Is the FXTH87 a gauge/differential or absolute? Since it is inside a tire I assume it is absolute, and therefore measuring close to atmospheric pressure. If I add a barometric sensor as a reference, I should be able to measure 0-1psi and up.

[solvioddsson]

I need to be able to measure close to zero accurately.  Is the FXTH87 good for this?

[reyes]

Hi,

Correct, pressure sensor inside the FXTH87 is an absolute type pressure sensor, however, due to calibration routines and parameters stored in the FXTH87, the pressure range (from 100kPa up to 1500kPa) is selected at production and cannot be changed in the field.

I’m not sure if I understand properly to your question, but, if you are planning to use the local atmospheric pressure as the reference, you should use a Gauge type pressure sensor instead, since we do not have any absolute pressure sensor in our current portfolio capable of measure such low pressure, most of our absolute pressure sensors can start measure pressure at 15 kPa (about 2 psi).

Using Gauge pressure sensor, you will have the local atmospheric pressure as the reference by default and you can start measuring from 0psi (0 psi compared to reference).

If using a Gauge pressure sensor is an option, please let me know the pressure ranges so I can recommend you the best part number option for your application.

Have a great day,
Jose

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[solvioddsson]

Jose

Thanks for your reply.

I need to explain my project a bit.

In Iceland we have a long history of low ground pressure vehicles. ie the

tirepressure is sometimes almost down to zero psi.

http://arctictrucks.is/#!breytingar-toyota-hilux/hilux-at44-6x6

I am in cooperating with Arctictrucks manufacturing a tire pressure system

that regulates the tyrepressure while driving in relation with changing

snow/ground conditions. The sensors I am currently using are MPX4250, 5500

and 5700 differential.

I am now building a new system where I would like to use a wireless sensors

in each tire, that can measure close to zero pressure.

for "normal" cars the pressure would be something like 1-36psi

for larger trucks the pressure could be 5-100psi.

the system is frequently used at high altitude 1000meters or more.

Therefore I assume I need a reference "barometer" since the relative

pressure will change drastically with altitude at such low tyrepressure.

as mentioned earlier, if I use an absolute pressure sensor that will start

at 100 kpa that is about an atmospheric pressure, I could correct the

altitude with the barometric sensor. right?

thanks for your time and thanks in advance.

have a nice day.

br

Solvi

On Mon, Oct 24, 2016 at 8:38 PM, reyes <admin@community.freescale.com>

[reyes]

Hi Solvi,

First of all, I have to say… OH WOW, what a beautiful truck!!

And thanks for the clarification, now I understand your application and the reason of the combination of the barometer with the pressure sensor.

Now, according to your pressure requirements, environment and end application, I do recommend you to use the FXTH87 family since these devices are not only certified as an automotive device, but also because of the wider temperature range and because it already have an embedded radio to make a wireless TPMS (Tire Pressure Monitoring System).

Based on the pressure range, you can use one of the following part numbers of the FXTH87 family:

* FXTH8709xxx which have a measurement pressure range from 100 to 900kPa (about 14.5 to 130 PSI).

* FXTH8715xx which have a measurement pressure range from 100 to 1500kPa (about 14.5 to 217.5 PSI).

And about your question “if I use an absolute pressure sensor that will start at 100 kpa that is about an atmospheric pressure, I could correct the altitude with the barometric sensor, right?” the answer is YES, you can use a barometer so you can measure the altitude and atmospheric pressure and correct it so you can use it as a reference for the TPMS.

I would like to suggest the NXP MPL3115A2 as the barometer for this application. Unfortunately this device is not certified for automotive applications, however, one key feature is that it includes an embedded temperature sensor to compensate the altitude/pressure drifts caused by temperature changes.

I hope this information can be useful.

Have a great day,
Jose

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[solvioddsson]

hi Jose

Thanks for your reply and sorry for my late one...

yes this is most helpful thank you. I have already ordered samples of the

sensors.

do you have any idea if I could get the sensors on PCB test boards?

thanks again and best regards

Solvi

On Tue, Oct 25, 2016 at 2:49 PM, reyes <admin@community.freescale.com>

[reyes]

Hi Solvi,

NXP do not sell any evaluation or breakout board for the FXTH87, however, there are free downloadable schematics and gerber files if you want to manufacture a board for testing purposes, or if you are working with a NXP sales representative or a distributor FAE you can possible order a sample board through them directly.

About the board for the MPL3115A2, things are much simpler, there are different board options for this device:

1. FRDMSTBC-P3115: Here you can get a Freedom compatible board or a Breakout board for the MPL3115A2
2. MPL3115A2 Breakout board: you can download simple free gerber files for this device and manufacture your own board.

Have a great day,
Jose

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