Two vacuum gauges fitted on one vacuum system do not have to agree, and that is normal. When the reading at the pump is compared with the reading at the chamber and the two numbers do not match, it is easy to conclude that a gauge is faulty or that the machine has a problem. In fact the position of the pressure tap decides which part of the system each gauge measures; the two gauges are not measuring the same quantity at all.
The tap position decides what the reading means
The gauge at the chamber is connected to the vacuum side of the degassing chamber and reads the pressure inside the chamber. The working vacuum of -0.08 ~ -0.0999 MPa in the machine specification refers to exactly this position. The ultimate vacuum of ≤5 Pa is likewise a figure measured in the chamber. Whether the degassing section is working properly is judged on the reading at the chamber.
The gauge at the pump is connected to the inlet of the vacuum pump and reads the pressure at the pump inlet. The oil vapour travels from the chamber to the vacuum pump through the pipework; the longer the pipe, the more bends it has and the higher the flow velocity, the greater the flow resistance along the way. The pressure therefore falls all the way along the pipe, so the absolute pressure at the pump is lower than at the chamber and the reading looks closer to a vacuum.
The tap itself also has to be chosen with care. It should be connected to the side wall of the chamber, where it reflects the gas phase pressure directly. A connection at the top of the chamber lets the head of condensate inside the pipework affect the reading, and a connection below the liquid level reads the pressure of the oil instead of the gas phase, which makes the number meaningless.
How large a difference is normal
The difference is not a fixed value; it follows the condition of the pipework. When the line is clear and the vapour flow is steady, the difference between the two readings is a relatively stable figure, and readings taken on the same duty on different days are close, which means the state of the system is consistent. If the difference keeps growing within a single day, it usually means that the resistance in the line has increased.
| What is observed | Likely cause | Where to look |
|---|---|---|
| The difference stays within a range | The resistance of the line is in its normal state | Record the difference as the baseline for later comparison |
| The difference keeps growing | Condensate has collected in the line or a section is blocked | Walk the line and check the low points, bends and reducers |
| The difference jumps suddenly | The pressure tap is blocked by dirt or sludge | Open the tap, clean it and measure again |
| The chamber reading is normal but the pump reading is higher | The two taps are connected the wrong way round, or the impulse line is blocked | Check the connection positions and the routing of the impulse lines |
Where to look when the difference grows
- Check the low points of the line first. The oil vapour produced by degassing condenses inside the pipework, the residual liquid runs back down the slope into the pump chamber, the flow resistance rises and the pressure difference between the two ends of the line grows.
- Then check the impulse line itself. It is a narrow tube, and over time sludge builds up on the inside wall, which delays the reading or distorts it. Clearing it and measuring again will tell the two cases apart.
- Compare the range and the accuracy of the two gauges. If their ranges differ a lot, the difference itself is unreliable; fit gauges of the same specification before comparing.
- Check the connection order of the two-stage vacuum. The ZYD-100 is fitted with the X-100 vacuum pump and the JRP-500 roots blower, the roots blower working as the booster in front of the vacuum pump, and the pipework has to follow that real order. Connected the other way round, the pressure distribution falls apart.
- Write the difference into the operating record. The difference is itself an indicator that shows the condition of the line early, long before the vacuum level drops and the problem has to be traced.
Which gauge decides
The process judgement is made on the chamber gauge. The -0.08 ~ -0.0999 MPa in the specification is the vacuum level inside the chamber; the reading at the pump is there to show whether the inlet conditions of the pump are normal. While a two-stage vacuum runs, the inlet and outlet pressures of the roots blower also change with the load, and looking at these readings together shows whether the pumping speed keeps up with the amount of vapour being produced.
If the two gauges never produce a stable relationship between them, the problem is most likely in the pressure tapping system rather than in the degassing process. Clear the taps and the impulse lines first, and only then go back to the degassing parameters.
Two readings that differ are normal; two readings that never settle into a stable difference are not.
