Heath Robinson or wot

[joe90]

As some of you will know I like to build stuff rather than buy it, with our MVHR I built my own terminals and manifold/silencer, I had to invent flow adjusters fir the distribution pipes and wanted them at the manifold end rather than room terminals (which can cause noise in the room). Well here is the prototype, works well (but looks crap). But what’s not to like, it will never be seen?

 

10mm dowel, 3mm acrylic sheet, 2” nail.

Edited February 13, 2020 by joe90

[Cpd]

Great stuff, one day I will be following your example.

[MJNewton]

Will be interested to hear how this works out!

 

I was surprised there weren't 'iris' type restrictors available which would be ideal for this sort of thing. (I think I may have found some, but they were very expensive) 

[joe90]

@Jeremy Harris kindly sent me a spare restrictor from his manifold and you break out rings to the desired hole but you can’t put rings back in, and I would have to glue them in. I will report back on whether this was a good idea (or not).

[SteamyTea]

As you are putting in a restriction to an airflow, there will be a pressure difference either side of it.

I wonder if it is enough of a drop to easily measure with a couple of cheap sensors (BMP/BME) on a RPi.

From that, airflow can be calculated.

What do people think?

Edited February 13, 2020 by SteamyTea

[dnb]

I think a solid state differential pressure sensor like a MPX2100DP, or a lower pressure alternative like MPX250DP would be easily good enough to see the pressure difference across the valve. The differential element is nice because only the pressure difference matters to the sensor - you aren't having to calibrate two sensors measuring absolute pressures. You can still drive them easily enough from a microcontroller or Raspberry Pi etc. (I used one once to make a high accuracy tank level sensor).

The maths to calculate airflow is easy enough too. More or less the same as some aftermarket car engine management systems I've played with.

 

I think I too will be following the example when I've got something built above ground.

[Temp]

Not sure if this is of any interest but.  

 

On another forum someone is using a MAF sensor designed for cars to measure air flow in a lab experiment. Perhaps they would work on mvhr if you needed a sensor.

[SteamyTea]

6 hours ago, dnb said:

I think a solid state differential pressure sensor like a MPX2100DP, or a lower pressure alternative like MPX250DP

That seems to be the ticket.  May get a couple to play with.

 

6 hours ago, Temp said:

On another forum someone is using a MAF sensor designed for cars to measure air flow in a lab experiment.

I considered this for another project, bit costly, even secondhand.  I like cheap.

[scottishjohn]

If you like home made and cheap --make yourself a water manometer 

basically a pipe set at 45 degrees with a u shape at bottom  washer piping on a board- it is also self calibrating to actual atmospheric pressure as it is open at top --

any sensor you use will not do that unless you have another one measuring actual baro metric pressure  all the time

used to have a commercial made one on my spray both --cos you needed to adjust input fan to extract fan to give very ,very small pressure difference from outside --so you could not suck in crap or WD40 spray,car polish dust etc-- in the air in the workshop which could contaminate the paint surface--too much and you pollute workshop with paint fumes 

 an altimeter from an  aircraft  would work as well -

 

Edited February 14, 2020 by scottishjohn

[Jeremy Harris]

1 minute ago, scottishjohn said:

If you like home made and cheap --make yourself a water manometer 

-u-shaped washer piping on a board-

used to have a commercial made one on my spray both --cos you needed to adjust input fan to extract fan to give very ,very small pressure difference from outside --so you could not suck in crap or WD40 spray,car polish dust etc-- in the air in the workshop which could contaminate the paint surface

 

 

Very hard to use for this sort of MVHR job, though, as the pressure difference is tiny.  I tried to use a water manometer sloped at 30° (about as much slope as is reasonably practical) with my home made pressure test fan (which works at the very much higher pressure differential of 50 Pa) and even with stripes behind it to try and read the level of the dyed water it was still way to small to be accurate.  The pressure differential across a restrictor is going to be a lot less than the 50 Pa use for an airtightness test, and 50 Pa is only about 5mm of differential height in a water tube manometer.  The pressure across an MVHR restrictor at trickle ventilation rate is probably going to be under about 10 Pa, so less than 1mm of height differential in a U tube manometer.

[scottishjohn]

6 minutes ago, Jeremy Harris said:

 

 

Very hard to use for this sort of MVHR job, though, as the pressure difference is tiny.  I tried to use a water manometer sloped at 30° (about as much slope as is reasonably practical) with my home made pressure test fan (which works at the very much higher pressure differential of 50 Pa) and even with stripes behind it to try and read the level of the dyed water it was still way to small to be accurate.  The pressure differential across a restrictor is going to be a lot less than the 50 Pa use for an airtightness test, and 50 Pa is only about 5mm of differential height in a water tube manometer.  The pressure across an MVHR restrictor at trickle ventilation rate is probably going to be under about 10 Pa, so less than 1mm of height differential in a U tube manometer.

If its just to see how much of a differential before and after flap  -then one either side of flap would show that I suspect --and cost nothing to make 

 and as you say if you use something without correction baro pressure-itwill be hard to compare one day to another

Edited February 14, 2020 by scottishjohn

[scottishjohn]

will it not depend on internal size of the piping on the measuring board as to how far it moves --eg smaller pipe --larger movement for same pressure diff/

[Jeremy Harris]

11 minutes ago, scottishjohn said:

will it not depend on internal size of the piping on the measuring board as to how far it moves --eg smaller pipe --larger movement for same pressure diff/

 

 

Just depends on the pressure across the restrictor, which will be determined by the flow velocity and the cross sectional area of the restrictor.   The flow velocity in the duct needs to be low to keep noise down, and at trickle ventilation rates is likely to be around  1m/s and should not exceed 2.5m/s.   The flow velocity through the restrictor will be higher, but not by the orders of magnitude needed to make a water filled U tube manometer work OK.  The dynamic pressure of air at 1m/s is roughly 0.614Pa, or about 0.063mm of water in a U tube manometer.