height control valves are set too high e.g., when I depressurised the system by pumping the brake pedal I noticed that the No.2 light would illuminate but then extinguish again, if I pumped the brake pedal more the light would illuminate again but extinguish again after a short while. I came to the conclusion that the height control was returning pressure as the weight of the car was compressing the rams which indicated that the valves were set too high and some was leaking back through the HCVs. This residual pressure can be let escape using the bleed screw, when the bleed screw stops releasing you know that you've reached the point where the car is resting on its springs without ram assistance and this helps in setting the height control correctly to suit the particular car's coil springs strength. | ||
| Geoff Wootton Grand Master Username: dounraey Post Number: 1709 Registered: 5-2012 |
Re: whether from the bleed screw or the port, is going to result in a fountain of brake fluid one way or another. I disagree. If for whatever reason the pressure has to be relieved through the hydraulic pressure gauge you can put a length of rubber tubbing over the nipple and direct the flow of brake fluid into a suitable container, as one would when bleeding the brakes. Without the bleed nipple a connection would have to be loosened with the resultant uncontrollable spray of brake fluid. | ||
| Brian Vogel Grand Master Username: guyslp Post Number: 2309 Registered: 6-2009 |
OK, gentlemen. I am all too capable of playing the "lets explore every conceivable possibility" game, but I really am trying to avoid that. Of course if one is doing everything in a controlled and considered way one should not have issues. If one let's one's attention stray sprays (from a loosened connection where fluid pushes through a very tight space) or a fountain (if pushed through a bleed screw opened sans a tube of any sort and opened too quickly) will ensue. Not that what's been mentioned does not deserve consideration, and I'm glad it's been mentioned, but much of it is avoiding my original question: Does it matter if there is air in the fluid feed line ahead of the pressure gauge or not strictly in terms of whether you'll have an accurate pressure reading? I say no, it should not matter one iota, but if there is counterevidence then I would love to have it presented. The question on my end has never been, "Could the bleed screw serve a purpose?," as it seems that the obvious answer, from multiple aspects, is "Yes." I apologize for having used the imprecise phrasing as part of my original message, with regard to the bleed screw, "it really serves no functional purpose," as that's not what I really meant to inquire about and it was sloppy on my part. Brian | ||
| David Gore Moderator Username: david_gore Post Number: 2564 Registered: 4-2003 |
Brian, Air in a hydraulic line has the same effect as the Nitrogen in the car accumulators - it stores energy until it is allowed to expand thus releasing some or all of the stored energy. When a pressurised hydraulic system containing gas is static a pressure gauge will give the true pressure in the closed system regardless of whether the gauge is upstream or downstream of the trapped gas. Once the fluid is allowed to move, the pressure recorded will drop and only start to increase again when the fluid flow is either stopped or is less than the output of the hydraulic pump pressurising the system. The danger in all of this is the energy stored in the compressed gas being accidentally released by component failure or operator/repairer mistake/negligence. This is a minor inconvenience in conventional low pressure car braking systems but it is obviously a serious safety consideration in high pressure systems such as the one installed in the Shadow/Spirit vehicles. | ||
| Brian Vogel Grand Master Username: guyslp Post Number: 2310 Registered: 6-2009 |
David, Thanks. I now have the answer I needed. I believe that somewhere I have a diagram or photo of the Crewe-issue gauge for checking the accumulators. It is not an elaborate piece of equipment. If someone does happen to have the Crewe-issue gauge or Crewe-issue documentation that shows one I'd love to have that just for my own archive. What I remember is something very much along the lines of the W. Hunter style gauge but with a very sharp (almost 90 degree) bend on the piece of flared brake line, but I could be entirely mistaken. Brian | ||
| Robert Noel Reddington Grand Master Username: bob_uk Post Number: 1437 Registered: 5-2015 |
Dave, safety is a concern where air is trapped because it behaves as a storage of energy device. Which if released gradually is safe. However due the small amount of air a cracking of the connection between the gauge and pipe should be safe providing that the ACVs are discharged FIRST. 100 pumps to be safe If making up the test gauge then the extra cost and effort for A bleed nipple version is small. |
