How to Choose the Process Instruments Range while Buying?

Hello ,

Could any one explain me how to choose the range of instruments while buying the device ( pressure /flow /level…etc)
means if i have max pressure =10 bar , shall i buy a pressure with range 0-10 bar ?
Similar question for level, flow …


Choosing the correct range for any process instrument is crucial as it directly impacts the measurement accuracy, device longevity, and process safety.

How to Choose the Process Instruments?

Here are some general guidelines:

  1. General Rule: The operating range of the device should typically be 1.5 to 2 times the maximum process value to allow for overpressure conditions. This rule, known as the “overrange” or “turndown” capability, helps prevent device failure due to unexpected increases in process conditions.
  2. Safety: Consider the maximum pressure that could possibly occur in the system, not just the normal operating pressure. This maximum pressure might be dictated by safety valves or the maximum pressure rating of the equipment. Ensure that your device can safely handle this maximum pressure.
  3. Accuracy: Instruments are usually most accurate in the middle of their range. If you choose a range that’s too large, the device may not be accurate at the lower end of the scale. On the other hand, if the range is too small, you risk damaging the device due to overpressure.

For example, for a system with a maximum pressure of 10 bar, you might select a pressure transmitter with a range of 0-15 bar or even 0-20 bar, depending on the specific safety and accuracy requirements of your system.

Similar principles apply to level, flow, and other types of measurements:

  • For level measurement, consider the height of the vessel, the specific gravity of the fluid, and any potential for overfilling. If you’re using a differential pressure transmitter for level measurement in a pressurized vessel, remember to account for the maximum vessel pressure as well.
  • For flow measurement, consider the maximum and minimum flow rates that can occur in your process. Be aware that some types of flow meters (like orifice plate meters) have a minimum flow rate below which their accuracy drops off significantly.

Always consider the specific requirements of your process and consult with the instrument manufacturer or a process engineer as necessary to select the best device for your needs.