12 Choosing Sensors for Temperature and Pressure
Mechanical Engineering measurements include quantities like temperature, pressure, stress, strain, force, position, velocity and acceleration, the fundamental quantities we need to know about for Fluid Mechanics, Solids, and Dynamics. These next few chapters provide a look at the background for all of these measurements. Still, most of your measurement decisions will involve choosing sensors from various manufacturers to add to your latest design. Unless you are really up to date, that should start with a survey of the latest technology options on the market. These chapters will address only some fairly traditional options to illustrate the decision processes. For details on specific sensor models you might use, check the Sensors chapter in Measurement Resources.
Some Temperature Sensor Options
Thermocouples and Thermistors or IC sensor like the TMP36 can be used to measure transient temperature response in a thermal system (video 3:47). Thermocouples have very low voltage output so you will need to remind yourself about the details of amplification to select an appropriate gain and configuration for your measurements.
The TMP36 has the advantage of being cheap and simple while introducing you to some of the issues that go along with all IC sensors, including limited temperature range (video 2:50).
Other Options for Temperature Measurement: There are many other ways to measure temperature with different advantages and disadvantages. This overview will provide you with some general guidance. Thermistors are very common in mechanical engineering applications where they provide reasonable accuracy, low cost and a wider temperature range than IC sensors. (video 17:33, slides in pdf)
Practical Examples in Pressure Measurement
Most practical pressure transducers involve a diaphragm or other deformable surface with different pressures on opposite sides.(video 7:04, pdf slides)
This video show some process pressure measurement instruments, illustrating both capacitive and piezoresistive diaphragm transducers. (video 4:52)
They can experience errors with remote sensing — ideally we would locate sensors as close to the flow being sensed as possible. (video 3:59)
A purely mechanical Bourdon tube gauge can have vibration reduced by adding damping with a viscous fluid fill. The mechanical response follows a second order relationship, so the added damping suppresses higher frequency pulsations while allowing the mean pressure signal to be displayed. (video 1:44)
Absolute Pressure for Determining Altitude
Fluid flow and pressure differences always go together and absolute pressure is important for properties and processes. We’re going to look at air pressures in this lab, even if we might measure them in millimetres or inches of water. Pressure in the atmosphere changes with elevation. The BMP180, BMP 280, and BME280 are advertised in their data sheets for measuring absolute atmospheric pressure accurately enough to find elevation. GPS is often much less accurate on elevation than on horizontal positions.
Could you measure elevation accurately enough with the BMP180 (or BME280) to land a drone safely? What measurement strategy or other equipment would you use to ensure a soft touchdown on a playing field? How would you account for the variation of atmospheric pressure over time?
Differential Pressure for Respiratory Technology
Mechanical measurements in medical equipment are extremely important in diagnostics and treatment for respiratory and circulatory flow systems. CPAP (Continuous Positive Airway Pressure) systems relieve sleep apnea for millions of people by providing breathing air at a pressure slightly higher than atmospheric pressure, usually prescribed as a whole number of “centimeters of water”, e.g. 14 cm H20, which seems strange for an airflow system. These units and the corresponding “inches of water” are leftovers from measurement of small air pressure differences using water manometers. You’ll run into them in many practical applications.
(1)
provides an easy conversion between pressure and elevation difference for water, so 14 cm H20 = 998 * 9.806 * 0.14 = 1370 Pa. Respiratory technicians usually use an “electronic manometer” to test pressures in CPAP systems. Google to find some examples of electronic manometers that look a lot like a multimeter. CPAP is an entirely mechanical technology that relies on pressure measurement to control a small blower, and is considered the “gold standard” of treatment for chronic obstructive sleep apnea. The same technology is used to provide full mechanical ventilation in the ICU
Could you measure air pressure well enough to control a CPAP blower with one of the transducers we will test? Describe your equipment configuration and measurement process. How would you account for the variation of atmospheric pressure over time?
Differential Pressure for Flow Systems
Heating and ventilation systems move air around with blowers that create enough pressure difference to drive the flow, usually totalling a few inches of water. There’s a pressure drop in every element of the system including the furnace filters in a typical household forced air system. The dirtier the filters, the higher the pressure drop. The same is true of the intake air filter on an automotive engine, or the face masks we have been wearing for protective filtration during COVID 19.
What are the typical pressure drops across automotive or furnace air filters, or face masks and respirators? Could you detect the difference between clean and dirty filters with one of these transducers? Describe your equipment configuration and measurement process. How would you account for the variation of atmospheric pressure over time?