Pressure transmitters are widely used in the process industry. Pressure transmitters have different accuracy components, such as the effect of re-ranging, ambient temperature, mounting position, static pressure, long-term drift, vibration, power supply, and more.
Typically, a pressure transmitter’s specifications include several accuracy components that must be considered when considering the total accuracy. The transmitter’s accuracy affects the accuracy of your calibration equipment. The calibrator should be more accurate than the transmitter you calibrate with it. Still, the accuracy ratio between the two have varied opinions.
Pressure Transmitter Accuracy Components
The accuracy components are broken down into several categories. It is critical to look at these components to understand the different things that affect the total accuracy you can expect in practice.
- Reference accuracy
- Different pressure ranges
- Ambient temperature effect
- Static line pressure effect
- Long term stability
- Orientation effect
- Vibration effect
- Total accuracy specification
- Contamination in usage
Reference accuracy is a baseline accuracy specification that does not include process effects that can contribute to transmitter errors, such as temperature and static pressure effects. The reference accuracy is established under reference conditions.
Different Pressure Ranges
The best reference accuracy is often valid only for specific pressure ranges, not all available ranges. Also, it may vary on the pressure type, and an absolute content may differ from a gauge range. It would help if you knew the accuracy of the exact pressure ranges/models you use.
Re-ranging an instrument to set the lower and upper range values responds with the desired sensitivity to changes in input. Often you can re-range a transmitter with a turndown ratio of 100:1 or even more. Accuracy specifications are commonly given to the full range or with a limited turndown ratio.
Ambient Temperature Effect
Most pressure transmitters are used in varying environmental conditions in the processes. Also, the temperature of the pressure media may vary widely during usage. Pressure transmitters, like so many measurement devices, have some temperature coefficient. This means there is an accuracy component that depends on the environmental temperature. If the temperature in your process varies a lot, consider this.
Static Line Pressure Effect
Differential pressure transmitters can be used under static line pressure conditions. This means that both inputs have a certain pressure, and the transmitter is measuring the difference between the two inputs.
Compared to a gauge transmitter measuring pressure against the atmospheric pressure or an absolute transmitter that measures pressure against a complete vacuum.
An ideal differential transmitter would measure only the difference between the inputs. Still, in practice, the common-mode static line pressure affects the output.
Line pressure may also affect the transmitter’s span, making it far more challenging to handle and calibrate. It requires a differential pressure standard for calibration.
Long Term Stability
All measurement devices will slowly lose their accuracy over time. Some pressure transmitters have one-year stability specified, while some have a five or 10-year specification. Depending on how often you re-calibrate your pressure transmitters, you should consider the long-term stability effect, as the transmitter may drift that much before the next calibration.
The mounting position typically has some effect on the accuracy of the pressure transmitter. Most pressure transmitters have a specification for the mounting position. The orientation should be considered if you calibrate the transmitter and then install it into the process or remove it from the process for recalibration.
Many pressure transmitters have a specification for the effect of vibration. This must be considered only if the transmitter is installed in a vibrating location.
Total Accuracy Specification
Some transmitters have some “total accuracy” specification that includes several standard accuracy components. These other components include reference accuracy, the ambient temperature effect, and the static line pressure effect. This kind of total accuracy has a more user-friendly value as it gets closer to the factual accuracy you can expect from a transmitter.
Contamination in Usage
When a pressure transmitter is used to measure pressure, there is a significant risk that the transmitter’s membrane gets contaminated by the pressure media or some dirt. This kind of contamination can substantially affect the transmitter’s accuracy. In addition to the transmitter’s long-term drift specification, this should be considered in the risk analysis.