Here is a procedure to check the leak rate of a RPM4 pressure monitor with range to 10000 psi. The pressure of the setpoints below can be adjusted up or down depending on the range of the device that you are testing. This procedure can also be used for pressure controllers (need to turn off control before checking leak rate), E-DWT-H pressure calibrators and comparators like P5514, P5515, P5510.
Note that RPM4 and PPC controllers show "Rate of change" on the main display screen (step 5 below). For other devices you will have to watch the pressure and see how much it is changing per second or measure the pressure change over a period of time and calculate "Rate of change".
- What is the pressure?
- Set the pressure to 9000 psi (or to 90% of full scale)
- Wait 1 minute
- If an automated controller, turn off active pressure control. Press ESC for a PPCx, press MEASURE for a 6270A, 8270A, 8370A or a 7000 series controller
- Wait 5 minutes or for the leak rate (Rate of change) to stabilize (get as good as it will get)
- What is the pressure? (e.g. psi, bar, kPa, inH2O)
- What is the pressure “Rate of change”? (e.g. psi/sec, bar/sec) and is it stable?
- Repeat steps 2 to 6 with setpoints of 6000, 8000, 9000 and 5000 psi (or 60%, 80%, 90% and 50% FS). Record all data!
For each test, the leak rate should be less than -0.002% per sec. (-20ppm/sec) for an automated pressure controller to work well. A leak rate better than -10 ppm/sec is preferred.
Leak rate is calculated by: (Rate of change, in psi/sec) divided by (pressure, in psi)
- Pressure is 0 psi
- Set pressure to 9000 psi
- Wait 5 minutes
- Pressure is 8994.14 psi
- Rate of change is -0.09 psi/sec. No, it is still changing but the value is getting better. Leak rate is -0.09 / 8994.14 = -0.000010 /sec (or -10 ppm/sec).
- Pressure is 8989.64 psi. Rate of change is -0.07 psi/sec. Yes, the rate of change isn’t changing anymore. Leak rate is -0.07 / 8989.64 = -0.0000078 per second. This is the same as -7.8 ppm/sec (parts per million per second) or -0.0078 % of reading per second. The leak rate is less than 20 ppm/sec so is good.
If you can capture this data electronically (by remote communications) and graph it, that will help also. Especially if the pressure is being influenced by temperature changes. If so, capture the temperature also. See the below example Excel file for a liquid filled RPM4. Note that closed, liquid filled systems are very sensitive to temperature changes.
The file shows that it takes about 2 minutes for the rate of change (and pressure) to stabilize after a pressure excursion from vent to 5000 psi. This is almost entirely due to temperature effects. When the pressure is increased, energy is added to the system. Most of this energy is added pressure, but some is heat (very slightly increase in temperature). When you stop increasing the pressure, the temperature stops increasing, and starts decreasing back to the ambient temperature. As temperature decreases in a closed system the pressure will decrease. This appears to be a leak but is actually pressure decreasing due to a decrease in temperature.
If you are familiar with deadweight testers you might note that they are more stable than closed, liquid filled systems. This is because when the temperature changes the piston moves up or down and maintains a very stable pressure if the piston does not move to its maximum up or down position. In a closed, liquid filled system there is no piston to move so the pressure changes with very small changes in temperature.