标签: pressure

About once a month, I check my car tyres, since correct pressure is necessary for good car handling, a smooth ride, and good gas mileage. When I checked my front tyres recently with my Topeak digital gauge (image below), one was at the correct pressure (30 psi), but the other was higher, at about 33 psi. What puzzled me was that they had both been at 30 psi the previous time. I know tyres can lose pressure, but I had never heard of a case where the pressure increased on its own.     This inexpensive digital-pressure gauge is a pleasure to use: It reads up to 160 psi/11 bar (useful for bike tyres and suspensions) to three significant figures; can be switched among psi, Bar, or kg/cm² readings; and handles Presta and Schrader valves -- a big improvement over the old "pencil-type" mechanical tyre-pressure gauges.   I gave this some thought and saw only two possibilities at first: Someone was playing a practical joke on me (very unlikely); or my previous reading for just that one tyre alone was in error (also unlikely, as all tyres were measured twice, and at the same time).   Then I looked at the car and saw that the tyre that read high was in full sunlight, while the other was in the shadow, and a black tyre certainly does heat up from solar radiation. Mystery solved -- or maybe not. I pulled out my custom-made "back of the envelope" pad and did a quick calculation using the ideal gas law :   P × V /T = K or P = K × T / V   ...where P is pressure, V is volume, T is the absolute temperature, and K is a constant, which depends on the amount and type of gas (here, the value is irrelevant).     My hand-made "back of the envelope" pad reminds me that doing quick, rough estimates is often a good first step to understanding the parameters of a problem.   Thus if the pressure I measured was about 10% higher than the original, and the volume was constant, then the temperature of the air in the tyre also should have gone up about 10%. The "cold" ambient temperature was about 77°F (25°C) or about 300K (remember, this is a rough estimate we're doing), so the delta rise would be 30K (30°C), or about 55°F.   Then I worried that perhaps a change in the tyre's volume would affect my estimate, but I realized it was a non-issue for two reasons. First, a car tyre is not an easily expandable balloon; it is a rubber enclosure restrained by steel-wire belts. Therefore, its volume stays fairly constant, especially for modest variations around a nominal value (this is a type of assumption we often use in many simplified models).   Second, even if the tyre did expand slightly due to the increase in internal pressure, that would actually cause a decrease in the resultant pressure -- again, the gas law. (I recall seeing a complex differential equation embodying the relationship among a tyre's construction, pressure, and volume, for more advanced modeling.)   Was solar heating the answer to my mystery? I don’t know, as I have no way of measuring the internal air temperature. I suppose I could do some thermal modeling, or even use an application such as COMSOL Multiphysics for a thermal/mechanical simulation, but it's not worth the effort.   So the question of sunlight heating the tyre and raising the pressure remains a slightly open mystery. My "gut" tells me that a 30°C/55°F rise for a black-rubber tyre in full sunlight is possible, but that's where I have to stop.   Do you think it was solar-heating effect? Can you think of any other causes? Have you ever had a similar "simple" measurement mystery, where you are not sure of the actual cause of the observed effect?

I check my car tyres about once a month just like many people, since correct pressure is necessary for good car handling, a smooth ride, and good gas mileage. When I checked my front tyres recently with my Topeak digital gauge (image below), one was at the correct pressure (30 psi), but the other was higher, at about 33 psi. What puzzled me was that they had both been at 30 psi the previous time. I know tyres can lose pressure, but I had never heard of a case where the pressure increased on its own.     This inexpensive digital-pressure gauge is a pleasure to use: It reads up to 160 psi/11 bar (useful for bike tyres and suspensions) to three significant figures; can be switched among psi, Bar, or kg/cm² readings; and handles Presta and Schrader valves -- a big improvement over the old "pencil-type" mechanical tyre-pressure gauges.   I gave this some thought and saw only two possibilities at first: Someone was playing a practical joke on me (very unlikely); or my previous reading for just that one tyre alone was in error (also unlikely, as all tyres were measured twice, and at the same time).   Then I looked at the car and saw that the tyre that read high was in full sunlight, while the other was in the shadow, and a black tyre certainly does heat up from solar radiation. Mystery solved -- or maybe not. I pulled out my custom-made "back of the envelope" pad and did a quick calculation using the ideal gas law :   P × V /T = K or P = K × T / V   ...where P is pressure, V is volume, T is the absolute temperature, and K is a constant, which depends on the amount and type of gas (here, the value is irrelevant).     My hand-made "back of the envelope" pad reminds me that doing quick, rough estimates is often a good first step to understanding the parameters of a problem.   Thus if the pressure I measured was about 10% higher than the original, and the volume was constant, then the temperature of the air in the tyre also should have gone up about 10%. The "cold" ambient temperature was about 77°F (25°C) or about 300K (remember, this is a rough estimate we're doing), so the delta rise would be 30K (30°C), or about 55°F.   Then I worried that perhaps a change in the tyre's volume would affect my estimate, but I realized it was a non-issue for two reasons. First, a car tyre is not an easily expandable balloon; it is a rubber enclosure restrained by steel-wire belts. Therefore, its volume stays fairly constant, especially for modest variations around a nominal value (this is a type of assumption we often use in many simplified models).   Second, even if the tyre did expand slightly due to the increase in internal pressure, that would actually cause a decrease in the resultant pressure -- again, the gas law. (I recall seeing a complex differential equation embodying the relationship among a tyre's construction, pressure, and volume, for more advanced modeling.)   Was solar heating the answer to my mystery? I don’t know, as I have no way of measuring the internal air temperature. I suppose I could do some thermal modeling, or even use an application such as COMSOL Multiphysics for a thermal/mechanical simulation, but it's not worth the effort.   So the question of sunlight heating the tyre and raising the pressure remains a slightly open mystery. My "gut" tells me that a 30°C/55°F rise for a black-rubber tyre in full sunlight is possible, but that's where I have to stop.   Do you think it was solar-heating effect? Can you think of any other causes? Have you ever had a similar "simple" measurement mystery, where you are not sure of the actual cause of the observed effect?

When it came time to select my senior design project it seemed fitting to do something along the lines of avionics since I had been working as a software engineer at an avionics manufacturer for four years. So I chose to design a rocket avionics card that would monitor flight characteristics, deploy two parachutes at the proper time, and help you find the rocket once it returned to the ground. Once the design was done and made it through the standard review process design by my professor, it was time to build and test the system. Due to the usual issues associated with supply chains I was already well behind schedule by the time the parts arrived and I began assembling the hardware. It basically came down to having 48 hours to assemble the circuit, test the software that I had mostly completed but never tested, and package the system into a payload bay for a high powered rocket launch. Since the launch event was coordinated with a local amateur rocketry club and the next event was well after my project due date, I could not miss the deadline or I would not be graduating that year. Needless to say, the phrase "failure is not an option" was forefront in my mind over that 48 hour period! Once I had the circuit assembled and had the USB communication link between it and my computer, I thought the rest was going to be smooth sailing. Any engineer reading this is probably laughing hysterically at this point since "smooth sailing" prior to an upcoming deadline is a fantasy held only by overly optimistic junior engineers. With only a day left, I began working the bugs out of my code and got the various features of the device working. After a short time I could save data from the pressure sensor and accelerometer to the EEPROM and dump that data over the USB port to the PC and I could manually force the electrical outputs that were designed to ignite the parachute deployment charges to cycle on and off. Everything seemed to be working great so off I went to the university's physics department to use the pressure chamber to test my system. Upon arriving at the pressure chamber I learned that it had broken the day before and it was the only chamber on campus. I now had no way to test if my algorithms to convert the sensor data through the ADC inputs were correct. If they were wrong then the parachute charges would be ignited at the wrong time which would present a serious safety threat to launch attendees. If I could not find a way to test the system then I would not be able to launch it and graduate the following month. So I did what any smart engineer would do at this point; ask an even smarter engineer for their advice. In this specific case, I drove over to my parents' house and consulted with my dad who was designing circuits long before I was even born. We needed to find or create a pressure chamber of some sort that would fit my circuit card and test it from ground level up to at least 20,000 feet ASL (which, in Central Florida, also happens to be 20,000 feet AGL). While we were sitting at the table brainstorming ideas my mom was working on canning (i.e. jarring) tomatoes from the recent harvest of their garden. Tomatoes... Jarring... Glass Jars... Eureka! With only 10 hours left before I had to be at the launch event we began hacking together a pressure chamber using two glass masonry jars, a few valves, and an automotive vacuum pump. One jar was the reservoir and the other held the avionics card. A little bit of physics and math determined the algorithm used to find the pressure needed in the reservoir to equalize with the other jar that represented a specific altitude. Thus, using the pump and valve system, we were able to simulate a rocket's accent and decent by creating a vacuum in the jar with the circuit card and then equalizing it with the normal atmospheric air around it. Using this hacked together pressure chamber I found a few bugs in my algorithm and was able to fix them prior to the launch event. After a successful flight and a few weeks of documentation writing, I presented my design along with the masonry jar pressure chamber to the senior design judges at the university's showcase event. A few of the judges were actual rocket scientists from NASA and they really enjoyed the story behind the pressure chamber. In the end, my project won the Best in Show award in the Computer Engineering category and it was all thanks to a jar full of tomatoes. - Terry Reinert Terry Reinert is a software engineer, karate instructor, and amateur photographer residing in Central Florida.