Internal Field Calibration in Pressure Transmitters

Several Viatran pressure transmitters have a feature called internal field calibration which allows a field to be rapidly set up. This offers multiple advantages, such as not requiring to take the transmitter out of service and the use of a defined pressure source is not needed. Instead, the field calibration (“Cal”) signal itself simulates the pressure that is to be tested. Using this mock pressure signal, zero and span adjustment can be performed which ensures the precision of transmitter measurements in turn. This feature can also be used to range down units so that they can take readings at lower pressure levels at Full-Scale Output.

What is the “Cal” Signal?

Figure 1 shows a block diagram of a pressure transmitter. The pressure sensor produces an electronic linear output in direct proportion to the applied pressure. The input/output plot has a slope which is the measure of the sensitivity of the transmitter, expressed as the ratio of signal to pressure or Volts/PSI.

The calibration or “Cal” signal is a preset electronic signal that is automatically added to the output signal generated by the sensor once it is activated. It is constant in amplitude, and therefore the resulting signal indicates a constant pressure. As an example, if a given sensor has a sensitivity of 1 volt/PSI of pressure applied, and if the Cal circuit generates 1 volt signals, the effect of activating the Cal feature is perceived by the device as the application of 1 PSI of pressure to the sensor.

The signal conditioner is the next component, responsible for transducing the sensor output into electronic format, such as 0-5 mA or 4-20 mA. It can also protect against radio-frequency interference, as well as provide compensation against temperature changes, correct errors, and make it possible to adjust for both zero and span (or sensitivity) settings.

Figure 1. Block diagram of a pressure transmitter.

Activating the Field Calibration Feature

The activation procedure depends upon the specific Viatran product. For instance, some Viatran pressure transmitters use a simple two-pin connection for calibration, as shown in Figure 2, which are shorted to activate the Cal circuit.

Other Viatran transmitters use a type of signal conditioner in which all the internal adjustments are connected to magnets, as shown in Figure 3. Both external and internal magnets are joined in a magnetic module, so that when the external magnet is turned using a screwdriver, the corresponding internal magnet also turns to follow it, automatically adjusting the controls. Thus turning control magnet number 3 by one-fourth of a turn to the 3 o’clock position activates the Cal feature.

The advantage of this type of operation is its safety and dependability. The presence of a durable hermetic seal keeps out fluids and flammable gases and prevents failure or explosion. This means that the transmitter can be calibrated without having to close down the operational workflow.

Figure 2. Connection diagram for a typical pressure transmitter.

Figure 3. Magnetic coupling safety drives control.

Figure 4. Connection diagram for a typical magnetic module transmitter.

Using Field Calibration

The only tools required to perform a field calibration on a Viatran transmitter are a calculator (to compute the strength of the required calibration current), a screwdriver to turn the magnet to activate the Cal feature, and a meter to measure the output signal from the transmitter.

Each transmitter has the calibration pressure value laser-etched on the body, and the actual value will be within 0.1% of the stated pressure.

The Procedure

The actual technique of calibrating a transmitter is as follows:

• First evacuate the transmitter of all pressure
• Adjust the transmitter’s zero control to range it down to zero
• Calculate the calibration current voltage by using the ratio below:

• Activate the Cal signal as detailed above
• Adjust the span control of the transmitter to get an output which is equal to that of the calibration current
• Deactivate the Cal signal

The original zero value of the transmitter can change after this process because of the adjustment of both zero and span controls, especially when the range is high or zeroes are suppressed. Therefore, to make up for this, the transmitter needs to have its zero set again freshly, repeating the process until the zero is reasonably constant. Typically, the span of a pressure transmitter can be adjusted to within ± 10% of its standard pressure range.

Example 1: Ranging a Transmitter

Viatran transmitters can be ranged from 3 to 5 times the standard output, depending on the model. Thus, a unit which has a pressure of 10 000 PSI can undergo adjustment to generate 2000 PSI at FSO. To achieve this, the transmitter is adjusted to give a 4-20 mA signal from a range of 1-3500 PSI. If the Cal pressure is 2042 PSI.

In this example, the preferred transmitter output is as follows:

This formula is used to calculate the calibration current:

The Cal current is thus 9.3367 mA, and this is added to the zero state current, namely, 4 mA, making the total Cal current 13.3367 mA.

• The procedure continues with the insertion of a current meter into the 4-20 mA loop, as shown in Figure 4
• The pressure is removed from the transmitter and the output is adjusted to 4 mA by turning the Coarse and Fine (#1 and #2, respectively) controls
• Control #3 is now turned a one-fourth turn to the 3 o’clock position so that the Cal signal is activated, increasing the output
• Control #4 is now adjusted to change the span until the output is 13.3367 mA
• The Cal signal is deactivated by turning the #3 control to 12 o’clock
• The transmitter can now be ranged back to zero if necessary, and the procedure repeated until both the zero pressure output and the Cal pressure output arrive at the calculated value

At this point, the transmitter has been calibrated to a range of 0-3500 PSI with an output of 4-20 mA.

Example 2: Non-standard Zero

In some cases the transmitter pressure cannot be removed, but the sensitivity is still capable of adjustment using the Cal signal.

In the present instance, a tank level transmitter with a 0-100"W.C. requires to be calibrated to a range of 0-80" but it still contains 12” of water. Viatran transmitters make this possible without having to remove the transmitter or empty the tank.

The transmitter in this example has a calibration pressure of 34.1” laser-etched on the housing. The 12” W.C. pressure recorded by the transmitter is considered “fake zero” and its output must be determined before the unit can be calibrated precisely. For this reason, the fake zero current is summed with the “true zero” current of 4 mA before the total is added to the calculated Cal current. The result gives the corrected calibration output. In summary, the transmitter output is as follows:

The “fake zero” output is now calculated by the following ratio:

FZ = 2.4 + 4 mA

The zero is thus adjusted to the total, that is 6.4 mA

The Cal current is now calculated as below:

The Cal current = 6.82 mA, which means that once the Cal feature is activated the output is expected to shift by 6.82 mA. However, this does not take into account the fake zero current of 6.4 mA, which means that on activation of the Cal the current output will actually be 6.82 + 6.4 = 13.22 mA.

The remaining steps are:

• Activate the Cal and make span adjustments for an output of 13.22 mA
• Deactivate the Cal and re-zero the transmitter for an output of 6.4 mA
• The calibration may be repeated as required, such that the output should be at 4 mA when the tank does not contain any water

Calibrating the Equipment Used for Data Acquisition

Once the calibration of both the transmitter zero and the span are achieved to the desired range, the Cal feature can be used again to confirm the reliability of operation of data acquisition equipment. Activation of the Cal feature should result in an output reading equal to the calibration pressure from the display which has been set for a particular special range. The panel meter may then be set to read the stated calibration pressure value on the body of the sensor.

Another use of the Cal feature is to test alarm activation when set thresholds are crossed. For instance, if the alarm is set to go off at or below the calibration pressure, merely activating the Cal feature should result in tripping the alarm, thus helping to establish that all emergency shut-down equipment is in good functional condition.

Precautions

The Cal feature on the Viatran differential pressure transmitter has several benefits, but some precautions must be observed. For one, the proper function of this function implicitly assumes that the sensors respond uniformly to pressure over time. However, it is possible that the characteristics of the transmitters may be affected by corrosive materials in the surroundings in the long term, leading to changes such as erosion of the sensing diaphragm leading to an unduly sensitive sensor.

Another caveat is that the Cal signal is one which is obtained from a simulated pressure, and may thus differ from the etched value by up to 0.1%. Whenever possible, it would be preferable to apply a known weight to the transmitter, such as using a dead weight pressure generator, to arrive at a closer model of the actual operating conditions.

This information has been sourced, reviewed and adapted from materials provided by Viatran.

For more information on this source, please visit Viatran.