Differential pressure transmitters (DPTs) are essential devices in industrial instrumentation. They help us measure the difference in pressure between two points and are widely used in applications like flow, level, and density measurement. But like any other measuring instrument, DPTs also need to be calibrated from time to time to maintain their accuracy.
This article explains the calibration process of Differential Pressure Transmitter in simple words, step by step, and also covers the basic working of DPTs. Whether you are a student, technician, or working professional, this guide will help you understand the process better.
What is Differential Pressure?
Differential pressure (often written as DP or ΔP) is the difference between two pressure readings taken from two points in a system.
For example, if the pressure at point A is 1000 Pa and the pressure at point B is 400 Pa, the differential pressure is:
ΔP = 1000 Pa – 400 Pa = 600 Pa
In many industrial processes, knowing this difference helps us measure things like:
- Flow rate in a pipe
- Liquid level in a tank
- Density or viscosity of a fluid
- Even temperature changes indirectly
So, differential pressure is not just about pressure. It is used in many indirect measurements too. It is very important to understand Calibration Process of Differential Pressure Transmitter.
How is Differential Pressure Created in Flow Measurement?
To measure flow using Differential Pressure (DP), we use two main types of instruments:
- Primary Elements
These create a pressure drop when fluid flows through them. Some common primary elements are:
- Orifice Plates – Simple, cost-effective devices
- Venturi Tubes – More efficient, used for cleaner fluids
- Pitot Tubes – Used for air and gas measurements
These devices are installed in the pipe and are designed in such a way that they create a pressure difference depending on the flow rate.
- Secondary Element – Differential Pressure Transmitter
The transmitter is connected to both the high-pressure and low-pressure sides of the primary element. It senses the difference and converts it into an electrical signal, usually 4-20 mA.
For example:
- At minimum differential pressure (0%), it gives 4 mA
- At maximum differential pressure (100%), it gives 20 mA
Digital communication protocols like HART, Modbus, or Profibus may also be supported.
Tools Required in Calibration Process of Differential Pressure Transmitter
Before you start, gather the following:
- 24V DC Power Supply – To power up the transmitter
- Multimeter – To measure output signal in mA
- HART Calibrator – To communicate with the transmitter
- Pressure Source – e.g. Scandura
Key Terms You Should Know
Before you start calibration process, you should know below terms:
- LRV (Lower Range Value): Output 4 mA = lowest value of measurement
- URV (Upper Range Value): Output 20 mA = highest value of measurement
- Span = URV – LRV
- LRL (Lower Range Limit): The lowest value the transmitter can measure
- URL (Upper Range Limit): The highest value the transmitter can measure
- Calibration Range: The range within which you calibrate (should be within LRL-URL)
- MWP (Maximum Working Pressure): Maximum safe pressure the transmitter can handle
Let’s take an example:
- Calibration Range: 0 to 1000 Pa
- LRV = 0 Pa → 4 mA
- URV = 1000 Pa → 20 mA
- Span = 1000 – 0 = 1000 Pa
Step-by-Step Calibration Process of Differential Pressure Transmitter
Step 1: Record Basic Details
Before doing anything, record:
- Manufacturer name and model
- Calibration range (e.g., 0–1000 Pa)
- Span
- LRL & URL
- Max working pressure
- Type of output: 4–20 mA, HART, etc.
This helps you understand what limits you must stay within while calibrating.
Step 2: Setup the Calibration Circuit
- Connect the 24V DC power supply to the transmitter
- Use a multimeter in series to read the output current
- Connect the pressure source to the high-pressure port of the transmitter
- Leave the low-pressure port open to atmosphere or connect it properly to simulate your desired low reference
If using a 3-valve manifold:
- Keep the equalizing valve closed
- Open both high and low-pressure block valves
Step 3: Apply Pressure and Take Readings
Apply pressure step-by-step as per your calibration span. Usually, we check at five points:
- 0% = LRV
- 25% of span
- 50% of span
- 75% of span
- 100% = URV
=> For each pressure input, record the transmitter’s output in mA
=> First, take readings while increasing pressure
=> Then, take readings again while decreasing pressure
Step 4: Zero Adjustment (LRV)
- Apply 0% pressure (no difference between high and low ports)
- Check output: it should be 4 mA
- If not, adjust using the HART communicator
→ Go to the configuration menu → Trim zero
Step 5: Span Adjustment (URV)
- Apply 100% pressure
- Check output: it should be 20 mA
- If not, adjust span using HART → Go to configuration → Trim span
Note: Sometimes adjusting the span affects the zero again. So…
Step 6: Recheck Zero
After adjusting the span, go back and apply 0% pressure again and recheck zero. If needed, do fine adjustments.
Step 7: Final Validation
- Once zero and span are set correctly, go through the 0% to 100% pressure points again (in both increasing and decreasing order).
- Record the actual mA output at each point
- Compare with expected values
- Calculate the error, if any
- If the error is within acceptable limits, your transmitter is now calibrated!
Example: Calibration Table (0 to 1000 Pa)
Pressure Applied (Pa) | Expected Output (mA) | Actual Output (mA) | Error (mA) |
0 | 4.00 | 4.02 | +0.02 |
250 | 8.00 | 8.05 | +0.05 |
500 | 12.00 | 12.01 | +0.01 |
750 | 16.00 | 16.03 | +0.03 |
1000 | 20.00 | 19.98 | -0.02 |
Tips for Accurate Calibration
- Allow the transmitter to warm up for 5–10 minutes before calibration
- Always bleed air bubbles from pressure lines—bubbles can affect readings
- Use proper fittings and leak-free connections
- Handle pressure sources carefully—do not overshoot the pressure
- Document everything – including environment temperature, pressure conditions, etc.
Summary
Calibration Process of Differential Pressure Transmitter may sound technical, but once you understand the steps and the key terms, it becomes a routine task. Accuracy in such calibration ensures the overall performance of flow, level, and pressure systems in a plant.
Remember, proper calibration improves:
- Process efficiency
- Product quality
- Equipment safety
- Compliance with industry standards
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