Dennis
The gauge you speak about is the filter pressure differential or (delta P)
The delta P is a function of pressure applied , and resistance to flow, meaning as the filter restriction increases the flow rate or resistance to flow rate increases causing the pressure drop or differential between the high pressure side of the filter and the low pressure side of the filter.
The reason that gauge is there is to allow a means of monitoring the filters restriction or dirt level .
Hydraulic filters are sized according to the type of service and equipment they are supplying as some equipment are far less tolerant of dirt than others.
The goal of the delta P gauge is to give you a warning indication to allow you to know when a filter is reaching a critical level and to help avoid filter failure caused by excessive pressure differential example filter media rupture.
The filter manufacturer will have an acceptable pressure differential figure for their filter, this is where your delta P gauge comes in to play as a rule of thumb the time to replace a filter is at approximately 1/3 of the max that the filter manufacturer lists .
As your heavy equipment mechanic friend pointed out these filters are often changed out long before there time but that's kinda the point as the filter is the only line of defense the hydraulic components have from their arch enemy ( dirt ).
There are other considerations here as well, one of the major ones is hydraulic oil degradation this is usually a function of heat
And in a hydraulic system any where there is a pressure change there is power being used and that equates to heat generated as components in the system ( ie. motors ,pumps ,valves ) become worn they tend to leak internally or by pass oil this by passing oil also creates heat this is why hydraulic systems of all kinds have oil coolers designed into them .
The problem arises when the heat reaches a point where the hydraulic oil begins to degrade at this point there is a very distructive chain reaction where the oil oxidizes and loses its ability to lubricate and the heat buildup gets out of control usually ending in pump and or motor destruction.
The truth is with clean oil in a closed system the filter should see no real dirt buildup and very little change in pressure from new to old but if for some reason the gauge reads change it can be an indication of something wrong in the system either dirt infaltration or a component failing if the pressure shows a shape increase or filter failure ( ruptured media )which would cause a shape decrease in gauge reading.
Remember that hydraulic fluid will oxidize on its own with time from expose to air and heat increases how fast it will oxidize.
The real reason for filter and oil changes in any hydraulic system is to maintain a good quality of lubricity and working characteristics of the hydraulic oil .
This is why manufacturers will set a time limit as well as a mileage or number of hours for an oil change on any given oil
Bottom line is it's best to change the hydraulic oil as scheduled by the equipment manufacturer.
To use your gauge it's best to start with new oil and filter and take a reading at say 2000 RPM to use as a base line so you can monitor any pressure change over the life of the hydraulic oil and filter.
Sorry for the long explanation
Eric
