Post by thread:Barometer using PIC, ADC0831, MPX5100A sensor

Hello! {Quote hidden}>> I have a Motorola MPX5100A sensor (Temp compensated, calibrated, >> conditioned sensor) that outputs 0.5- 4.5V for 0-15PSI. Since >> atmospheric pressure is around 14.6 PSI, the output voltage from >> this sensor will only sweep a small range for a 29"- 31" pressure >> change (4.2 - 4.7V) >> Can I use a differential serial ADC like the National ADC0831 >> and feed it the output of the pressure sensor and hold the V- >> input at 4.2V? Or is that too tight of a range (4.2 - 4.7V) for >> a full 8 bit count? > > I built a barometer using a 16C84A and the Motorola MPX4115A (which > has > a pressure range of 15-115kPa), and have been very pleased with the > results. However, there are some things you might want to consider: > > - The Motorola MPX family of pressure sensors are not guaranteed to > give valid results outside of their rated pressure range. The > high end of the MPX5100A range is 100kPa, which is slightly below > the average sea level pressure (101.32kPa). I haven't seen any > problems when that close to the rated range, but you might > consider > the MPX4115A, which ranges up to 115kPa (33.96" of mercury at sea > level). I beg to differ here - although the differential (guage) models like the MPX5100D support a differential pressure of 100 kPa, the absolute pressure sensors (MPX5100A) use an internal reference that sits at about 15 kPa. The absolute range measurable with the MPX5100A is thus also 15 - 115 kPa. I am in the process of building an altimeter using a PIC and the MPX5100A sensor, which is very well suited due to the fact that it is fully temperature compensated. Is there any reason for using an external A/D converter instead of using something like the PIC16C71 which has a four channel 8-bit A/D built in (besides the '84 EEPROM vs '71 EPROM)?? Cheers Roland

Roland Andrag wrote: >I beg to differ here - although the differential (guage) models like >the MPX5100D support a differential pressure of 100 kPa, the absolute >pressure sensors (MPX5100A) use an internal reference that sits at >about 15 kPa. The absolute range measurable with the MPX5100A is >thus also 15 - 115 kPa. You would be wise to heed the advice. A Motorola Data-sheet states that the MPX5000 series is only useful for the stated range because the calibrated amplifier hits the rail at the end of the range and no further change in output will be observed. Regardless, of what you think _should_ happen. What is your objection to the MPX4115A? >I am in the process of building an altimeter using a PIC and the >MPX5100A sensor, which is very well suited due to the fact that it is >fully temperature compensated. Is there any reason for using an >external A/D converter instead of using something like the PIC16C71 The external A/D converter is 12-bit; the internal, 8-bit. It's a question of granularity. Do you want your altimeter to register 256 separate Feet (Meters) or 4,096 separate Feet (Meters) of altitude? -- Hank

Roland Andrag <spam_OUTANDRAGTakeThisOuTBUDGET.MECH.WITS.AC.ZA> > I am in the process of building an altimeter using a PIC and the > MPX5100A sensor, which is very well suited due to the fact that it is > fully temperature compensated. Is there any reason for using an > external A/D converter instead of using something like the PIC16C71 > which has a four channel 8-bit A/D built in (besides the '84 EEPROM vs >'71 EPROM)?? As someone who is also using the MPX5100A sensor in Variometer product that I built for Paragliding and Hangliding enthuisiasts, I feel that that I also ought to say my little bit here... :) My vario uses a PIC16C71 with some analog circuitry to condition the signal before it goes to the the AtoD. You will find that without this analog circuitry, the 8 bit A/D of the 16C71 would not provide sufficient resolution and sensitivity. However, if you want to use some higher resolution external A/D's like the MAX195 which is a serial I2C 16 bit A/D, then you can probably do away with any external analog circuitry and still have exceptional accuracy. So I agree with Roland, I would also go for an external A/D, unless cost is a big issue. Another popular technique would of course be to use the technique of converting the analogue signal to a frequency using a voltage to frequency converter and measuring this frequency. This is generally less expensive than external A/D, but with additional problems such as temperature compensation and measuring range problems. Regards Werner -- Werner Terreblanche Tel +27 21 7102251 Fax +27 21 721278 .....wterrebKILLspam@spam@plessey.co.za (work) OR wernerKILLspamaztec.co.za (home)

>I am in the process of building an altimeter using a PIC and the >MPX5100A sensor, which is very well suited due to the fact that it is >fully temperature compensated. Is there any reason for using an >external A/D converter instead of using something like the PIC16C71 >which has a four channel 8-bit A/D built in (besides the '84 EEPROM >vs '71 EPROM)?? it depends on the accuracy you want, 8 bit A/D gives you 256 levels, assuming you want a range of (say) 0-25,000ft this means the resolution is 100ft, and the accuracy is +- 1lsb = 200ft. If this is good enough for your requirements then the internal A/D is fine otherwise you have to do a bit better. Chris Cain > >Cheers > Roland > >

According to Roland Andrag: > > I beg to differ here - although the differential (guage) models like > the MPX5100D support a differential pressure of 100 kPa, the absolute > pressure sensors (MPX5100A) use an internal reference that sits at > about 15 kPa. The absolute range measurable with the MPX5100A is > thus also 15 - 115 kPa. > > I am in the process of building an altimeter using a PIC and the > MPX5100A sensor, which is very well suited due to the fact that it is > fully temperature compensated. Is there any reason for using an > external A/D converter instead of using something like the PIC16C71 > which has a four channel 8-bit A/D built in (besides the '84 EEPROM > vs '71 EPROM)?? > > Cheers > Roland > I also have an altimiter in the pipeline. I chose to go with an external 12bit A/D converter. The quantiums of the 8 bit were > 40 feet (if i remember correctly). I wanted something with less than 2' quantiums. Other than that the issue might be size. The 84 + small serial A/D is less realestate than the 71 -- Les Troyer Sr. Analyst Siemens Power Corp 2101 Horn Rapids Rd. Richland, Wa. 99352-0130 Voice (509) 375-8695 Fax (509) 375-8940 Operator (509) 375-8100 email .....ljtKILLspam.....nfuel.com Ad Hoc, Ad Loc, Quid Pro Quo; So Little Time SO Much To Know. -Jeromy Hillery Dillery Boo, PHD, MS and Q