Strange pressure readings on filter

[Jim]

I installed a pressure gauge on my Motorguard filter that is in my 1991 Benz diesel. I was expecting to see a fairly constant pressure reading. I have found that the pressure ranges from 50 PSI when the oil is cold to zero when the oil is hot. (can’t see that the needle has moved) I was thinking that the temperature or viscosity would not matter since the oil has to go through an orifice first before the oil getting into the filter housing. If the oil is thick it should enter slowly to compensate for the fact that it is also harder to get through the filter. The pressure change with temperature therefore seems very strange. I was hoping to see a reading when the oil was hot in order to see if the filter plugs with time. Maybe someone with fluid dynamics training can explain this. My only explanation is that when the oil is thicker it packs the fibers together harder making it more difficult for the oil to get through.

I made a little machine to re-wind my rolls in order to get tighter rolls of paper. I can fit one and a quarter Scott 1000 rolls in the housing after they are wound tight.

 

[deeter16317]

Are you sure you are plumbed upstream of the orifice? That's what it sounds like to me...your down stream of the orifice and reading the pressure across the media, not the orifce.

I have a pressure guage in-line (upstream) of my bypass...I get from 60 to 80 psi cold and 25 to 60 psi hot. But this guage is between the orifice and the oil source, so the orifice is hold pressure, so to speak.

 

[Gary Allan]

If the gauge is on the supply side and the reading drops off to near zero ..then your supply line is too long. It's a curious state no matter how you shake it up on the supply side. You should read the same as your oil pressure in most cases. Again, length can alter the pressure in a dynamic flow. When cold, it's almost (like) a static chamber ...like a tank. The pressure is the same due to no (or near no) flow. Once stuff gets in motion ..then the line can offer a pressure drop and you can, figuratively speaking, "overload the supply". Some who remote mount their pressure sender get lower readings at a remote location. That is, instead of teeing off at the sender port and putting their sender at the normal location, they just move the tee out to the remote locations and relocate the sender too. It's fine when everything is cold. When you get warm and a larger amount of oil can pass through the bypass filter (it's like a brickwall when the oil is cold) ..then the sender would read a good bit lower across the span of hot pressures.