How does Ambient Temperature influence Gas Measurement Uncertainty?

The accuracy of metering instruments, like temperature and pressure transmitters, is affected by fluctuations in ambient temperature. This is known as “the ambient temperature effect”.

In custody transfer metering, this directly impacts the reporting of quantities. In certain cases, this can cause a seemingly compliant metering system to exceed measurement uncertainty requirements.

Despite its potential significance the topic is occasionally overlooked or misinterpreted, so let’s take a deeper dive!

How does this affect measurement accuracy?

The impact of ambient temperature effect on measurement is more pronounced in gas metering systems, which are sensitive to pressure measurement uncertainty (and of course differential pressure too, in many cases).

Here are some key factors that drive ambient temperature effect on overall gas measurement uncertainty:

1.??? The ambient temperature (duh!) – not just the ambient temperature; rather, it's about the variance between the current ambient temperature and the ambient temperature during the instrument’s last calibration. Thus, the effect is more significant in field locations with greater ambient temperature fluctuation, for example, the temperature shift from day to night in the desert.

2.??? The Transmitter –

Some instruments are more sensitive to ambient temperature changes than others. Furthermore, for a given model of transmitter, ambient temperature effect specifications often depend on the instrument ‘range’.

For example, Pressure Transmitter “A” with a 0-500barg range is likely to have a greater ambient temperature effect than the same Pressure Transmitter “A” with a 0-100 barg range.

This is because the ambient temperature effect uncertainty is generally expressed by pressure transmitter manufacturers as a function of Upper Range Limit (URL); higher URLs generate higher ambient temperature effect uncertainties. As a result, ambient temperature effect uncertainties are generally more significant in high-pressure gas metering applications.

?The magnitude of ambient temperature effect on measurement accuracy is defined in the manufacturer’s instrument specifications.

3.??? Transmitter Calibration Schedule –

As mentioned above, the ambient temperature at the time of calibration is a very important factor. Therefore, the time of day (or year) that calibration checks are performed also has an influence. If checks are always performed in the cool of night, then ambient temperature effects on gas measurement uncertainty will be higher during the peak heat of the afternoon.

What can be done to limit this effect?

If you meter gas in a location with high ambient temperature swings, your measurement integrity could be at risk. The below recommendations should help mitigate that risk:

1.??? Transmitter Selection – At the design stage, try to select pressure transmitters that exhibit the lowest ambient temperature effect whilst fulfilling measurement range requirements. It is important to make considerations for other sources of instrument uncertainty however, in general, the ambient temperature effect is the largest source of pressure transmitter measurement uncertainty when a large temperature fluctuation is present.

Making use of a dynamic measurement uncertainty model at the design stage enables engineers to determine ambient temperature uncertainty contribution to overall volume uncertainty for various design cases.

2.??? Calibration Strategy – Attempt to perform calibration checks at a time of average ambient temperature to minimise uncertainty exposure in custody transfer. At the very least, avoid calibrating metering instrumentation during the extremes of the day/season if the geographic location is subject to significant temperature swings.

3.??? Control the temperature – If the above steps have been implemented as far as possible and the ambient temperature effect on the metering station uncertainty is still significant; consider a temperature-controlled instrument enclosure for metering pressure and/or differential pressure transmitters.

However, take care to ensure enclosure temperature control is adequate and stable, or else the effect may be inadvertently worsened.

If you are concerned about your gas metering uncertainty, or would like further information, please get in touch!

The example included above is from a real uncertainty analysis of a high-pressure (~100barg) ultrasonic custody transfer gas metering station, located in the Middle East. Specifics will be unique to each system’s design, operating envelope and performance. However, the general uncertainty trends will apply to many metering systems. This article focuses on gas metering, however the conclusions also apply to differential pressure liquid metering.

Written by Gareth Lake, Lead Flow Measurement Engineer, SOLV Ltd.