View Full Version : Electronics question

09-14-2007, 05:29 PM
I am trying to figure out how to get a gauge that shows a difference in pressure between two senders.
The first use I have for this is oil filters, to see when it is time to change them.

What I need to know is how electric gauges work. Obviously, the more power going through from the sender, the higher the reading, but is there a way to use another sender to counter that reading?

What I need is to get a read on only the difference between the outputs of two senders. If both have 50 psi then the gauge should read 0, if one is 60 and the other 50 psi then the gauge should show 10 psi.

Anyone know electronics here?

09-14-2007, 05:49 PM
I am not sure this can be done. But if it can... I am sure that there will be a 'wheatstone bridge' involved. That is the only circuit that will deal with true analog in a balanced circuit.

Ill have to scrape the gray matter off the skull and think about this for a few... but if I think of anything, Ill let you know.

09-15-2007, 08:58 AM
Of course there is one other option, but I don't know if the proper sender exists. If I had one that has increasing resistance and one that has decreasing resistance (for the same pressures) then they could be installed in parallel to counter each other.

Heck, if I could find a mechanical gauge that would show differential pressures I'd go with that.

At least I have something new to research. I've never heard of a wheatstone bridge, so I'm about to learn something new. ;)

David Pozzi
09-17-2007, 07:10 PM
You need a differential pressure gauge. We use them in drip irrigation filter systems to check how much pressure drop we have.

There are differential pressure sensors that are used to trigger the electronic back flush systems but it's all controlled by a small computer board.

The gauge would look like an ammeter with needle in the center and positive and negative numbers on each side of center, OR like a normal pressure gauge for systems where you know one input will always be higher than the other.

12-01-2007, 05:59 PM
too bad its worthless as most filters have inside pressure relases and the setup you propose would require special filters and would be firtually worthlessas filters need to be serviced when the oil is or at regualar intervals, not as they get supposedly less flow as the filters may come out of the box may have slightly different flow rates anyway and the internal reliefs might be different also.
Kind of moot point as filters arent that expensive and to me it would be much easier to simply use synthetic oil and run oil analisis at specific intervals.

12-01-2007, 06:56 PM
I would be using a remote filter mount, with two inlets and two outlets, so I could take a pressure reading before and after the filter, so no special filters would be required.
Some filters do, indeed, have built-in bypasses. The pressure setting varies, on what I don't know, but it is more than a couple psi, so it shouldn't be too difficult to read..
I'd probably go until the differential increased a certain amount.

The reason I wanted to do this is because I plan on running synthetic oil, so I won't be changing it as often as if it were conventional oil, and I wanted to be sure the filter wasn't getting clogged in the meantime. After all, if the bypass opens I'll still have oil running through the engine, but it'll be unfiltered. I wouldn't know thee was an issue until it wore out the engine.

I've never had an oil analysis done, so I don't know how to utilize that information. I'm guessing I'd get the engine broken in, so wear metals stabilize and then have the oil analyzed every 1,000 miles until it determines the oil should be changed, and then just change it at that interval from then on?

12-02-2007, 12:40 PM
Ok, for the first fun fact you dont switch to synthetic to get longer oil changes, period.
With a synthetic you should switch to it to give extra protection under stress and the ability to use tighter clearances,thinner oil for less resistance to motion/economy.
It by no means lets you double your mileage between changes. If you were told that then you listen to someone still stuck in the AMSOIL 80's.
I can tell you for the fact you may take a oil change from say 3000-3500 to 4000-4500, but it does not let you get away with 7500-15000 mi oil changes.
I use synthetic for multiple reasons and one is Ican stretch a change a few extra(say for one pay period) days. Not months or thousands of miles.
As an engine builder please dont get fed hype on change mileage advantages as there are few.
Additives are depleted expotentially as the oil ages.
AS for the reason for changing filters say mid change,,,no no, no. If its worth changing filters change oil.
If you really want to manage your oil install an hour meter on the negine and watch your hours per week driven, and per oil change. Do a few analysis on the oil and plan a little. Donthink synthetic makes your oil bullet proof,,, it doesnt.
But good luck if you want o mess with a system that wont help maintain your engine any better than just pullingthe plug and screwing on 2 new filters.
Now if your say 2000 into a 3500 mi oil change and want to autocross, check the condidition ofthe oil(color) and maybe swap filters for a cheap mental pat on the back, BUT if I wasgonna do it $25-$30 in oil and what $10 in filters before a race/etc would be cheap.
Besides I have NEVER seen an engine lost because the filter bypassed, failed yes but more are due to oil break down due to lack of changing (ie all oils "wear out" some are just better out of the bottle) or a filter failure in the outer casing.
The setup your thinking about wouldrequire many thousand miles of experimenting AND monitoring ,preferably by electronic means.
See to do this you would have to have oil temp bias figured, differences in your connections(ie pressure/flow loss on each side) engine temp bias figured, type of miles driven (ie engine speed).
you would also need a way to qualify both filters before doing so.
My question is, have you ever had an engine fai ldue to filtering?
If no this is a waste of time/resources and simply change your oil at say 4000 vs 3000 or 4000 vs 5000.
Besides being of a very limited benefit its just not gonna protect like a regular change schedule.
Trust me I switched to sythetics al ong time ago and only use extended change times if I have to wait to buy the oil/filter no more.
Good luck what ever you do.

David Pozzi
12-02-2007, 09:44 PM
You can get a bypass oil filter that will filter out smaller particles than your regular filter does. It bleeds off a small amount of oil that passes very slowly through a much finer filter. It's an old trick that is still being used. The Amsoil guys like to sell them.

I don't think synthetic oil is so good you can extend it's drain interval way out. You may be able to extend it a little though. Most synthetics are blended with regular oils and not pure Synthetic.

A good test would be to run your regular oil to the normal oil change date, then test it. That would be your standard to at least equal with your extended drain interval. I wouldn't waste my money testing every 1000 miles, the oil will barely change in that small a time. 2000 or 2500 would be better and begin tests at the regular oil change time.

I don't use synthetics in any of my cars, including the Lola T70, I don't push the oil temperatures high enough to need it.

12-04-2007, 08:57 PM
The truth about synthetics is thatthe molecule is engineered to a specific purpose, its still made from petroleum products, its just design from the ground up instead of a distilation byproduct then fortified with aditives.
Synithetics are not super technology products just engineered lubricants.
A big bonus from synthetics is the fact that they resist thermal breakdown, have better cold flow characteristics and the base being a much better product overall doesnt seem to breakdown as easily as "dino" oil. And they usually take a lot less(number and amounts) additives(ie the ash and zinc) that other require to do a specific job.
As for dual filters, those are a real pain to package, it adds double the plumbing and complexity and wont save you unless your gonna go run the Baha 1000, its overkill.
Again, use a good brand, a proper brand(but not Fram, super junk restrictive filters) filter Wix,Purolator,K&N, are good and carried by several companies. Run enough oil to allow the oil to spend some time in the pan or sump to reject heat (an oil cooler would be a better choice and single filter) adequately.
Again oil analasis and maybe an hour meter, and just watch the guage a little. I can tell when my oil starts breaking down or gets low(remember a well sealed engine will use a certain amount of oil).
I try to run 3000-4000 mi oil changes. but have run it to 4000-4500 and oil still looks good.
But if you still want to try to do this good luck.