Calculating LRV (Lower Range Value) and URV (Upper Range Value) for a Level Transmitter

The LRV and URV are crucial parameters for configuring a level transmitter. They define the range of the level measurement that corresponds to the transmitter's 4mA and 20mA output signal, respectively. Here's how to calculate them:

1. Define Level Span and Transmitter Output Range:

• Level Span (LS): This is the full-scale range of the level that the transmitter will measure. It's typically expressed in meters (ft for imperial units) and can be found in the transmitter's specifications or the process tank dimensions.
• Transmitter Output Range: This is the standard output range for most level transmitters, which is 4mA to 20mA.

2. Identify Zero Point Reference:

• This is the level value that corresponds to the 4mA output (LRV). It depends on the type of level transmitter and its installation:

• For bottom-mounted level transmitters with wet leg (pressure at the transmitter is always higher than atmospheric pressure): The zero point reference is typically the bottom of the tank.
• For top-mounted level transmitters with dry leg (pressure at the transmitter can be lower than atmospheric pressure): The zero point reference can be the bottom of the tank (for negative pressure measurement) or the transmitter mounting flange (for positive pressure measurement).

3. Apply the Formula:

• Here's the formula to calculate LRV and URV:

LRV (mA) = 4mA + [(Level Span (LS) * Zero Point Reference) / Level Span (LS)] * (20mA - 4mA)
URV (mA) = 4mA + Level Span (LS) * (20mA - 4mA) / Level Span (LS)

Example:

• Let's say you have a bottom-mounted level transmitter with a wet leg in a tank with a height (Level Span, LS) of 10 meters. The zero point reference is the bottom of the tank (0 meters).
• LRV calculation:

• LRV (mA) = 4mA + [(10 meters * 0 meters) / 10 meters] * (20mA - 4mA)
• LRV (mA) = 4mA + 0 * (16mA)
• LRV (mA) = 4mA

• URV calculation:

• URV (mA) = 4mA + 10 meters * (20mA - 4mA) / 10 meters
• URV (mA) = 4mA + 16mA
• URV (mA) = 20mA

Therefore, in this example:

• LRV = 4mA (corresponding to the bottom of the tank at 0 meters)
• URV = 20mA (corresponding to the top of the tank at 10 meters)

Additional Considerations:

• Some level transmitters allow you to configure the zero point reference and output range in the transmitter settings. This can be useful for adjusting the LRV and URV to specific needs.
• Always refer to the transmitter's manual for specific instructions on calculating LRV and URV and for configuration procedures.

By following these steps and considering the additional points, you can calculate the LRV and URV for your level transmitter and ensure it operates within the desired measurement range.