PR235: How to Perform Field Auto-Calibration? What Are the Steps?

In modern industrial automation, process instruments, particularly pressure transmitters, are the cornerstone of ensuring precise, reliable, and safe system operation. Emerson's Rosemount PR235 pressure transmitter is widely used in various demanding environments due to its exceptional performance and stability. However, even the most precise instruments can experience minor drifts in measurement accuracy over time, due to changes in ambient temperature, or the influence of process media characteristics. To ensure it maintains optimal performance in the long term, regular calibration is essential. The "Field Auto-Calibration" feature is a powerful and user-friendly function provided by the PR235, allowing technicians to quickly restore its measurement accuracy without removing the transmitter from the process line or using complex external pressure sources.

This article will delve into the field auto-calibration function of the PR235 pressure transmitter, explaining its working principle, prerequisites, and providing a step-by-step operational guide.

#### **Understanding the Concept of "Auto-Calibration"**

First, it is crucial to understand precisely what "Auto-Calibration" means for the PR235. It does not mean the transmitter can calibrate itself completely without any user interaction. Instead, it is a **simplified two-point calibration procedure based on user input.**

Its core principle is as follows: In most process applications, the process media creates a static pressure (e.g., the liquid column pressure always sensed by a transmitter installed at the bottom of a tank). This static pressure point can be considered a stable "zero" reference. When process conditions change (e.g., a pump starts or a valve switches), the pressure rises to a higher, stable value. The user can utilize these two naturally occurring, stable process pressure points to perform the calibration.

Therefore, auto-calibration is essentially **teaching the transmitter**: "When I see pressure X (low point), you should output 4mA (or 0%); when I see pressure Y (high point), you should output 20mA (or 100%)." The microprocessor inside the transmitter then recalculates its output curve to match these two known points.

#### **Prerequisites for Performing Field Auto-Calibration**

Before starting the operation, the following conditions must be met. Otherwise, the calibration results will be invalid and may even introduce greater errors:

1.  **The Process Must Be Stable:** This is the most critical condition. You need to wait for the process to be in two distinct and very stable states. Typically, this is a "zero" state (such as static pressure when the pump is stopped and valves are closed) and a "span" state (such as rated pressure during normal pump operation). At these two states, the pressure reading must be absolutely stable, without any fluctuations.

2.  **Know the Reference Pressure Value:** You must read the actual current process pressure value through a **known accurate** reference standard gauge (or field indicator) with higher accuracy than the PR235. The PR235's auto-calibration function is based on this reference value you input. If the reference value you enter is itself incorrect, the calibrated transmitter's output will also be incorrect.

3.  **Have Operation Permission:** It is usually necessary to connect to the transmitter via a hand communicator (like Emerson's AMS Trex Device Communicator) or software supporting the HART protocol (like Emerson AMS Device Manager). You need the necessary permissions to modify calibration parameters.

4.  **Transmitter is in Good Condition:** Ensure the PR235 sensor itself has no mechanical damage, and the diaphragm is not dented, clogged, or corroded. Auto-calibration cannot fix physical hardware faults.

#### **Detailed Steps for Field Auto-Calibration (Using a HART Communicator as an Example)**

The following is a typical step-by-step operational procedure:

**Step 1: Safety Preparation and Connection**

*   Follow all site safety procedures, perform Lockout-Tagout (LOTO), and ensure the process is in a safe state.

*   Connect the HART communicator in parallel to the transmitter's loop (usually connected across the transmitter's signal terminals or the load resistor). Ensure there is at least a 250Ω load resistance in the loop to ensure proper communication.

*   Power up the transmitter and establish HART communication.

**Step 2: Access the Calibration Menu**

*   In the communicator's main menu, select the connected PR235 transmitter.

*   Navigate to the "Calibration" or "Setup" menu.

*   Select "Calibrate" -> "Sensor Trim" or a similar option. In the PR235, this is often called the "4 and 20 mA trim" or "Re-range" function, but its essence is to perform a two-point calibration.

**Step 3: Perform the Lower (4mA / 0%) Calibration**

*   Let the process stabilize at the first reference point (lower value). For example, ensure the pump is stopped, the discharge valve is closed, and the system is at full static pressure.

*   Use your **reference standard gauge** to accurately read and record the current actual pressure value (e.g., 0.5 bar).

*   On the communicator, select "Lower Sensor Trim" or "Set 4mA Point".

*   The communicator will display the pressure value currently measured by the transmitter. At this point, you need to **input the accurate value read from the reference standard gauge (0.5 bar)** into the communicator.

*   Confirm the entry. The transmitter will align its currently read pressure value with the standard value you entered and adjust its output accordingly to output an accurate 4mA signal at this point.

**Step 4: Perform the Upper (20mA / 100%) Calibration**

*   Change the process state so it stabilizes at the second reference point (upper value). For example, start the pump, open the valves, and let the system reach normal operating pressure.

*   Wait for the pressure to become completely stable.

*   Again, use your **reference standard gauge** to accurately read and record the current actual pressure value (e.g., 10.0 bar).

*   On the communicator, select "Upper Sensor Trim" or "Set 20mA Point".

*   Similarly, **input the accurate value read from the reference standard gauge (10.0 bar)** into the communicator.

*   Confirm the entry. The transmitter will align its upper range value with this standard value and adjust its output to output an accurate 20mA signal at this point.

**Step 5: Verification and Completion**

*   After completing the two-point calibration, the communicator will usually prompt that the calibration was successful.

*   **A crucial step:** Verify the calibration results. Vary the process pressure throughout the entire range (if possible), or at least check a few points between the lower and upper values. Observe whether the pressure value read on the communicator matches your reference standard gauge.

*   If the verification passes, the calibration is complete. Save the settings and disconnect the communicator.

*   If the verification reveals significant errors at mid-range points, it may indicate sensor non-linearity. In this case, consider sending the transmitter to a laboratory for a comprehensive multi-point calibration.

#### **Conclusion**

The field auto-calibration function of the PR235 pressure transmitter is an extremely powerful tool that significantly reduces instrument maintenance time, lowers costs, and avoids process interruptions caused by instrument removal. However, its effectiveness depends entirely on **stable process conditions** and **accurate reference pressure values**. Correctly understanding and executing the steps above ensures your PR235 transmitter can quickly regain its measurement accuracy and continue to provide reliable data for your process, thereby ensuring safe, stable, and efficient production. Remember, if there are concerns about the sensor's health itself, field auto-calibration is not a panacea; comprehensive laboratory calibration remains the gold standard for guaranteeing the highest accuracy.