Sensible Sensor Installation

Well it is half time, and I have about 20 minutes to write something sensible about sensors for my blog while on vacation so off the top of my head I offer the following rules of thumb.

The best sensitivity from a temperature or pH sensor can generally be achieved by an installation where the tip of the thermowell or electrode is in the center of the pipeline. This is particularly important when there is a high viscosity fluid such as a polymer for temperature control or concentrated sulfuric acid reagent for pH control. For temperature, it is also desirable to maximize the insertion length in the center line to reduce the thermal conduction error from the tip to the flange. The insertion of the thermowell into an elbow affords this opportunity.

The location of thermowell or electrode should be at least 20 pipe diameters downstream of a heat exchanger or static mixer to provide blending of the exiting stream. When there are bubbles, the distance should be increased from 30 to 50 pipe diameters so that the bubbles have time to dissolve, which is important to insure a representative measurement and to avoid damage to the thermowell or probe from bubble impingement and collapse.

To make the response of the temperature measurement faster it is important to minimize the air gap between the sensor sheath and the inside thermowell wall. Sometimes the focus is more on the sensor than the total installation, which leads to erroneous conclusions, a popular one being an RTD is too slow. For a bare element installation the RTD might be a few seconds slower than a thermocouple but this is insignificant when other sources of lags and delays are considered (e.g. resolution limit of control valve). Furthermore for most chemical plants, a bare element raises safety concerns. An RTD with a tight fit will be faster than a thermocouple with a loose fit in a thermowell. Also, the actual response time depends upon the fluid velocity, process conditions, and the cleanliness, thermal conductivity, and size of the thermowell. The more important stuff is overlooked because it is not as easy to pigeon hole like sensor type or mass.

For both temperature and pH a pipeline location and size should provide a sufficient fluid velocity to reduce the fouling rate and increase the heat transfer coefficient for temperature and the mass transfer coefficient for pH. I go for about 5 fps but the probe installation and process conditions should be evaluated for each application because too high of a velocity can cause vibration failure and erosion from solids. For pH, the electrode signal gets noisy and the life expectancy shortened as the velocity is increased.

What impresses me the most is how the response time of a pH electrode can increase by an order of magnitude for low velocities (< 0.5 fps), low pH (< 2 pH), high temperatures (> 50 deg C), and thin films (> 10 mm). Thick or flat glass can aggravate the problem.

Why isn’t there information out there on these important installation effects? The information I have all dates back to Dupont and Monsanto technologists in the 1960s who had the luxury of picking fundamentals to investigate for long term importance. We can only hope manufacturers solve these problems so we get to know the rest of the story.