For the internal plumbing system of a rocket which is expected to reach 100 km above the ground, industrial pressure sensors with a 4-20 mA current output and a 0-10 VDC output are available. What would be a more reliable choice?

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    \$\begingroup\$ That's impossible to say, and the reliability of the sensor as is has little to nothing to do with the output being current or voltage. Read the datasheet. Check especially for temperature, pressure, and vibration ratings. If you want to put something on a rocket, contact the manufacturer of these sensors \$\endgroup\$ – Marcus Müller Jun 21 '19 at 21:49
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    \$\begingroup\$ The 4-20mA option has the advantage of being able to detect an open in the sensor connection (the current goes to zero) where the 0-10V output can't distinguish between 0V and an open fault. The two schemes will differ in EMI susceptibility, but which is better depends on conditions. Other than that, all of @MarcusMüller 's comment apply. \$\endgroup\$ – John D Jun 21 '19 at 22:07
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    \$\begingroup\$ Related but not to the question per se :-) : - Ejection charges often fail at altitude due to chemical reactions proceeding differently under low (or here no) pressure. Be CERTAIN that zero pressure and zero-cooling by convection do not compromise your craft. || You are probably not using LOX, but, LOX valves often freeze - usually closed but open is possible. || Probably solid, but, if liquid, be sure that end of burn handling of propellant and oxidiser do not allow unexpected mixtures and re-ignitions which compromise craft. Early Atlas missiles were lost that way. \$\endgroup\$ – Russell McMahon Jun 22 '19 at 0:20
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    \$\begingroup\$ Current loop is fine. Digital bipolar is also fine. Avoid single ended voltage (you know that) but even that can be made fine enough. Beware spurious boundary states (glitches during staging or launch) - "full size" rockets have been lost that way. Digital interconnects with suitable layers of integrity / error checking where possible give you a better chance of bulletproof data integrity. || Ensure cooling does not become an issue. Be aware of max max real world vibrations and g forces. \$\endgroup\$ – Russell McMahon Jun 22 '19 at 0:25

As far as signal integrity, 4-20mA is almost always better. Two reasons why:

  • Noise is usually less of an issue.
  • Circuit discontinuity is instantly detected.

As for its use in high g applications, that is more up to the sensor itself, both electrical and mechanical construction. Note if it is more then just a sounding rocket, i.e. entering LEO, then you need to start considering radiation from the environment as well and it's effect on the electronics.


There are several situations where choice of 4-20mA current is necessary

a) Where the receiver already exists and is 4-20mA
b) Where you need to transmit over a substantial distance of wiring
c) Where there are particularly high levels of EMI

Other than those, you have a fairly free choice. The reliability of the sensor and the control electronics themselves will be similar, especially if made by the same vendor.

Most people find it easier to design around a voltage interface, and you may well use fewer components doing it. In which case you'll least chance of messing it up going the 'easier' route of a voltage interface.

Bear in mind that in electronic design, getting an interface to do what you want when everything else is behaving as you expect is the easy bit. Anticipating what can go wrong, and handling it without causing other things to blow up, that's the hard bit. With a rocket, you usually only get one chance to test it live, so figuring out how to test it adequately before flight ought to consume >90% of your research.

  • \$\begingroup\$ In efforts to develop their equivalent of the Saturn5 moon-launch-rocket, the Soviet bundled 20+ engines into the N1 rocket. The large number of engines required a maze of plumbing to route fuel and oxidizer, with many turbo-pumps to implement the forced-feed rates. Each launch ended with explosions, fortunately later and later after launch, indeed even after stage1 separation, for the 4rth launch. Thus the Soviets retained hope. For 5th launch, they used 130,000 sensors and over a Billion-bits-per-second of downlinked telemetry. After that 5th test also exploded, they gave up. \$\endgroup\$ – analogsystemsrf Jun 22 '19 at 7:17

I assisted very senior engineers in producing 6" BY 6" BY 4" HIGH black telemetry boxes for TRW. In assisting them, I had the opportunity to examine the schematics.

The inputs were:

....high-level analog single-ended,

....low-level analog differential-ended into amplifiers

....and digital switch-closures (logic levels)

MOSFETS (tho sometimes special-doped bipolars) were used as the analog multiplexors.

Analog Digital Converters, using 0.01% red-cased wirewound resistors in R-2R-4R-8R etc DAC structures, using the UA710 single comparator, were the state-of-art and extremely reliable. [I'd written UA711, but was 710]

These small telemetry systems had to, after being conformal coated against fungus, sit in storage at Cape Canaveral for 25 years without failing, then be launched in extreme vibration and shock, and then perform for 25 years in orbit --- all sorts of particles impinging on the circuits to degrade current gain and degrade junction leakage. Glass sealed tantalums were the favored large-value capacitors; glass capacitors used in precision timing.


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