SuperMID calibration methods ? / overinflation valve stem failure

[Cd]

Yesterday, I aired my tires up to what I thought was 50 PSI. It turns out it was closer to 65-70 PSI - but this measurement was taken after a blowout on a ( very ) hot tire. ( Maximum inflation listed on the tire is 44 PSI )
I was in a hurry and did not check the PSI using more than one guage. I have found ( and especially yesterday ) that tire guages are horribly inaccurate sometimes.
I guess I have found my limit, because I had the blowout. Thankfully, what occured was that my valve stem developed a large leak around its' base - so there was no dramatic explosion like I always feared.
( I'm not saying that an explosion could never happen, but thankfully this time around it didn't )
The tire is fine, but needs the valve stem replaced.

A question that I'd like to ask you folks about, is how to go about finding your MPG without a SuperMID or ScanGuage. I actually have a SuperMID, but need to calibrate it using some sort of accurate method.
What I normally do is fill my tank up till the pump shuts off,set my odometer to zero, drive for a few hundred miles, fillup again and divide the gallons it takes to fill the tank by the miles traveled as shown on the odometer.

Simple right ? Yah, but apparently not too accuarate, since the pump sometimes shuts off before the tank is really full.
To account for this, in the past I have filled the tank to the point where that it spills over. This way I know that the tank is truly full.

There has to be a better way .

[ankit]

So did the PSI go up because of the heat or solely the problem of the faulty gauge? How long were you driving with this high tire pressure, just want to know how long it took for the tire to warm-up. Either way glad you are okay.


As for the MPG, I just rely on the pump. I usually don't overfill but just use the same pump everytime.

[AXMonster]

Fill up on the same pump at a low flow rate to exactly the same level every time.
If you fill at a fast flow rate then different pumps (and sometimes the same pumps) can shut off at different levels due to fuel back-up in the fuel tank filler neck.

When conducting comparison high speed FE testing in Germany a few years back this is the method we used, even refuelling at the same time of day over the week to try to get the same temperatures. And also refuelling on a cold tank (i.e. the morning after a test run) so that the fuel in the tank had stabilised to ambient temperature.

Fill the tank until the fuel is at a point in the filler neck that you can repeatedly see, and once it has auto shut off then trickle fuel into the filler neck until it reaches that exact point. Rocking the car a bit using your butt can help get air out of the high points in the tank to ensure there are no air pockets

[brucepick]

My guess is maybe a bad valve stem was the cause.
Valve leaked causing tire to lose pressure.

I'm not sure if it can go this way but - - -
Pressure loss causes temperature rise which causes heat built up due to expansion of air in tire. This causes increased pressure - until the rubber fails, then you lose the pressure.

I found a pressure gauge rated accurate to +/- 1 psi. From Accu-Gage company in Ohio or thereabouts, brass case, dial gauge. I found them by a web search so that works - but I bought it locally. Now made in China but they still say accurate +/- 1 psi. Nearly $15 but money well spent.

[RH77]

Originally Posted by brucepick

My guess is that a bad valve stem was the cause.
Valve leaked causing tire to lose pressure.

Pressure loss causes temperature rise which causes heat built up due to expansion of air in tire. This causes increased pressure - until the rubber fails, then you lose the pressure.

I'll have to agree with the assessment. Heat is generated from underinflation. BTW, was the tire inflated to 50 psi cold? I'm sure you know about the speed/heat/increase in pressure process after highway speeds -- could have increased pressure unknowingly and put pressure on the stem creating a leak.

Sorry to hear about the failure...

RH77

[fanamingo]

How did you measure/estimate the tire presssure initially? And how did you measure it to get the 65-70 psi reading (digital guage, pencil guage, guage on the air compressor, etc.) I've never had a blowout. What happened in your case? Was it caused by a pothole, debris, or something? Did it slowly deflate while you pulled off the road? How old and worn were the tires?

Sorry for all the questions. I'm trying to get an idea if there's anything I need to be wary of to avoid the same situation.

[fritz]

Hi folks,
I'll reply to this one and then go back to lurking. Afterall this site is for the pros and I belong with the amateurs at gassavers.org

"Simple right ? Yah, but apparently not too accuarate, since the pump sometimes shuts off before the tank is really full.
To account for this, in the past I have filled the tank to the point where that it spills over. This way I know that the tank is truly full.

There has to be a better way" .

Set the kms to 1/2 miles and the fuel used to quarts and then just double the kmh number to get your MPG

This will take a while but it's what I did with my supermid.

Record a few tanks usage, miles gone, supermid gas usage, and supermid KMs gone. The longer you wait the more accurate your result will be.

I adjusted the litres used to quarts used

Use yoshis formula, if you want to stick with litres use 3.8 instead of 4 in the formula

New fuel paramater = (supermid total fuel usage/(total gallons used*4))*old fuel parameter

This will adjust the supermid fuel usage to quarts

Next step adjust the distance. If you adjust the KMs accuratly you will end up having to multiply the KM/Quarts by a factor of about 2.5 to get MPG as in

MPG=KM/Quart*2.35*(4.0/3.8) or MPG = KM/Quart*2.47
2.35 for conversion from KM/L to MPG 4.0/3.8 from L to Quarts

If you left the setting in litres it would be MPG=KM/L*2.35

So if you want to bother doing tough multiplication in your head you use yoshi's formula

New distance parameter=((total miles traveled from odometer*1.61)/old displayed distance)*old distance parameter

If you want to do it like me a not worry about displayed KMs on the supermid you fudge the distance parameter to make 1KM/L displayed= 2MPG. So if the mid says 25 KM/L you are getting 50MPG

New distance parameter=((total miles traveled from odometer*1.61)/old displayed distance)*Old parameter*(2.47/2)

The last number being a conversion factor to make the KMs traveled off by enough to make the display correct for miles per 1/2 gallon

good luck. If your numbers are way off you will be pushing that little button a lot of times

[LostCause]

The pump shut-off is designed to prevent the tank from being completely filled...mainly to prevent spilled fuel which is a fire hazard and pollutant.

The fuel nozzle usually has a bunch of tiny holes drilled near the tip. These holes are used for sucking up old fuel vapor. When the fuel level rises in the tank to cover these holes, the pump automatically turns off. There shouldn't be too much of a variance between pumps. If there is, it is probably because the pumps are calibrated slightly differently.

Your method is pretty much the best you can do. You can test the accuracy of your odometer by using highway mile markers, gps, or Google Earth. You can test pump accuracy by filling up a graduated container. Pumps will always vary slightly because of temperature variations (they measure volume, not mass)...but underground storage tanks keep fuel pretty stable.

- LostCause

[brucepick]

I've been filling same was as .Cd.
Except .Cd didn't say how hard he squeezes the handle. I pump a bit on the slow side. I don't want a lot of splashing and a lot of fuel backing up into the filler neck.

First time it clicks off, I'm done. I figure there's more variation in trying to fill the neck manually than in trusting the auto shutoff. I try use the same pump all the time but of course can't always get it.

Pump handles (at least here in CT) have a "shoulder" you feel when squeezing. Squeezing is hard at first and all of a sudden gets easier. I squeeze just enough to get to that point and keep it there. Release it a bit to get on the other side of the shoulder and it pumps very slow, that's not for me.