Home > NCU Calibration
NCU Calibration
1328 Azalea Garden Rd.
Norfolk VA. 23502
757-855-6006
Fax: 757-855-3761
Date: Febuary 10, 1998
Contact: Customer Service ( )
This information bulletin affects - Nitrogen Control Units NCU1/ NCU2/NCU4
Overview
In addition to the cycle control programming, the calibration tables for the Nitrogen Control Unit are stored in the lower module programmable logic controller. This allows the lower module to operate separately from the programming module. The lower module, and its calibration set points however must be accessed through the upper module PC, specifically through the programming software for the PLC which resides in the c:/eagle directory of the Upper Module. In order to access this software and perform a calibration the service technician will need the following:
A personal computer keyboard with a standard keyboard connector (not ps2)
A pre-calibrated pressure gage
Prepare for calibration.
1) Disconnect the power supply cable from the rear of the NCU and depressurize the incoming high pressure gas supply.
2) Loosen and remove the flat head screws holding the rear cover to the upper module and remove panel
3) Lower the front access door to use it as a holding tray for the PC keyboard and insert the Keyboard plug into the keyboard connector on the motherboard inside the upper module. (Note: the keyboard connector is most often found adjacent to the motherboards power connectors at the right rear corner of the board while facing directly into the back of the upper module. Be sure to grasp the board connector with one hand to offset any undue force as the plug is inserted with the other hand)
4) Reconnect the power supply cable to the upper module and power up the NCU
DO NOT APPLY INLET PRESSURE TO THE UNIT AT THIS TIME
Input the set points to be calibrated
5) With the NCU powered up, select the channel to be calibrated on the touch screen and enter the following parameters while clearing out the other channels ( select the channel and press the "Clear" Button)
| Step | Pressure | Time | Ramp |
|---|---|---|---|
| Step 1 | 1000 | 30 | 500 |
| Step 2 | 2000 | 30 | 500 |
| Step 3 | 4000 | 30 | 500 |
| Step 4 | 5000 | 30 | 500 |
| Step 5 | 1 | 30 | 500 |
Then download these values to the PLC by pressing the "Download active Set points" button. At this time one should check to see if the active values are properly set by clearing out all the channels in the programming module and pressing the "retrieve active set points". When pressing the graph button, one should only see the channel to be calibrated with the desired set points listed above.
6) Stop the upper module touch screen program by pressing the Ctrl and Break keys at the same time. A message will be displayed over the upper screen "press any key to return to system". After pressing any key the "c:\" prompt will appear. For ease of reading the screen one should then type the letters CLS and then press the enter key. This will clear the screen.
7) We must now start the PLC programming software. To do this we must change to the eagle subdirectory by typing cd\eagle and pressing enter. The screen should indicate c:\eagle
8) Start the Symbolic relay logic programming by typing SRL and then pressing the enter key. The SRL program will then start and give us several options via the function keys on the keyboard. The function we will be requiring will be F5 Monitor. By pressing the F5 key we will start to monitor the program running in the lower PLC. The screen will initially show us the program name running in the PLC and by pressing enter we will see the program operating through a ladder logic overlay.
9) We now need to get into the calibration tables for the individual channels. We do this by setting the program into what is called the "write mode"by pressing the Alt key and the F2 key at the same time. When we do this the F5 key (on the lower screen menu) will state "Calibrate analog" When we press the F5 Key a box will appear with the cursor flashing at the INPUT ANALOG NUMBER area. Input the analog number that corresponds to the channel being calibrated per the following chart.
| NCU type | Channel | Analog Number |
|---|---|---|
| NCU1 | 1 | 26 |
| NCU2 | 1 | 25 |
| NCU2 | 2 | 26 |
| NCU4 | 1 | 25 |
| NCU4 | 2 | 26 |
| NCU4 | 3 | 2011 |
| NCU4 | 4 | 2012 |
After entering the appropriate analog number, pressing the enter key will bring up the current calibration table for that channel.
Calibration table
The calibration table is used to equate the actual input value coming from the pressure transmitter for the channel being calibrated to real world values ( in this case PSIG) that we and the PLC can understand. We ourselves do not see the first step in the pressure reading process, that being the output of the pressure transmitter. The pressure transmitter , depending on system design, will output a signal in a range of either four milliamps to twenty milliamps (4 to 20 mA) or zero to ten Volts (0 to 10 V) depending on the pressure it has at its input. The standard NCU transmitter will generate a signal of 4mA at 0 psig and 20 mA at 5000 psig. (Note: the 6000 psig NCU will incorporate a 7000 psi transmitter which is designed to operate at 4 mA at 0 psig and 20 mA at 7112 psig.) This signal is introduced to the PLC through an analog to digital converter that, as its name implies converts the 4 to 20 mA signal to a digital value. At 4 mA this "raw value" will equate to 0 and at 20 mA this value equates to 2047 ( 2 to the 11th power, as this is an 11 bit converter). We must then "equate" these raw input values to our real life "pressures".
We will in no doubt notice that with the outlet of the NCU open to atmosphere and the pressure disconnected from the unit, the "raw input " is not actually 0. This is normal as most of the transmitters used will actually put out a signal of approx 4.04 to 4.1 mA at 0 psig. This generates a raw input of between 88 to 120. It has been found that in some circumstances in which a good ground it not supplied to the NCU this value can be even higher. If this occurs, the NCU will appear to not fully depressurize on its main screen even when fully disconnected from pressure sources.
Editing calibration table
With the system fully depressurized ( outlets not connected and no incoming gas supply) the cursor can be placed at the first "engineering unit" which must be 0 and the F8 key depressed. This will place the value from the "current raw input " into the "raw input" location for 0 psig. By pressing the enter button we can now go onto the next engineering unit however we must now pressurize the transmitter / channel being calibrated.
Connect the calibrated gage to the channel in question and supply the inlet gas to the unit. We now see why we programmed the channel in question with pressure steps that spanned the full range of outlet pressures while each step last for 30 seconds, this will give us time to input the data. Manually close the initiate contacts to start a cycle and watch the outlet pressure gauge. During each step, when the pressure stabilizes on each set pressure, input the actual pressure seen on the gage at each engineering value and press the F8 key to insert the current raw value into the table adjacent to said engineering value.
Note: During the calibration process the first set of numbers in the table MUST be 0 and the raw value associated with no output pressure. The last set of values entered MUST be the max pressure of the NCU with its associated raw value however not all the calibrate table has to be filled out. Since the pressure transmitter used are fairly linear in output, the basic system will normally function quite accurately with the following values. Filling the table completely will increase the system accuracy slightly.
| Eng. Units | Raw input (0-2047) | |
|---|---|---|
| 1 | 0 | F8 at 0 psig |
| 2 | 5000 | 2047 |
| 3 | ||
| 4 | ||
| 5 |
If the NCU is being used to control a process and a particular pressure in that cycle is critical, it wise to include that pressure and its associated raw value in the body of the calibration table.
When the required calibration points are entered in the table, press the F5 key again to save the table. Then press the Alt and F10 keys to exit from the monitor screen, the ESC key to go back to the DOS prompt and press the system reset button the restart the upper module program.
Then repeat the calibration process for each channel to be calibrated.
Note: It may be possible that even though the NCU is programmed to go to its Max.pressure, the system will not quite reach this pressure. This is normally due to the low pressure control is not being set high enough. If this is the case, increase the control pressure by rotating the lower control pressure regulator clockwise in small increments (1/16th to 1/8 turn) untill the system can optain its max output.