water inline low pressure gauge at output

i don't know if they sell them , water Inline  pressure Gauge  made for 5 lbs and under, or is there an easy way t to make one, doesn't have to be inliine, it just needs to  measure the psi at 5 gallons and at 1 gallon.. Even A balloon that burst at 3psi would be good/.
it is for a medical device. i spent the last year going crazy cause it was defective brand new.  it was putting out the correct  pressure at the start, 5 gallons, at the half way point it was putting out about half the pressure, didn't know it, almost had  surgery because of the poor results
thanks for any ideas

lemonie5 years ago
Yes they do sell them, "water Inline pressure Gauge" is what to search for.