Europa #435 G-RODO Build Journal - 2021 12
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|2||Shoot-bolt stop mod parts delivered from LAA. Store in box 26 in garage.
While updating online journal, notice that I have not modified the ATR833 wiring diagram to reflect the new line-out socket in the headset socket box, nor the deviation from the installation manual in respect of earthing the unused dynamic mic inputs. Edit the CADintosh file as required and update the wirebook.
|3||4mm and 5mm ID polyurethane tubing delivered.|
|4||Drop U/C for access to tunnel. Check the blanking plugs recently received from Vital parts for fit in the starboard hole of the throttle box; the 13.5mm x 1.6mm BLA024 fits just about perfectly.
Pull the pressure tube for the fuel weight sensor back through the hole in the seatpan and check what size of bush will fit it. This tubing is larger OD than the stock I used for the vent port of the sensor so needs the 8mm OD bush SFB005. Drill out the hole to 8mm. Push tube forward through the hole again. Slide a bush onto it from the free end and push it into place in the hole.
Pull vent tube out from the sensor box and check that the 7.1mm OD bush will fit the hole. Re-insert the tube through the bush. Enlarge the hole at the other side of the box to 8mm for a bush to fit the pressure-side tubing. Slide another 8mm bush onto the pressure tube.
With the pressure tubing so short and emerging so close to the bottom of the seatpan, it won't be possible to warm it in hot water to soften it as I did for the vent tube. Experiment with the nail mandrel and the 8mm x 5mm PU tubing recently bought. Heat the point of the nail with the heat gun and try pushing the tube onto it. Can't make much progress. Try gently warming the end of the tube also and then it goes on OK, but the end of the tube has gone too soft and distorts. Wonder if it might be possible to somehow lubricate the nail (petroleum jelly?) to let the tube slide on more easily.
|13||Remove the melted end of the PU tube test piece from the nail mandrel and cut it off. Re-heat the nail with the hot-air gun and dip it in wax as a lubricant before pushing into the fresh end of the tube. Goes in quite easily. Cool it in water. Difficult to remove nail from tube - need to use pliers, but care needed to avoid damaging the tube. The enlarged end of the tube fits easily onto the sensor - in fact it's almost too big. As the test tube is larger bore than the one installed in the seatpan, the process should be OK on the real thing.
Re-heat the nail, dip in wax and push into the PU tube in the port seatpan. Surprisingly hard to push it in any distance. It's a bit awkward reaching across into the port seatpan from the starboard door aperture. Leave to cool. Once the nail has well cooled, pull it out of the tube. The 3mm Eliza Tinsley clip is now a close fit on the expanded end of the tube, so will have to slide it on before trying to fit the tube to the sensor. To make room for that, take the PCB out of the box slots and well out of the way. Insert the tube into the box and push the ET clip onto it. Re-position the PCB and start to push the tube onto the sensor port. Initially it goes OK but then resists going onto the larger-diameter section at the base of the port. Manage to get it about half-way onto that section. Levering with screwdriver and long-nose pliers, slide the ET clip along onto the port. Crimp one side of the clip with nippers, but as I do so the tube slides off the port. The clip must not have been far enough inboard.
Cut off the clip with wire cutters and start again. Remove the PCB from the box and fit a fresh ET clip to the tube, inside the box. Re-heat and wax the nail. Push it into the tube, managing to get it slightly further this time. Rest the nail on pliers to cool it and prevent it melting the box. Once cooled, re-seat the PCB and start pushing the tube onto the sensor - always remembering to support the sensor against the push, not the box! Manage to get the tube almost home against the sensor face on the larger-diameter section of the port. Slide up the ET clip using long-nose pliers and manage to get it on sufficiently far that the outboard end of the port is just visible through the tube. I think I am ready to crimp the clip now, but notice that the tube has somehow slipped back and is no longer covering the larger-diameter section. The end of the tube is still clear of the clip, so decide to go ahead and crimp. Once the clip is crimped, push the pre-installed bush along the tube and into the box hole. Fit small cable-ties inside and outside the box on both pressure and vent tubes to prevent movement. Check the wire IDs and connect the sensor box wires to the harness in the seatpan.
Power up the panel. The MGL FF-1 fuel gauge initially shows about 40l but gradually winds down to about 11-12l. Try blowing gently into the vent tube (to simulate a reduction in fuel level) but there is no reaction. However, gently sucking on the vent tube (which should simulate an increase in fuel level) causes the indication to reduce below 10l, combined with a flashing low fuel warning. This is the exact opposite of what I would expect! It is obvious now that it might have been helpful to do a simple pressure test on the sensor BEFORE I connected it to the fuel system! I will need to re-check the polarity of all the components to make sure that no unintended reversal has been made. I will also need to check the integrity of the connexion just made for leaks - but I will need to double-check that I have got everything else in the fuel system secured before I add even a little fuel to the tank.
After leaving the garage, re-check all the component specs and circuit diagrams. Can find nothing amiss.
|14||Decide that I need to check the sensor properly. Cut off the cable ties and the ET clip on the pressure side of the sensor. Pull the pressure tube off and remove the PCB from the box. Fit a length of silicon tube temporarily to the pressure side of the sensor. Power up the panel. MGL FF-1 gauge initially shows about 40l, falling to about 12l. Gently blow on pressure side of sensor - no effect. Gently suck - still no effect. Suck on vent side of sensor to see if it's still behaving as yesterday - but now no effect! Also no effect on blowing into the vent tube. looks as though the sensor may be dead.||3580.1|
|15||RS have stopped stocking the exact sensor but show an equivalent Honeywell one. Farnell have the same thing cheaper so order one from them.|
|18||Honeywell sensor delivered from Farnell.|
|20||Power up the panel and measure voltages art various points on the fuel weight sensor PCB. 5V rail is OK. Across the output terminals (2 & 4) of the sensor bridge there is 0mV with no pressure applied. Blowing into the pressure port tube gives between 5mV & 10mV across pins 2 & 4. The output of the INA118 (pin 6) remains at 30mV irrespective of sensor port pressure changes or adjustment of the gain trimpot. Looks as though that amplifier may be the problem, but will need to investigate further - there may be a fault on the PCB. I think I need to take the PCB out of the aircraft and examine it on the bench for easier probing of the circuit.||3580.6|
|21||Check availability of INA 118 and order one from Farnell.
Disconnect fuel weight sensor from seatpan wiring harness and remove it from the cockpit. Set up on PSU in office and check voltages at various points. Discover that the voltage across the legs of the sensor does not match the voltages on the PCB tracks. Pin 4 has fractured and is only making intermittent contact - must have fatigued with all the movement caused by trying to get recalcitrant tubing fitted to its port. Melt the solder on its track; remove the excess and the broken pin with a solder sucker. Lay the soldering iron across all the pins and melt the solder to allow pushing the wires slightly further through the PCB. Re-solder them all securely and clean up. Check for solder bridges with a magnifying glass. Power up and check the output voltage - now quite easy to get around 5V at the output by blowing on the pressure side. (So the INA118 is going to be spare, like the too-hastily-ordered sensor.) Wind the 100 ohm trimpot to each extreme to check the gain; less pressure needed to show 5V with pot wound CCW. Checking resistance with the meter shows it is zero ohms which agrees with the documentation - lower resistance between pins 1 & 8 of the INA118 gives higher gain. Cut the pressure-side tubing to about 400mm and try filling with petrol while holding the tube vertical. First attempt no good, tube ID too small to admit the end of the syringe I am using. Swap to a larger-bore tube and that is more satisfactory. Filling it to a head of about 300mm above the sensor gives and output of about 4V. That seems OK, but if the MGL display needs a full 5V swing I could change the padding resistor in the gain link from 20 ohm to (say) 15 ohm. That would change the maximum gain available from x2500 to x3333 and should allow the output to swing close to the 5V rail.
All tonight's work done in the office so didn't get logged on the garage timer.
|22||Unsolder the 20 ohm resistor from the fuel weight sensor gain circuit and replace it with a 15 ohm.||3580.9|
|23||Remove all the blue foam pieces from the seatpans to keep them clear of any fuel spills. Connect the 400mm length of larger-bore tubing to the pressure port of the fuel weight sensor. Connect the sensor leads to the seatpan harness and power up the panel. MGL FF-1 now shows about 13 litres. Fill the pressure-side tube with petrol using the syringe as before. FF-1 now shows low fuel, so obviously setup is wrong. Enter setup menu and set max fuel to 60 litres. Go through the calibration procedure, adjusting the height of the tube to roughly the right level for the calibration points (60, 48, 36, 24, 12, 0). Hard to get sensible values, and to avoid slope errors, but eventually get a series that are at least in value sequence, if not very linear. Exit setup menu and FF-1 now shows 34 litres with tube still filled. Drain tube of petrol and remove it; FF-1 now shows 0 litres, alternating with a low fuel warning. That seems fairly reasonable, given the very rough nature of the calibration.||3581.5|
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