Adjustable Air Pressure Switch

[ntsqd]

I set up an unused ARB solenoid valve as the unloader on the 4rnnr's Puma compressor OBA system. It vents the line between the compressor and the tank when not energized and is paralleled with the contactor that I use to turn the compressor on and off.
To Mike's suggestion I can see such a solenoid valve holding air pressure up to the set point, where it could either open and vent the supply air (that would seal the tire at that pressure) or it could close the line and seal that tire from the supply.

Man, both of those are getting really complex. May as well go portals and CTIS.

 

[ducktapeguy]

You guys really like to overcomplicate things.

 

[ntsqd]

I haven't even gotten started!

 

[alia176]

You guys really like to overcomplicate things.

I know, right? . This is coming from an automation controls engineer who makes things way complicated, but I get paid to do that!

 

[ntsqd]

I set up an unused ARB solenoid valve as the unloader on the 4rnnr's Puma compressor OBA system. It vents the line between the compressor and the tank when not energized and is paralleled with the contactor that I use to turn the compressor on and off.
To Mike's suggestion I can see such a solenoid valve holding air pressure up to the set point, where it could either open and vent the supply air (that would seal the tire at that pressure) or it could close the line and seal that tire from the supply.

Man, both of those are getting really complex. May as well go portals and CTIS.

I realized while at lunch that either of these options still falls victim to the need to read the pressure in one area of the system while having a supply pressure that, ideally for speed, far exceeds the desired set pressure. Basically you can't simultaneously monitor the tire's air pressure and increase it. Just like charging a battery, your supply pressure (charging voltage) must be greater than the desired set point (battery voltage), and the closer the two pressures (or voltages) are the longer it will take to reach the set-point.
So, if you're determined to automate this process you will need a method to shut off the air supply long enough for the pressure to stabilize and then be measured, before resuming with inflation. I like thinking about things like this, but I really don't see a simple solution. If time were not a critical variable, then it would be a lot easier, but since the goal is to save time it will be complicated.

Ultimately I think figuring out some sort of CTIS would be necessary as this would allow for a slower fill, but not require sitting around waiting for the fill to occur.

< me, bummed that no one got the Princess Bride reference....

 

[ducktapeguy]

I realized while at lunch that either of these options still falls victim to the need to read the pressure in one area of the system while having a supply pressure that, ideally for speed, far exceeds the desired set pressure. Basically you can't simultaneously monitor the tire's air pressure and increase it. Just like charging a battery, your supply pressure (charging voltage) must be greater than the desired set point (battery voltage), and the closer the two pressures (or voltages) are the longer it will take to reach the set-point.
So, if you're determined to automate this process you will need a method to shut off the air supply long enough for the pressure to stabilize and then be measured, before resuming with inflation. I like thinking about things like this, but I really don't see a simple solution. If time were not a critical variable, then it would be a lot easier, but since the goal is to save time it will be complicated.

Ultimately I think figuring out some sort of CTIS would be necessary as this would allow for a slower fill, but not require sitting around waiting for the fill to occur.

< me, bummed that no one got the Princess Bride reference....

That happens no matter which method you use to air up. The offset isn't that big and you just compensate for it with setting the pressure slightly higher. It's just tires, no need for high precision. It also depends on the airflow, the capability of the compressor and restrictions within the system. With smaller compressors that can't put out much cfm, the static reading isn't that much different than the dynamic. But with something like the maxitrac or ARB dual, you'll probably need to dual or quad tire inflation and/or have a buffer tank ready to handle the air output

 

[ducktapeguy]

I didn't read the whole thread so that obviously makes me an expert to reply here .

I love complexity as much as the next guy, but man O man, fellas seriously? One of the reason why anyone, including the OP, doesn't want to kneel down next to his tire for those extra few minutes is because the OBA system is quite possibly underpowered, and/or under delivering the air given their expectations. I'm kneeling at MOST four min per tire or less, depending on how much I aired down to begin with. I can hear the air going into the valve stems on both SUVs.

The 80 has a York belt driven air comp, the 4runner uses Air Zenith elec air comp. The 80 uses both sliders plus a 1.5g air tank. The 4runner only uses both sliders for storage with the plans of installing two alum 1.5g air tanks that were salvaged from a Land Rover.

Might I suggest any or all of the following for increasing inflation performances?
- get a 100% duty cycle compressor with its own electric pancake cooling fan on top of the head(s)
-run large gauge wires directly from the battery to the air comp. Use quality crimps, not from Amazon! The alum crimps open up over time due to heating/cooling cycles.
-if a relay/solenoid is being used, use a 100amp relay that uses ring terminals (not spade) on the high voltage side.
-keep the air comp cool during inflation duties by popping the hood (if located inside the engine bay)
-install air tanks to help out the air compressor. I use sliders as air tanks on both of my SUVs. Some folks use 2gal air tanks under their vehicles as additional storage capacity.
-fyi both of my systems have 80/105 psi fixed pressure switches. This range is suited for operating ARB air lockers w/o the use of a second step down regulator. Been doing this for 20 years.
-i'm using 3/8" air brake DOT hoses to supply air from the air comp to all storage locations. a 1/4" hose is used for tire inflation duties.


Cheers and happy blowing.

Agree with almost everything you said. Make sure the electrical is up to par. This happened to one of my battery clamps recently, I didn't get the picture of it on fire but you can imagine what it looked like. I had been meaning to rewire it for a while, this just motivated me to do it sooner.

The duty cycle depends on your needs. If you have a decent compressor, you probably won't come close to hitting the duty cycle unless you're filling up really large tires, or have to do a large group. I can probably inflate a dozen 33" tires before I come close to hitting to duty cycle of my compressor.

 

[ntsqd]

That happens no matter which method you use to air up. The offset isn't that big and you just compensate for it with setting the pressure slightly higher. It's just tires, no need for high precision. It also depends on the airflow, the capability of the compressor and restrictions within the system. With smaller compressors that can't put out much cfm, the static reading isn't that much different than the dynamic. But with something like the maxitrac or ARB dual, you'll probably need to dual or quad tire inflation and/or have a buffer tank ready to handle the air output

The point that I was trying to make is that when the supply pressure greatly exceeds the desired tire pressure that the filling of the tire will go faster (up to the point of the compressor not being able to supply enough volume to maintain that pressure) than when the supply pressure is just barely above the desired tire pressure. The closer the two pressures are to each other, the longer it will take for them to equalize.

The offset in set pressure that you prescribe is a symptom of this. How much you offset the pressure is a direct measure of how long you're willing to wait.

In my old Xcab shortbed I could tell the difference of 2 psi in the rear tires by how it drove. The fronts not so much, closer to 5+ psi there. A Detroit on pavement will do that for you.

Thru this exchange I have become convinced that the only really viable solution for the OP's use is a CTIS. It is the fastest means to get moving again because you don't really need to stop in the first place. Design and execution of that in an existing, non-portal axle should keep someone out of the pool halls for quite a while.

 

[ducktapeguy]

The point that I was trying to make is that when the supply pressure greatly exceeds the desired tire pressure that the filling of the tire will go faster (up to the point of the compressor not being able to supply enough volume to maintain that pressure) than when the supply pressure is just barely above the desired tire pressure. The closer the two pressures are to each other, the longer it will take for them to equalize.

The offset in set pressure that you prescribe is a symptom of this. How much you offset the pressure is a direct measure of how long you're willing to wait.

In my old Xcab shortbed I could tell the difference of 2 psi in the rear tires by how it drove. The fronts not so much, closer to 5+ psi there. A Detroit on pavement will do that for you.

Thru this exchange I have become convinced that the only really viable solution for the OP's use is a CTIS. It is the fastest means to get moving again because you don't really need to stop in the first place. Design and execution of that in an existing, non-portal axle should keep someone out of the pool halls for quite a while.

All this discussion has been interesting listening to different ideas, but simple solutions exist and have been proven.

Supply pressure will always exceed tire pressure. You don't need those two pressure to be equal nor do you want them to be. With no pressure difference, there is no air flow. So in contrast to your statement, the greater the difference in pressure, the faster the tire will inflate, which is the goal for most people. They don't want to sit around for hours waiting for air pressure to equalize so they can get an exact reading of tire pressure in millipascals, they want to fill up as fast as possible and move on.

The offset is dependent on the output of the compressor, the restriction in the air system, and the flow rate. There are some other minor factors, but for the purpose or airing up tires we can ignore those. At a certain pressure (i.e. around 30-35psi, a typical tire pressure), the output of your compressor is essentially constant, the restrictions in the air lines are also constant, the flow rate of a Schrader valve is constant, so it's safe to say the offset is constant.

CTIS is the only viable solution? There are hundreds of companies making auto shutoff compressors, used by tens of thousands of people all over the world. It's a simple solution to the problem that works. From what you're saying (and maybe I'm misunderstanding), you think the compressor needs to stop, read static pressure, automatically calculate the difference between static pressure and desired pressure, then automatically compensate set the new cutoff point and turn on the compressor again. What I'm saying is that's all unnecessary. If you want a tire pressure of 30psi set your cutoff to 35psi. When the compressor stops, watch the pressure drop while everything equalizes. If it drops to 28psi, then increase the cutoff pressure 2 psi to 37psi and you're done. Next time you fill up keep the cutoff at 37psi, and you'll end up with 30psi in the tires every time.

 

[ntsqd]

Perhaps you should re-read my posts for their content. You've repeated what I've already said.

CTIS isn't the only solution, it is simply the one that best fits my understanding of the OP's goal.

 

[ducktapeguy]

The goal is to have an adjustable air pressure switch so that I could change the cut-off pressure to suite the tires.

Thru this exchange I have become convinced that the only really viable solution for the OP's use is a CTIS.

I guess I just don't follow the logic on how CTIS is the only viable solution when a simple adjustable pressure switch is the goal. You kind of lost me when you suggested using a pressure regulator and timer vs a simple pressure switch.

 

[ntsqd]

As I understand the goal, it isn't a simple pressure switch though that was put forth as a possible option. It is to minimize how long the OP sits on the side of the road or trail airing back up. Given that this happens many times a day, the time consumed by the repetition of this process is considerable. CTIS means little to no sitting on the side of the road or trail. It just has one small problem. Well, not so small. How do you implement it? It gets a LOT easier with Portal Axles, but that itself isn't so easy.....

I wasn't presenting a tried and proven solution, I was brain-storming possible solutions.

The problem with a simple pressure switch is that you can't directly monitor the actual tire pressure while inflating the tire. We've beaten that into the ground. Since you can't directly monitor the pressure in the tires, and the inflation pressure needs to be something greater than the desired tire pressure (we've also beaten that into the ground), how would a simple pressure switch work?
If the inflation pressure is more than the switch's set-point then it will switch off before much of anything happens. So the set-point has to be greater than the inflation pressure. Only that will never work because the system pressure, much less the tire pressure, will never get there.
A pressure limiting blow-off valve (BOV) has the same problem that a switch set below the system pressure has, it will do it's thing just from the inflation pressure and the tires won't get re-inflated.

Were I determined to automate this I would set up the system with a really high inflation pressure with large plumbing (for speed) and a timer that could automatically shut off the inflation pressure, read the actual tire pressure, and then turn the inflation pressure back on. Let it run some set time period and repeat the cycle ad nauseum until the desired tire pressure is achieved. An Arduino or Raspberry Pi could do the logic part, pressure transducers are easy, turning the inflation pressure on and off might get tricky just because you'd want that valve to have a low cv and those tend to be expensive, power hogs, and are large and clunky. Unless you're using an A/C compressor for your OBA, then it could cycle the clutch.

 

[ducktapeguy]

The problem with a simple pressure switch is that you can't directly monitor the actual tire pressure while inflating the tire. We've beaten that into the ground.

Agree on this point. Although my point is you don't have to monitor the actual tire pressure, you can estimate what it will be from the dynamic pressure. The difference between static pressure and dynamic pressure (i.e. when the compressor is on vs off.) will be consistent for a given compressor and air system, so you can account for that offset when you set the target pressure.

Since you can't directly monitor the pressure in the tires, and the inflation pressure needs to be something greater than the desired tire pressure (we've also beaten that into the ground),

Also agree

how would a simple pressure switch work?

The same way a person can sit by their compressor and turn it off when they see it's reached a certain pressure. Set cutoff pressure above target pressure to compensate for the offset. If you want 30 psi, you may need to set it at 35 or 37 psi to account for the drop in pressure after the compressor is shut off.

If the inflation pressure is more than the switch's set-point then it will switch off before much of anything happens.

So the set-point has to be greater than the inflation pressure.

I'm not sure I understand what you mean by inflation pressure. If the tire pressure is already higher than the set-point, then yes the compressor just won't swtich on. But the whole point is that you're always starting with tire pressure below your set point otherwise you wouldn't be inflating your tires in the first place. The only time you might have a problem is if you have a really, really high output compressor and a lot of restriction in your system, then there's enough backpressure to prematurely trigger the switch. That's a sign of a poorly designed system regardless of the type of switching because you're putting a lot of strain on the compressor and limiting it's potential.

Only that will never work because the system pressure, much less the tire pressure, will never get there.

Again I don't understand this point. Why won't system pressure or tire pressure get there? I can say from experience with multiple type of auto-shutoff compressors it works. As I said there are thousands of people using these auto-shutoff compressors so I'm not sure how you can claim it won't work

 

[ntsqd]

I think that I now see the disconnect. You're approaching this from the inflation air supply source starting at zero pressure. I am approaching this from a more typical OBA or stationary air system that is already at some set pressure.

If starting from zero pressure in the supply system then a simple pressure switch will get you to some pressure that may or may not be within the acceptable range of tire pressures. It will work, more or less, but it will not necessarily be consistent or accurate. The reason it won't be is because of the compressor's discharge air temperature. With a stationary system (home, shop, whatever) this isn't too important because the receiver tank moderates it. In the field that temperature will be a lot higher than ambient and it can vary widely depending on the conditions. When that hot air cools down the tire(s) will 'lose' pressure. The air pressure vs. temperature curve is very predictable, what isn't is the ambient conditions and how they will affect the compressor. Placing an aftercooler in the supply system between the switch and the compressor should make it more consistent. The more consistent the air temperature at the switch is, the more consistent the results will be.

It may well be that the OP's usual ambient conditions are consistent enough for this to work. I know that mine vary way too much for it to work for me.

 

[Lo2aY]

I appreciate the discussion and all is good info!

When you turn on the compressor, let's say an ARB compressor. And there is nothing connected to it. It runs for a few seconds (builds up pressure) then the pressure switch kills power. The pressure I think is around 100 PSI (not sure on this number). So, can't we down stream this pressure switch with a variable switch? I do understand that there will be pressure error due to length of line but we can adjust for the error with a regular gauge and verify the actual number?

I'm assuming this ARB pressure switch is sensing the supply side. is there a way to make it sense the tire side?

It's the same thing I was suggesting we do, Just this time replace the ARB switch?

 

[ducktapeguy]

I think that I now see the disconnect. You're approaching this from the inflation air supply source starting at zero pressure. I am approaching this from a more typical OBA or stationary air system that is already at some set pressure.

If starting from zero pressure in the supply system then a simple pressure switch will get you to some pressure that may or may not be within the acceptable range of tire pressures. It will work, more or less, but it will not necessarily be consistent or accurate. The reason it won't be is because of the compressor's discharge air temperature. With a stationary system (home, shop, whatever) this isn't too important because the receiver tank moderates it. In the field that temperature will be a lot higher than ambient and it can vary widely depending on the conditions. When that hot air cools down the tire(s) will 'lose' pressure. The air pressure vs. temperature curve is very predictable, what isn't is the ambient conditions and how they will affect the compressor. Placing an aftercooler in the supply system between the switch and the compressor should make it more consistent. The more consistent the air temperature at the switch is, the more consistent the results will be.

It may well be that the OP's usual ambient conditions are consistent enough for this to work. I know that mine vary way too much for it to work for me.

How often do you measure the air outlet temperature when you're airing up? I've never met anyone that does that and yet somehow they're still able to air up.

I'm also approaching this from the aspect that I know it works, I've been using it for the past year and a half and many other people have successfully used the same method. I'm not sure why you're so positive that it won't work when it's an already tested solution. I'm not disagreeing with any of the theory you've presented, I'm just saying it's already being done so none of those arguments really mean anything in the real world when the end result has already been proven.

When you turn on the compressor, let's say an ARB compressor. And there is nothing connected to it. It runs for a few seconds (builds up pressure) then the pressure switch kills power. The pressure I think is around 100 PSI (not sure on this number).

I don't have an ARB, but it sounds has a closed flow chuck and is relying on a pressure cutoff switch to protect the compressor. Before pressure switches were widely used in these portable compressors, most compressors would use an open flow chuck otherwise the compressors build up pressure very quickly, possibly damaging it. But with this switch as soon as you connect to a tire, the pressure drops, the compressor turns on, and when you're done you disconnect the pressure builds and the compressor turns off automatically.

So, can't we down stream this pressure switch with a variable switch? I do understand that there will be pressure error due to length of line but we can adjust for the error with a regular gauge and verify the actual number?

Yes. See my original post on modifying a Maxi trac with the same thing. The pressure error is due to a variety of factors, but you can compensate for that by setting a higher cutoff pressure.

I'm assuming this ARB pressure switch is sensing the supply side. is there a way to make it sense the tire side?

Not easily. The pressure difference is mainly due to the restriction of the schrader valve. Other than using a TPMS within the tire there's no easy way to get that pressure. But there's really no need to sense the tires side since you can accurately predict the final pressure from the supply side.

It's the same thing I was suggesting we do, Just this time replace the ARB switch?

If it were me I would just leave the ARB switch in place and wire an adjustable pressure switch in series with it. The stock ARB switch will be doing nothing most of the time, but will still act as an overpressure cutoff in case something fails.

 

[ntsqd]

snip......
I'm assuming this ARB pressure switch is sensing the supply side. is there a way to make it sense the tire side?

Not that I know of. This is the fundamental problem with a system that is already pressurized. If it starts out at or less than tire pressure then it will shut off at the setting since both the supply system and the tire are at the same pressure (roughly).

 

[Ramzizaz]

How often do you measure the air outlet temperature when you're airing up? I've never met anyone that does that and yet somehow they're still able to air up.

I'm also approaching this from the aspect that I know it works, I've been using it for the past year and a half and many other people have successfully used the same method. I'm not sure why you're so positive that it won't work when it's an already tested solution. I'm not disagreeing with any of the theory you've presented, I'm just saying it's already being done so none of those arguments really mean anything in the real world when the end result has already been proven.


I don't have an ARB, but it sounds has a closed flow chuck and is relying on a pressure cutoff switch to protect the compressor. Before pressure switches were widely used in these portable compressors, most compressors would use an open flow chuck otherwise the compressors build up pressure very quickly, possibly damaging it. But with this switch as soon as you connect to a tire, the pressure drops, the compressor turns on, and when you're done you disconnect the pressure builds and the compressor turns off automatically.


Yes. See my original post on modifying a Maxi trac with the same thing. The pressure error is due to a variety of factors, but you can compensate for that by setting a higher cutoff pressure.


Not easily. The pressure difference is mainly due to the restriction of the schrader valve. Other than using a TPMS within the tire there's no easy way to get that pressure. But there's really no need to sense the tires side since you can accurately predict the final pressure from the supply side.


If it were me I would just leave the ARB switch in place and wire an adjustable pressure switch in series with it. The stock ARB switch will be doing nothing most of the time, but will still act as an overpressure cutoff in case something fails.

Can you please share an example of an adjustable pressure switch. I assume it would be quicker and more accurate than a pressure regulator.

 

[ntsqd]

Not necessarily. Switches and regulators have different roles. One can be used in place of the other in fairly tightly constrained circumstances, operate outside of those and it'll work, just not well.

This is the adjustable pressure switch that I use on OBA systems: https://www.automationdirect.com/ad...sure_sensors/pressure_switches/mps25-1c-p200d There are likely less expensive options, but I've used these for years at both work and at home and never had one give any trouble. I use the 40-200psi version which has a dead band of 3-30psi. The dead-band tends to track the set-point, so with a 120psi set-point I can roughly expect a dead-band of about 20psi. Set-point repeatability is ±4psi. So while the switch will repeat a pressure setting ±4 psi, if the pressure is already roughly within 20 psi of the set-point it isn't going to do anything. An 8psi tolerance range is fine for system pressure control, but it isn't terribly good for tire pressure setting.

Contrast that with this regulator that I use for ARB & OX air locker control systems that typically operate at 80-90psi: McMaster-Carr - https://www.mcmaster.com/8520T511/ It has an operating range of 7.5psi-123psi with an accuracy of ±2.5% So if one of these were set to regulate tire inflation pressure to 30psi, the actual pressure would range from 28.5psi to 31.5psi. Which still isn't great, but is better than the switch.