Choose Pressure Transmitters Wisely

Pressure transmitters are widely applied for a variety of measurement purposes, and they cause a lot of trouble. Zeros shift, lines plug, and readings become erratic.

But fundamentally, there's no reason to have pressure measurement problems. All over the world there are transmitter installations giving consistent, accurate pressure readings in the most difficult applications. The key is to specify and install pressure transmitters wisely.

The heart (and in many cases, the limitation) of a pressure transmitter is the transducer (an overview is in the sidebar, Transducer Technologies, and a more complete treatment can be found in Reference 1). The critical factors used to select a transducer are its accuracy, the system pressure and temperature, and fluid characteristics.

Controls experts such as Bala Liptok have said [Control Design--June '01, p68] inaccuracy is probably a better measure of instrument performance. Engineers must recognize that many accuracy statements may be formulated based on laboratory-like conditions that are much more benign than the dirty and electrically noisy environment seen by a pressure transmitter inside a plant.

The engineer should ask the transmitter manufacturer two questions:

1. Is every transmitter tested to meet the accuracy specification?

2. If only a representative sample of transmitters is tested, what statistical method was used to develop the statement? Statistical criteria include the size of the population, statistical limits of error, and confidence.

Ask about effects of factors such as humidity and vibration on hysteresis, linearity, drift, and repeatability (Table I). Each pressure transmitter has an accuracy envelope determined by base accuracy, ambient and process exposure, and drift.

It is common for manufacturers to combine linearity, hysteresis, and repeatability into the base or nominal accuracy. The primary contributors to error are temperature and, in the case of differential pressure measurements, static pressure.

Examine the pressure application and determine the maximum pressure that the transmitter will see.

Consider the following values:

1. The normal operating pressure range, low and high.

2. The maximum abnormal operating pressure range, low and high.

3. The maximum safe overpressure range (burst or damage limits).

4. Peak and frequency of pressure pulses.

The low limit of pressure must be considered if the transmitter can be damaged by vacuum and could be exposed to vacuum. Users should also pay attention to the hydrotest pressure associated with the line, though this does not tend to be a problem. Also, if the sensor is directly exposed to high temperature, the pressure rating will be limited by that temperature. Exposure to abnormal events (such as water hammer) must also be taken into account.

Once the pressure limits are determined, it is good practice to allow a 20% safety factor. If there is an overpressure or safety device, the pressure rating of the transmitter can be at the trip setting of the overpressure device.

Over-rating a transmitter can negatively impact its range and sensitivity. These two parameters have a direct correlation to the accuracy of the measurement. The installation of a pressure snubber or dampener could alleviate the need for the safety factor, as we'll discuss later.

Select a transmitter with the operating pressures at 50-75% of the calibrated range. This assumes that there are not many expected upsets that could cause large swings in pressure. If there are large swings (widening the range) due to process conditions, installing a second transmitter to handle the additional range should be considered.

Consider both the process temperature range and the ambient temperature range. The process temperature range represents the normal and abnormal temperatures the transducer will be exposed to via the process. The ambient temperature range gives the amount of temperature error to allow for without degrading performance outside the limits set by the application.

Accurate, reliable pressure measurements depend on correct installation. Start with location: A transmitter with a small display 50 ft. in the air next to a ladder becomes an operability issue. If maintenance personnel can't reach the transmitter, there is a maintainability, and possibly a safety, issue.

The manufacturer's installation literature describes how much vertical and horizontal distance is required around the transmitter to ensure it can be placed in and out of service easily.

Determine if and how the instrument will be taken out of service for maintenance and calibration without shutting down the process or injuring personnel with leaks or spills. A root valve and secondary block valve can isolate the instrument from the process. Some manufacturers will furnish this item as part of the transmitter assembly, or a manifold can be furnished as a separate item.

To keep maintenance costs down, look for a transmitter with modular components. For example, modularity would allow one piece of electronic circuitry inside the transmitter to be replaced in lieu of replacing the entire transmitter.