The Difference between a BME-280 and a BMP-280      Jed Margolin          6/23/2024

 

The Bosch BMP-280 reads the temperature and the absolute air pressure. The Bosch BME-280 reads the temperature, the absolute air pressure, and the humidity. Some of the times that I have bought a BME-280 on eBay the sellers have sent me the BMP-280. Here is how to tell the difference.

 

BMP-280

 

The BMP-280 is rectangular and the vent hole in the can is on the right side. This part number is OFZKU. The K means it is a BMP-280. This is from the BMP-280 datasheet.

 

 

The Device ID for a BMP-280 is 0x58 which is programmed into the part and which you can read.

 

BME-280

 

The BME-280 is square and the vent hole is on the left side. This part number is 079UP. The U means it is a BME-280. This is from the BME-280 datasheet.

 

 

The Device ID for a BME-280 is 0x60 which is programmed into the part and which you can read.

 

 

There are two main types of breakout boards currently being sold.

 

One has 6 pins, the other has 4 pins. Both support either the BMP-280 and the BME-280.

 

The BMP-280 and the BME-280 support both the SPI interface and the I2C interface but the 4-pin board supports only the I2C interface.

 

The pins on the 4-pin board are:

 

 

The pins on the 6-pin board are:

To use the I2C interface with this board:

 

1.  Connect CSB to VCC;

2.  Connect SDO to GND for I2C slave address 0x76 or connect it to VCC for I2C slave address 0x77.

3.  The SDA pin requires a 4.7K pullup resistor. There might or might not be one on the breakout board so be prepared to put one on your hardware.

 

Either part can come on either board. My I2C interface port has an extra Ground and +3.3V so it can use either one.

 

The BME-280s that I got from AliExpress were the correct parts. Since I only bought them from AliExpress one time that is too small a sample to generalize so be careful.

 

 

Air Pressure v. Barometric Pressure

 

The BME-280 and BMP-280 both read absolute air pressure.

 

The absolute air pressure is a measure of the density of the atmosphere.

 

If you are at sea level the barometric pressure and absolute air pressure are the same.

 

But if you are above sea level they are not. The air pressure goes down as the altitude increases. The barometric pressure reported by weather-people assumes you have dug a hole down to sea level and measured the air pressure down there. Why do they do that? I will present two theories in a little bit.

 

At sea level the nominal atmospheric pressure established by the National Institute of Science and Technology (NIST) is 14.696 psi (you can round it to 14.7 psi) at a temperature of 20 °C (68 °F).

 

The standard atmosphere (atm) is 1,013.25 millibars = 101,325 Pa (Pascal) = 101.325 kPA (kilo Pascal) = 760 mm Hg (Mercury) = 29.9212 inches Hg = 14.696 psi (pounds per square inch).

 

What is a millibar?

 

A millibar is 1/1000th of a bar. A bar is the amount of force it takes to move an object weighing a gram, one centimeter, in one second. (Aren’t you glad you asked?)

 

This Web site gives a good explanation of air pressure (and units): https://www.weather.gov/source/zhu/ZHU_Training_Page/winds/pressure_winds/Pressure.htm

 

This calculates the air pressure at altitude: https://www.sensorsone.com/us-standard-atmosphere-altitude-pressure-calculator/ Select Pressure, Feet, and  mbar. Then enter the altitude in the box for it.

 

As an example, I live in the mountains about 22 miles SE of Reno, Nevada. My house is at an altitude of about 6,000 feet. At an altitude of 6,000 feet AMSL (Above Mean Sea Level) the standard air pressure is 811.99 mbars (make it 812 mbars) which is 812/1013 which is about 80% of what it is at seal level.

 

Reno is at an average altitude of 4,500 feet AMSL so the standard air pressure is 858.96 mbars (make it 859 mbars) which is 859/1013 which is about 85% of what it is at sea level.

 

Denver is at an average altitude of 5,280 feet AMSL (which is why it is called the Mile High City). The standard air pressure is therefore 834.27 mbars (call it 834 mbars) which is 834/1013 which is about 82% of what it is at sea level.

 

Now, why do the weather people report the barometric pressure which assumes they have drilled a hole down to sea level and measured the air pressure down there?

 

Years ago I read an explanation that the computer programs for predicting the weather assume the air pressure is at sea level and would be hopelessly confused if it isn’t. I have my own additional theory, that if the absolute air pressure was reported people could be confused and even alarmed. If someone from San Francisco (which is literally at sea level) were to visit Reno to gamble or ski or do business and hear that the air pressure is 859 mbars (not 1013 mbars as it would be in San Fran) instead of knowing that it is another great normal day in Reno they might think they are in the eye of the biggest hurricane in recorded history. That would be alarming.

 

There is another consequence to the air pressure at altitude being lower than it is a sea level.

 

Because the air pressure is lower so is the oxygen level so the oxygen level at my house is 80% of what it is at sea level, the oxygen level in Reno is 85% of what it is at sea level, and the oxygen level in Denver is 82% of what it is at sea level.

 

There are two consequences to that.

 

1. Someone with COPD or other respiratory problem will find it harder to breathe. You should take that into consideration if you are thinking of moving to a place at a higher altitude than where you are now.

 

2.  Everything that burns fuel will operate at reduced power. That includes car engines that burn gasoline or diesel. In the old days when cars used carburetors, at higher altitudes they combine less pressure with less oxygen in the air, but the same amount of fuel being delivered, and you have a rich mixture that can cause the engine to knock, especially when you give it more throttle to compensate for having less power. The carburetors could be retuned to compensate for that to some extent but you would still have less power. Modern cars with electronic ignition have oxygen sensors so the engine controller automatically maintains the correct mixture. This is actually part of the pollution control system since a rich-burning engine produces more pollution.

 

Other equipment that burns fuel are generators (gas, oil, or propane). At high altitudes they will produce less than their rated power. Some generator manuals will explain that and explain how to calculate the power reduction at altitude. This is especially important if you are buying a whole house generator. That is probably why some people who buy generators complain that it doesn’t produce its rated power.

 

Another piece of equipment is a gas furnace (natural gas or propane). They have to be derated, too. For example, at an altitude of 6,000 feet a gas furnace rated for 60,000 BTU would be able to produce only 48,000 BTUs. That doesn’t necessarily mean that the furnace is less efficient but, as with carburetors, unless the amount of gas is controlled it will run rich and produce more pollution. Unless things have changed in the last several years the manufacturers of domestic gas furnaces do not provide a mixture control. They produce only one or two “high altitude” orifices. This should be a national scandal.

 

Pure electric furnaces do not have this problem. An electric furnace rated for 48,000 BTU will produce 48,000 BTUs regardless of the altitude. BTW, the term “Electric Furnace” means an electric furnace for a mobile home. An electric furnace for a house is called “an Air Handler with an Electric Heating Element”.

 

It seems to me that if you have central A/C you might as well have a Heat Pump. At really cold temperatures you will need some auxiliary heating such as with an electric heating element. But I am seeing heat pumps advertised with auxiliary heating that use gas. I think that is a really bad idea.

 

As long as the compressor is not too noisy the only downside to a heat pump is that the compressor runs more often so it will have a reduced life span.

 

Heat Pumps (and air conditioners) have their own problem with higher altitude. The lower air pressure produces less heat transfer at the heat exchangers which will reduce the heating (and cooling) efficiency. See this article from the Denver Gazette. https://denvergazette.com/news/business/heat-pumps-perform-below-par-at-high-altitude-cold-weather-xcel-nrel-research/article_776f0ddc-2740-11ee-88d9-2f1035a177e5.html

 

But all in all I think heat pumps (with electric heating elements for really cold temperatures) are the wave of the future. As more and more electricity is produced by solar, less will be produced by using fossil fuels, and the effect on climate change will slow down. Hopefully this will happen before it’s too late and there is runaway global warming like took place on Venus maybe as early as 500 million years ago. (The surface temperature on Venus is about 465 degrees Celsius which is more than enough to melt solder which is why Venus landers don’t last very long. The solder on the PCBs melts.)

 

Anything with a heat exchanger, like the heat sinks on a stereo system power amp, will be less efficient so the electronics will run hotter. The failure rate of semiconductors doubles (or more than doubles) with every 10 degree C increase in temperature. See  https://www.electronics-cooling.com/2017/08/10c-increase-temperature-really-reduce-life-electronics-half/

 

This is a good discussion of why that is: https://www.analog.com/en/resources/analog-dialogue/articles/high-temperature-electronic-pose-design-challenges.html

 

Do you remember when TVs with plasma displays were popular, expensive, and a status symbol? The word was that you should not get one if you live at a high altitude. The problem wasn’t the plasma display itself. It was the power supply. Plasma panels are power hungry. The power supply runs hot and needs to be cooled. If you design it to be properly cooled at high altitude it will be overdesigned for use at sea level where most people live. That means either charge even more for the TV or less profit for the manufacturer.

 

How many people live at high altitude? A number of years ago I called the Census Bureau and asked if they had that information. They said no, they don’t. Maybe I could look up the altitude of every county in the U.S. and make my own database. That would not work. I live in Storey County, Nevada, where the altitude of the county goes from about 6,500 feet AMSL in the south to 4,500 feet AMSL in the north. No doubt there are other counties like this.

 

I think the altitude information of the population would be valuable for, as an example, epidemiological studies. How does altitude affect the diseases that people suffer from? The reduced oxygen level would almost certainly increase respiratory problems. On the other hand, although fleas can live at high altitudes they prefer not to, so there are probably fewer flea-borne diseases at high altitudes.

 

What effect, if any, does the altitude (and therefore a person’s oxygen level) have on the efficacy of meds?

 

BTW, if you live at a high altitude, and are healthy, your body will produce more red blood cells to compensate. Does having more red blood cells affect anything else in your body either for good or ill?

 

 

Why I Started Using the BMP-280

 

Then there is the reason I started using the BMP-280 in the first place. I had designed and built a monster subwoofer. I did this because the Storey County Board of Commissioners was planning to pass an ordinance setting the maximum sound levels that wind turbines would be allowed to produce. I thought their levels were way too high so they would benefit from a demonstration. I wrote about it here: http://www.jmargolin.com/newprojects/woof-tester/woof-tester.htm  (Scroll down to the bottom for my subwoofer.)

 

Or you can skip the details. This is Big Fred 2. It goes down to 12.5 Hz and rattles things in my house with only 30 Watts. Did you know that the port for a ported subwoofer can be outside the cabinet? That way it can have a large throat area without taking up a huge amount of space inside the cabinet. The reason you want a large throat area is because at high sound levels with a too-small throat you can hear the air rushing in and out of the port. (That was the problem with Big Fred 1.) This phenomenon is called, appropriately, Port Noise. (You can make up your own joke about Port Noise Complaint.)

 

 

Big Fred 2. The dimensions are 25.06” wide x 40.55” high  x 15.49” deep. The external port is on the right. The speaker is an Alpine SWE-1043.

 

Afterwards I wondered how the altitude (air density) affects speaker systems. I made a board that used a MSP430G2402 to read a BMP-280 and display the temperature and air pressure on an LCD. Then I built a large box, put a subwoofer speaker in it, and using my trusty Harbor Freight Tools air compressor, tried to pump the air pressure up to sea level pressure. I failed completely. The difference in air pressure between 1013 mb at sea level and 812 mb at 6,000 feet was too much for my box to hold.

 

I was able to do it with a 55 gallon drum using 12 clamps to hold the lid shut. I measured the subwoofer speaker’s parameters at different air pressures. I did not find any measurable difference between sea level pressure and the air pressure at 6,000 feet. I was puzzled by that. Since then I found https://sengpielaudio.com/calculator-speedsound.htm . It turns out that the speed of sound (and therefore its wavelength) in a particular material depends only on the particular material (even if it is just air) and the temperature.

 

Too bad. If it did have an effect I had an idea for compensating for it. Instead of a ported cabinet use a passive radiator. https://en.wikipedia.org/wiki/Passive_radiator_(speaker) but use an actual speaker.

 

This is a simple electrical model for an active speaker:

 

 

This was the Plan. Note that because the air pressure didn’t affect the parameters of the subwoofer speaker I haven’t tried it.

 

You don’t drive it with audio. You use an appropriately fast processor to simulate Resistance, Capacitance, and/or Inductance to modify the compliance of what would have to be called a dynamic passive radiator. There are limits to how much you can do but with modern processors you can do a lot. It would look like this:

 

 

R3 is a low value resistor that samples the current to the speaker. The Difference Amp measures the difference and goes to the ADC which is read by the Processor. The Processor determines the voltage to be delivered to the Speaker and sends it to the Power Amp that drives the speaker.

 

Since the Drone Speaker is in the sealed cabinet with the Audio Speaker it picks up the audio signal like a microphone. This is the signal that is operated on. For example:

 

1.  For a simulated resistor:  V = I * R    Iout is I, R is selected in software, V is calculated by the processor and becomes Vout.

 

 

2.  For a simulated capacitor:

 

   so

 

C is selected in software.

 

 

3.  For a simulated inductor

 

    V = L * di/dt

 

L is selected in software.

 

 

It isn’t quite this easy because you don’t want it to oscillate.

 

Note that because the air pressure didn’t affect the parameters of the subwoofer speaker I haven’t done it. But it occurs to me that it might be useful anyway. When you have a subwoofer, the room is part of the subwoofer “cabinet”. This method could be used to tune the subwoofer to more closely match the rest of the cabinet (the room). It would be called Subwoofer with Dynamic Tuning.

 

There are people who will pay $100 for special speaker cables that make no measureable difference but which they say they can hear. (That doesn’t mean they are not hearing a difference, only that it cannot be measured with traditional instruments.) If this works it would have a measureable effect so it might be a successful product.

 

Even without that, all is not lost.

 

Although the air density does not affect the sound, the humidity does. In the 1970s when I was an engineer at WUOM I read an article in the Journal of the AES (Audio Engineering Society) explaining how the humidity affects the absorption of sound and how that affects the acoustic properties of a recording studio. When you build a recording studio you don’t want it to be too reverberant but you don’t want it to be “dead” either. That is why you use acoustic sound panels on the walls like these which can be flat:

 

 

or can have things sticking out to help diffuse the sound.

 

The problem is that changes in humidity can change the sound of the studio.

 

The following chart comes from: https://ntrs.nasa.gov/api/citations/19670007333/downloads/19670007333.pdf

(For a local copy Click Here)

 

Look at how much the absorption changes at the high frequencies (like 12.5 KHz) as the humidity changes from just 40% to 20%.

 

 

You can compensate at least somewhat by using a BME-280 to measure the humidity and use it to control an equalizer. That takes care of direct sound. Reflected sound is what produces reverberation so you would also want to add frequency-dependent reverberation determined by the humidity.

 

While you are at it you can also measure the temperature and compensate for that, too. This might be really good for outdoor concerts. The changes in temperature will change the speed of sound so you can easily measure that and compensate for it.

 

You might think that the problem can be solved by just having a humidifier to tightly control the humidity but if you live in the desert (I live in the high desert in Nevada) you become accustomed to the low humidity. I would be uncomfortable with a high humidity. Right now the humidity in my house is 25%. It gets higher when it rains or snows which is not very often. (Did I mention that I live in the high desert?)

 

Besides, at outdoor concerts that last all day (or for several days) you can’t control the humidity anyway.

 

I have not checked if someone has thought of this already and even gotten a patent for it. If you are going to do something with this you should check it yourself.

 

And if you do file a patent application for it you should list me as one of the inventors and give me some reasonable (not excessive) compensation .   :-)

 

Jed Margolin

Virginia City Highlands

Nevada

6/25/2024

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