Calibration of DP Transmiter in Vacuum Service


We have a DA vacuum tower which is used for removing the oxygen content of the seawater and used for water injection back to the reservoir in the Oil & Gas industry. We have the Yokogawa DRS (Digital Remote Sensor) installed on our DA tower. The Master Tx is directly coupled to the Lower nozzle and the Slave Tx is directly coupled to the Upper nozzle. This arrangement eliminate the capillary effect and the problem associated with wet leg etc.

The trends are shown in the attached after we first installed the two Tx's. You can see the Tx for control (Blue Line) and the Tx for trip (Orange Line) are aligned with each other for the first two months after first installation of both Tx’s. The trip Tx is then start to drift lower. The inst tech then completed a re-calibration of the Tx’s, the trip Tx trend then jumped back but it reads always about 6% higher than the Tx for control. Offshore techs have done a re-calibration of both Tx’s last week, but there are still a discrepancy of 6% between the two Tx's as before.

Another problem is that both Tx’s read artificially higher under normal operations (when the vacuum pump is on). The level in the tower drops back to the true level when they switch the vacuum pump off.

Both Tx's have a range of 5725mmWG. The LRV is set at -5725mmWG and URV is set at 0.

What could cause the behaviours we have seen? The tech calibrate the master Tx by having the LP side vented to the atmosphere. should the LP / Slave side connected to the process to see the vacuum while doing the calibration? Much appreciated for your help.


I'm on page one, and you're page 8.

The typical application for DRS is hydrostatic liquid level measurement, where the slave goes up top and measures the vapor space pressure (in your case a vacuum) and the master is mounted down low and measures the combined vessel pressure and hydrostatic head.

The dual transmitters (like a single DP transmitter) has the low side/top pressure value subtracted from the high side/bottom pressure to get raw hydrostatic head corrected for vessel pressure. Typically, the receiver divides the pressure in water column unit by the specific gravity to get the physical level. So in the end, the output is a single level signal, even though there's two transmitters.

The DRS manual shows this kind of application.

But your graph has two level (?) trends. Are there two sets of DRS on this vessel? Where both tracked for awhile then one drifted?

The trends do not appear to be separate high and low measurement values. The absence of units adds to the problem.

Please explain what the two trends are beyond the cryptic 'control' and 'trip' designations. Where do the signals originate from and what is their scaling?