I'm designing a probe to provide a stable and continuous stream of exhaust gas sample to an analyser. The probe has to be inserted into the exhaust for about 20–30 cm to keep outside air from entering the probe. The static pressure there is about 1000 Pa above atmosphere. The dynamic pressure is probably way higher. The material of the probe is brass.

The problem here is that the fluctations in the environment inside the exhaust may have significant influence on my sampling. The analyser precisely dilutes the exhaust gas to match the sensor's specs, and environmental changes may alter the dilution ratio, resulting in incorrect readings. The probe has to offer a sample at a pressure as constant as possible.

The pressure inside the exhaust will force exhaust gas into the probe, and with changing engine load, the temperature, static, and dynamic pressure will change, forcing a different amount of exhaust gas into the probe. I want to have that pressure as stable as possible, so that I can correct for it in my calculations.

I have studied the pitot tube and my design is now as illustrated below: probeShape

I have tried to eliminate the influence of dynamic pressure, but I'm not sure if this is the best way to create a sampling of constant pressure.

So tl;dr, consider image above, is there a better way to offer samples of constant pressure, in an environment that has fluctuating static and dynamic pressure, and temperature?


It sounds like you are unable to affect the analyser end of things. My first suggestion would be to adjust the analyser software to undersample the probe.

Since this does not sound possible and without knowing more about your design constraints, I would suggest adding an accumulator. With only 1K pa over Atmos. you should be able to calculate the volume of a small accumulator that could smooth pressure spikes and allow the gas to cool slightly for a better read. I've seen designs in applications similar to what you are describing which have a two port accumulator (more of a settling chamber) where one is intake from the probe front end and the second contains an orifice calculated to provide a set flow for a given pressure range to the end sensor.

  • $\begingroup$ It is indeed an idea to add an accumulator, from what i've found on google, it's kind of like a buffer, but it can only smooth out peaks. It's good for a start though. But if the pressure stays elevated for a longer period, the buffer will become saturated. I could make a pressure actuated valve that keeps the pressure stable(which i believe is what you describe), but i'm affraid the thing will get too big and heavy. This is excellent and inspiring input though, so thanks! $\endgroup$ – Bart Oct 24 '17 at 12:26
  • $\begingroup$ I just realised that i can simply bleed off the surplus pressure with a little spring loaded valve, combined with an accumulator, it should work. This is fairly simple, I was just thinking too difficult.. $\endgroup$ – Bart Oct 24 '17 at 12:54
  • $\begingroup$ Great idea with the bleeder valve. Best of luck on the design! $\endgroup$ – rdautel Oct 24 '17 at 15:20

One solution to your problem I saw was to take the gasses through a water trap, at least that controlled the temperature for them. Did it add extra moisture ? I don't have the answer for that, but it did give them a smooth gas flux into the analyser...

  • $\begingroup$ Thanks for the tip. A water trap is commonly used indeed, but moisture and a high temperature itself are luckily not a problem for my application. The problem is the changing pressure in the exhaust affecting my dilution ratio. If the probe offers a sample at a higher pressure, the dilution ratio will be richer than anticipated for. It's similar to a carburetor. $\endgroup$ – Bart Oct 23 '17 at 15:38

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