Operating environment plays a major part in the ongoing operation and accuracy of pressure transmitters. If not properly specified to handle environmental conditions, such as sub-arctic locales and freezing liquids, pressure transmitters can fail prematurely and cause catastrophic failure to equipment.
While pressure transmitter specification sheets provide information regarding sensor performance at ambient conditions and expected readings over a given temperature range,
a deeper understanding of the transmitter construction will help define how the sensor performs in extreme environments.
Pressure transmitters typically use some form of strain gages, mounted to a diaphragm to measure pressure. These gages can be applied using glue, thin film deposition, encapsulated in oil, or a glass firing process. As the diaphragm deflects, the resistance values change. In all cases, the effects of temperature can also change the resistance of this output signal, causing errors in the sensor.
Sensor survivability can be tested in cold weather climates (temperatures below -20°C can cause oil-filled sensors to gel and harden). With ceramic technology, the O-ring between the machined port and diaphragm can harden and become brittle. This compromises the integrity of the sensing element and creates a potential leak path. To prevent the sensor from reaching below its operating temperature range, their location and environment must be altered. Heated boxes or rooms can be used to protect the sensor and prevent the conditions at the sensor from reaching freezing temperatures.
In remote locations, power may not be readily available for instrumentation operating on solar power. Thus, the ability to heat the sensor is limited or not available. For example, some well equipment can experience temperatures as cold as -50° C. In this case, the sensors are often installed directly on outdoor piping to monitor the hydraulic, casing, and tubing pressure from the well head.
Dying of Cold
Process media icing is an indirect result of cold climates. In certain natural gas drilling applications, water can be found in the same pipes as the gas. When the system is shut down and the temperature drops below freezing, the water inside the pipe can freeze and expand, causing an overload on the pressure sensor for a prolonged period of time. The expansion can mirror a pressure spike of 500 PSI (35 bar) to 1,000 PSI (70 bar). For a 100 PSI system, the pressure can increase to 1,500 PSI (100 bar).
For many sensor technologies, this strain on a lower pressure diaphragm will cause a failure of the strain gage or rupture of the diaphragm. To protect against sensor failure, the pressure increase must be sustained by the sensing element for a period of time without affecting the accuracy of the sensor after thawing. Special cavity design as well as special proof pressure capability and calibration are the best ways to guarantee that the sensor will not fail from the media freezing in the cavity.
Compensating for Temperature
In order to compensate for the changes in temperature electrically, pressure transmitter manufacturers test sensors over both pressure and temperature to compensate for the effects of temperature. Because each sensor and strain gage is unique, it is best to test each individual sensor for its specific properties.
Constructed of very thick diaphragms and state-of-the-art silicon strain gages, these Explosionproof Pressure Transmitters offer repeatable results even in the hostile environments of deep well drilling.
The traditional method is to trim (reduce) the raw output signal using resistors to optimize performance over the tested temperature range. The sensor would then use a circuit board assembly that amplifies the millivolt signal to the required output signal (ex. 4-20 mA). Certain pressure transmitters will offer zero and span adjustability. This function is commonly needed for drifted output signals after diaphragm fatigue.
As the cost and size of digital electronics have decreased, digital compensation has increased through the use of an ASIC (application-specific integrated circuit). In cases of low temperature, the ASIC is programmed as the pressure sensor is tested over temperature, with some designs correcting non-linearity or deviations from the ideal output signal. The ASIC temperature can be compensated at the gages, using a temperature sensor, such as a thermistor or at the ASIC itself. The main difference is media temperature. If compensated based on the ASIC temperature, the temperature reading is not as accurate, due to its proximity to the media. In cold climates, the ASIC could be reading close to ambient temperature, whereas the media could be a hot liquid or gas. Measuring the temperature of the gages produces the fastest response and dynamic compensation, optimizing performance.
Filed Under: Industrial automation