Gauges how to? Gauges how to!

[Crazy2287 1,886]

I was going to call it "hacking gauges" but decided it sounded dumb =P

 

So i save money wherever i can. This also means getting some of the extremely cheap nasty gauges off ebay, from hongkong with fwree dirivery.

Here is some of the things i have run into and done to make them work as specified. I'll also break down how they work which in of itself will justify why i don't think normal gauges should cost more than $50

 

Starting with, the boost gauge.

 

As you can see a mechanical boost gauge is a very simple device. A copper tube is rolled almost flat and curled. When a pressure is applied inside it the tube will move. one end of the tube is fixed, the other is connected to 2 adjustable linkages, one shaped like a bellcrank controls the scale, while the other controls the set point (or eg, zero) these linkages are then connected to a half gear which works on the pointer. The helical spring holds the needle against the gears and linkages to reduce errors from the gaps between the teeth and linkages.

 

Circled in green you see the bellcrank, This can be manipulated or bent to change the ratio or movement of the tube vs movement of the needle, Or scale. Pinching this will shorten the scale. Ie, between 0-5psi the needle will move 45 degrees. Spreading the bell crank increases the scale. Ie, between 0-5psi the needle will move 90 degrees.

In red is the setpoint. Pinching or spreading the hump in this linkage will set what zero actually is. It will set the needles zero degree point.

 

This gauge cost me about $15. I used regulated valve and my air compressor with a calibrated gauge to check the accuracy of this gauge. After setting to zero and doing a full sweep i found it was within 0.5 of a psi at 20psi. Giving about 2.5% error across its positive pressure sweep. I could have adjusted to bell crank to correct for this but this gauge is more than accurate enough for my purposes.

Congratulations, you can now calibrate mechanical gauges.

 

The next gauge is an electronic gauge. The brand is dragon gauge and can be had for about $25 inc post depending on the gauge type. Understand that all mechanical gauges operate the same as above. Be it boost, vacuum, oil pressure, temperature (with a capillary tube). and just the same, most electronic gauges operate the same too. They use an analogue circuit to modify an input signal so that it will provide the desired output on the needle. the input signals are small currents/voltages given by Potentiometers/rheostats (vairable resistors), thermistors (temperature sensitive resistors), thermocouples (temperature sensitive current generators) or crystals (pressure sensitive oscillators (?)) and the accuracy of these devices are usually determined by the quality of components and their isolation from interference. Typically, 2 wire sensors have isolated themselves from earth for a stable ground and 3 wire sensors also isolate themselves from the cars varying 10-14v by taking a stable voltage from the sensing device.

 

They take this input signal and modify it to provide electro motive force to the needle via a coil or electronic magnet. The magnitude of this force is controlled by current which is typically calibrated by Potentiometer. The helical springs you can see in the above pics do 2 things. The give a no resistance way of getting the current for the coil onto the armature, without using brushes and sliprings, and they provide force to oppose and return the pointer.

 

The first thing i noticed about my cheap "dragon gauge" was it's failure to dim. Because of the smoked glass design the illumination was always on. Otherwise you could not see the dial face. The downside was, they were bright so you could see them during daytime, and at night they were blindingly bright. So i built a simple circuit to allow dimming when the lights were turned on and also give a dimmed brightness control. It was also a requirement that it would draw very little current when not in use (vehicle lights were not on) and not interfere with daytime intensity of the gauges.

 

The transistors in this circuit will supply up to 3 of these dragon gauges, It's tested and works feel free to build and use at will. The transistor has 0.4-0.7v drop in saturation (daytime operation), the difference to daytime intensity it is negligible.

 

Shortly after i got an oil pressure Gauge. I installed and was testing/calibrating it off my high accuracy test pressure gauge . When i tested the dimmer circuit i noticed on dimming the lights past a certain point the display reading would reduce dramatically. Realizing this was a result of the signal modifying circuits voltage dropping i pulled the gauge apart to investigate.

 

Using a soft tool work under the bezels edge as pictured. go all the way around a few times working more until it is spread enough that you can pop the bezel and front glass off.

 

Undo the nuts on the back and push the gauges internals out. This picture was taken AFTER i had done the work, here you can see i have already filed out the access hole on the rear to allow proper access to the calibration potientiometer and the extra wire is already added (white)

 

First sign is a good one, I was worried it might not even have a voltage regulator to give a stable 12v to the pressure sensor. But it does, you can see here a 6v regulator.

 

Next, how to get around the issue with the dimmer changing the reading. Initial investigations confirm what i suspected. The gauge takes it illumination and sensor power form the same source. Simple fix is to split these sources, and add a wire that alone will power the illumination LED's. Very simple fix, Remove the supply wire for the illumination circuit, and add my own in.

 

Remove the white wire that termiantes to the same solder pad as the red one.

 

and add a new wire for the Led's

 

put it back together and way you go.

 

See? gauges are simple!

These gauges were made quickly. Overall however i was surprised at the quality of the solder joints, i expected a lot worse. I would not use these in anything requiring high accuracy/reliability. For obvious reasons. But who cares of the difference between a reading of 40psi or 45psi oil pressure? Or 15psi boost vs 15.2psi which you wouldn't be able to make out on the dial anyway.

 

Forgive errors, my cap is on sideways again yo. And by that i mean im drinking homebrew so im quite jovial Atm. and by that i mean a great place for left for dead.

 

I'm gonna play l4d

 

Peaceout.

[Crazy2287 1,886]

I should mention to get the bezel back on use a soft tool to roll the edge of the bezel back over till the glass and bezel are tight on there again.... Yup

[RAWDEAL 222]

Cool thread man! Ive always though that guages that cost 200,300,400$ were horseshit lol. IMO if your oil pressure reads a few psi off because the guage isnt accurate, it doesnt really matter, it will still tell you if it drops to low, like to 5psi or something . might not actually be 5, could be 1-10. but at least you know you need to shutdown the car lol

[bear351c 10,270]

Cool thread. Cheers.

[Mikgan 32]

Your write ups have been awesome mate keep them up, I'll probably never pull apart a gauge but it's great to read about someone doing some cool digging around.

 

 

Sent from my iPhone using Tapatalk

[Crazy2287 1,886]

Cheers guys. Good luck.

[Mixalis 266]

Mad write up mate very informative!