Pressure Transmitter Accuracy Specifications: The Small Print

Background We see “number games” being played with some transmitters’ specifications, where the advertised accuracy number is just part of the truth, i.e., it is just one of the many accuracy components that you should consider. In some cases, these advertisements are confusing and give the wrong impression of the total practical accuracy you will get in your application. Maybe the competition and race for the best accuracy numbers have led to this situation, where some manufacturers make a “limited” accuracy figure and put that on the cover of the brochure and advertise that on their website, while the full specifications are found in the user manual.

Typically, a pressure transmitter’s specifications include several accuracy components that you should take into account when considering the total accuracy. As mentioned, this article will review some popular pressure transmitter’s specifications to give you an idea of the kind of important factors you should consider and be aware of. Also, it will list some typical specification numbers for the different partial accuracy components. This content is by no means trying to put down or depreciate any transmitter.

Because the transmitter accuracy affects the accuracy of your calibration equipment, we do get these accuracy questions from customers. Certainly, the calibrator should be more accurate than the transmitter you calibrate with it, but different people have different opinions on the accuracy ratio between these two. Anyhow, you should be aware of the total uncertainty of the calibration and document that during the calibration.

The selection of your process transmitter’s tolerance should be based on the process requirements, not on the specifications of the transmitter that is installed in that location. Time to dive in. 

Pressure transmitter accuracy components

Reference accuracy 

Often there is a separate, “limited” accuracy statement, typically on the cover of the brochure or on the website. This may be called “reference accuracy” or something similar; it includes only some parts of the accuracy, not all parts. It includes, for example only linearity, hysteresis, and repeatability. This “best-case accuracy” does not include all the practical accuracy components you should consider (mounting position, ambient temperature, etc.). So, do not think that this specification is what you can expect in practice from the transmitter when you install it in your process. This “best-case accuracy” may be, for example, 0.04 percent or even 0.025 percent of range for the most accurate pressure ranges for the most accurate transmitters. 

Different pressure ranges

Often the best (reference) accuracy is valid only for certain pressure ranges, not for all the ranges available. Also, it may vary on the pressure type, i.e. an absolute range may be different than a gauge range. While the best ranges can have, say even a 0.04 percent range accuracy, some other range of that same transmitter model may have, for example, a 0.1 percent accuracy. Accuracy specifications may be doubled or tripled for the different pressure ranges available. So, make sure you know what the accuracy is for the exact pressure ranges/models that you are using.

Re-ranging

HART (smart) transmitters can be re-ranged with a wide ratio. 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.

If the HART transmitter (with a mA output) is re-ranged for a smaller range than the full range, the accuracy typically worsens. So, if you re-range your transmitter to a smaller range than the max range, please make sure you find out how much error that adds to the accuracy.

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. Like most measurement devices, pressure transmitters typically have some kind of temperature coefficient, i.e., there is an accuracy component that depends on the environmental temperature. The temperature dependency often seems to be specified in a pretty difficult-to-understand format. But try to understand it, and ask the supplier if you cannot figure it out. Anyhow, looking at different transmitters, this value may vary from say 0.01 percent of range to even up to 0.5 percent of range. The worst models seem to specify the temperature effect as more than 1 percent of the range. If the temperature in your process varies a lot, you should take this into account.

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. Compare this to a gauge transmitter that measures pressure against the atmospheric pressure or an absolute transmitter that measures pressure against a full vacuum. An ideal differential transmitter would measure only the difference between the inputs, but in practice, the common-mode static line pressure has some effect on the output. If you have both inputs open to atmospheric pressure, the differential pressure is naturally zero. Also, if you have the same pressure (say 50 bar/psi) applied to both inputs, the differential pressure is still zero. In practice, that static pressure has some effect on the transmitter output. So, the output changes a little when the line pressure changes. Typically, the line pressure effect can go from 0.025 percent of range up to 0.4 percent of range, depending on the transmitter model. Commonly, the line pressure mainly changes the zero of the transmitter but does not make a significant change to the span. So, in calibration, you can test this effect by applying the same pressure (a low pressure and a high pressure) to both inputs and see how much the zero changes. Line pressure may also have some effect on the span of the transmitter, which makes it far more difficult to handle and calibrate. It requires a differential pressure standard for the calibration.

Long-term stability

All measurement devices will slowly lose their accuracy over time. Some more, some less. That also goes for pressure transmitters. Some pressure transmitters specify one-year stability; some have a five- or 10-year specification, or even longer. For example, a transmitter that has a reference accuracy of 0.04 percent of range can have a one-year stability of 0.2 percent of range. Some other models have a similar 0.2 percent range level of specification valid for five or even 10 years. The best one I found was as low as 0.01 percent of the range as a one-year stability. Depending on how often you recalibrate your pressure transmitters, you should consider the long-term stability effect, as the transmitter may drift that much before the next recalibration (and possible trim).

Mounting position (orientation) effect

The mounting position typically has some effect on the accuracy of the pressure transmitter. Most pressure transmitters have a specification for the mounting position. Typically, a change in the orientation changes the zero and does not affect the span accuracy. In practice, the orientation of the transmitter does not change during normal usage. The orientation should, however, be considered if you first calibrate the transmitter in a workshop and then install it to the process, or if you remove the transmitter from the process for recalibration. Certainly, if a transmitter has a remote seal, the location of the capillary tubes will have a big effect on zero value. Again, this is something that does not change during normal usage, but it may affect the calibration if the transmitter is removed from its install location.

Vibration effect

Many pressure transmitters have a specification for the effect of vibration. Naturally, this needs to be considered only if the transmitter is installed in a vibrating location. The vibration effect to accuracy is often relatively small and can be, for example, specified as being “less than 0.1 percent of range.”

Power supply effect

A two-wire transmitter needs an external power supply to work. Typically, the power supply is a 24 VDC supply. Transmitters can commonly work on a wide supply voltage range, going even down to 10 VDC. A supply voltage change during the operation can have a small effect on the accuracy of the transmitter. The effect of the power supply voltage is typically small and can be specified as being “smaller than 0.01 percent of span per 1-volt change,” for instance. In practice, if you have a normal good power supply, this is not an issue.

Total accuracy specification

Some transmitters have some kind of “total accuracy” specification that includes several of the common accuracy components. This can include the earlier mentioned “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 is closer to the real accuracy you can expect from a transmitter. As an example, the “total accuracy” specification can be 0.14 percent of the range, while the reference is 0.04 percent. So as soon as you include the temperature and line pressure effects, the reference accuracy is multiplied by a factor of 3 to 4. Another example model offers a 0.075 percent range reference accuracy, and when the temperature effect is included, it increases to 0.2 percent. When static pressure effects are also included, it goes up to 0.3 percent of the range. If the transmitter has this kind of “total” accuracy specification, it helps you to get a more realistic picture of what kind of accuracy you can expect in practice. Even though the “total” accuracy is often still missing, some accuracy components are listed here.

Contamination in usage

When a pressure transmitter is used in a process to measure pressure, there is a big risk of the pressure media or some dirt contaminating the transmitter’s membrane. This kind of contamination can have a huge effect on the transmitter’s accuracy. This is, of course, not something that can be specified, but is anyhow a big risk in normal use, especially if you decide to have a very long recalibration period, such as several years. So, in addition to the transmitter’s long-term drift specification, this should be considered in the risk analysis. If the transmitter gets very dirty and starts to measure significantly incorrectly, you will normally see that in the measurement results. But if it only starts to measure slightly incorrectly, it is difficult to notice in normal usage.

Best-case and worst-case examples

When you add up all the different accuracy specifications listed above, you come to the real total accuracy specification you can expect in practice. Generally, when you combine independent uncertainty components, the common rule is to use the “root sum of the squares” (RSS) method. Just adding all components together as a straight sum would be a worst-case scenario, and statistically, it is not very likely that all components will be in the same direction at the same time. Therefore, this statistical RSS method is used. To get a best-case summary, take all the smallest accuracy components and neglect the ones that may not be relevant. For the worst-case scenario, take all the accuracy components as their max and assume they are all present. After reviewing the specifications for several different transmitters, the smallest combined accuracy I can find takes me down to about 0.15 percent of the range. For most other models it seems that the best case is double that, so about 0.3 percent of the range at best. Many models have bigger best-case accuracy.

Again, looking at the different specifications, it seems that adding these worst-case accuracy specifications brings us somewhere around 1 percent to 1.5 percent of range accuracy with the most accurate transmitters. But this figure can also go higher with some models.

Accuracy in practice 

As mentioned earlier, modern pressure transmitters are very accurate instruments. However, it is good to read the accuracy specifications carefully, including all the different components that affect accuracy. It is easy to miss these and just look at the one accuracy (e.g., “reference accuracy”) that is shown in marketing and other materials. The purpose of this article is to raise your awareness about the different things that affect the total accuracy that you can expect in practice. Of course, the same goes for all measurement equipment, not only for pressure transmitters. It is always good to read all specifications, including all the footnotes with small print.