Topic: Davis 7747 FARS interior dimensions

[ginahoy]

Hello, this is my first post. I'm looking for a solar powered aspirated radiation shield. Davis Instruments #7747 FARS seems to fit the bill, and in any case, it's the only one I've found that fits my budget.

I intend to install a 3rd party wireless temperature sensor / data logger with integrated bluetooth radio & battery. I'm currently evaluating several options. Obviously size is a big consideration. I would be most grateful if someone could post the approximate clear interior dimensions (diameter x height).

David

[ValentineWeather]

Don't have the dimensions but I can assure you it's large compared to most any other shield and deep too. Check this nice view Scaled Instruments has done. So the sensor board size PCB Temperature Humidity Sensor Mounting Board - 3" x 2.25" x 1/16". But it can go much deeper for a probe.


https://www.scaledinstruments.com/wp-content/uploads/2020/12/24hr_fars_Exploded_Diagram.pdf

[ginahoy]

Thanks for your reply! Although the linked diagram doesn't show dims, it lists the part #'s for individual components. With that I was able to locate the radiation shield plate (7342.236) on the same site. One of the product images shows the interior diameter (4 inches), which is the dimension I was most concerned about. Very helpful.

[CW7491]

The interior dimensions of the sensor chamber for the FARS is as ValentineWeather says: 2.26” in diameter. Definitely not 4”. See the bottom of the linked document.
https://cdn.shopify.com/s/files/1/0515/5992/3873/files/7755_spec.pdf

[CW2274]

I can't stand it, I have to post here. The inside of the 24hr FARS sensor chamber is just about 4 1/8" in diameter. I just pulled out my spare case fan and that's exactly what it's trimmed to to fit inside. Obviously the diagram is wrong as well.

[ValentineWeather]

I can't stand it, I have to post here. The inside of the 24hr FARS sensor chamber is just about 4 1/8" in diameter. I just pulled out my spare case fan and that's exactly what it's trimmed to to fit inside. Obviously the diagram is wrong as well.

 

I've been visiting a little more too.

[CW7491]

I can't stand it, I have to post here. The inside of the 24hr FARS sensor chamber is just about 4 1/8" in diameter. I just pulled out my spare case fan and that's exactly what it's trimmed to to fit inside. Obviously the diagram is wrong as well.

Well I’m glad to have you both back …

The diagram is correct. The fan mount is larger than the actual sensor chamber as it sits above the insulating sleeve and the “three wall” chamber.

But now as I read it again, the 7747 is the daytime FARS, not the 24hr FARS and I can’t speak to that. It may very well be the full 4 1/8”.

[CW2274]

I can't stand it, I have to post here. The inside of the 24hr FARS sensor chamber is just about 4 1/8" in diameter. I just pulled out my spare case fan and that's exactly what it's trimmed to to fit inside. Obviously the diagram is wrong as well.

It may very well be the full 4 1/8”.

It's not "very well may be"... it is!   And I wonder why I don't post here anymore.

[CW7491]

I can't stand it, I have to post here. The inside of the 24hr FARS sensor chamber is just about 4 1/8" in diameter. I just pulled out my spare case fan and that's exactly what it's trimmed to to fit inside. Obviously the diagram is wrong as well.

It may very well be the full 4 1/8”.

It's not "very well may be"... it is!   And I wonder why I don't post here anymore.

Thank you for clearing up my initial confusion on which model FARS was in question and for confirmation of the daytime FARS dimensions. I’m sorry it causes you so much anguish.

That said, the info you put out on the 24hr FARS sensor chamber dimension is wrong and Davis’ diagram of their product is correct.

Sorry ginahoy. Hope your question was answered at least.

[CW2274]

I can't stand it, I have to post here. The inside of the 24hr FARS sensor chamber is just about 4 1/8" in diameter. I just pulled out my spare case fan and that's exactly what it's trimmed to to fit inside. Obviously the diagram is wrong as well.

It may very well be the full 4 1/8”.

It's not "very well may be"... it is!   And I wonder why I don't post here anymore.

That said, the info you put out on the 24hr FARS sensor chamber dimension is wrong and Davis’ diagram of their product is correct.

You're joking, right? You actually think that I'm not capable of measuring the diameter of the sensor chamber of the ISS that I've owned for 15 years, put multiple stock and case fans in, and can dismantle and reassemble blindfolded, right? You even said yourself "But now as I read it again, the 7747 is the daytime FARS, not the 24hr FARS and I can’t speak to that."..and yet you have the unmitigated audacity to sit there and tell me I'm wrong. Unbelievable...

[ginahoy]

Wow, I didn't realize my question would generate such controversy!

I spent some time digging into available documentation and now see there've been several versions of SPARS/FARS shields over the years. The 7755 version posted by ValentineWeather and CW7491 are based on the SPARS welded core (7345.029), which has a 2.26" diameter sensor chamber with notches to accommodate the similarly sized sensor PCB's.

The 7747 kit instructions depict what appears to be a different fan plate without the internal sensor chamber. Instead, the instructions show the sensor gets mounted on a solid plate at the bottom of the stack, so the open baffle plates form the sensor chamber. This is what CW2274 confirmed (baffle plates have an interior diameter of 4 inches). @CW2274, I assume you must have purchased the 7747 kit, right?

[CW2274]

vantage-pro2-24-hr-spars-welded-core/]7345.029[/url]), which has a 2.26" diameter sensor chamber

@CW2274, I assume you must have purchased the 7747 kit, right?

The diameter is not 2.26"....it is just as I stated ~4 1/8". Believe me or don't.

No, I do not have 7747 kit. I have a 24hr VP2 ISS that has been aspirated by a MEASURED AND INSERTED CASE FAN INTO THE SENSOR CHAMBER FOR 6 YEARS. I can't make it anymore clear than that.

[ginahoy]

The diameter is not 2.26"....it is just as I stated ~4 1/8". Believe me or don't.

You may have misunderstood my reply. I only said I confirmed the SPARS welded core (used in the version referenced by ValetineWeather and CW7491), has a 2.26" diameter sensor chamber. The welded core holds the fan on top and its sensor chamber fits down inside the baffle plates, thus the "three walled" chamber.

Based on your posts, I discovered the difference between the 24-hr SPARS and the 7747 modification. I clearly understood (and agree) with what you said about the diameter (although I missed the part that you mounted case fans rather than use the 7747 kit). Thanks again.

[ginahoy]

After reviewing the 7747 instructions, I realized what should have been obvious from the product images -- the solar panel that comes with the kit is intended to replace the cover on the Sensor Integrated Module (SIM) cover. Since I don't own a SIM, I'll need to devise a way to mount the cover at the appropriate angle. And since the 7747 kit is intended to upgrade the standard Pro2 radiation shield, I'll need to buy a passive shield (7710) and mounting bracket. If I decide later that I need 24-hr fan operation, the fan plate includes provisions for two Ni-Mh batteries and charging PCB. (In fact, the top side of the 7747 fan plate appears to be identical to the welded core that comes with the 24-hr SPARS models.)

Due to cost considerations, I ruled out the Davis ISS and receiver. I just need to log temperature data, which I can do with an inexpensive bluetooth temperature data logger. The one I'm considering only cost $17 and its phone app supports csv downloads. The sensor isn't as precise as the SHT31, but I have a highly accurate reference for calibration. My objective is to calculate Cooling Degree Hours and Heating Degree Hours and correlate that with HVAC energy consumption.

[CW2274]

The diameter is not 2.26"....it is just as I stated ~4 1/8". Believe me or don't.

You may have misunderstood my reply. I only said I confirmed the SPARS welded core (used in the version referenced by ValetineWeather and CW7491), has a 2.26" diameter sensor chamber. The welded core holds the fan on top and its sensor chamber fits down inside the baffle plates, thus the "three walled" chamber.

This is the EXACT replacement part for the 24hr VP2 ISS. It DOES NOT have a 2.26" diameter, it is 4 1/8".

I wonder how in the hell I separated airplanes for 30 years...but yet can't read a tape measure.

[CW2274]

Just for fun, I dug out one of my old stock fans and measured (if I am to be believed... ) and guess what the diameter of the outside of the fan housing that slips inside the sensor chamber is??? Drum roll....exactly 4 1/8". Imagine that.

[ginahoy]

This is the EXACT replacement part for the 24hr VP2 ISS. It DOES NOT have a 2.26" diameter, it is 4 1/8".

This site has better pictures of the SPARS Welded Core sensor chamber that I was referring to when I acknowledged that the 7755 SPARS has a 2.26 inch chamber. Looking at the center (top-down) image, the fan collar is obviously a lot larger (4-1/8") than the sensor chamber (2.26"). I never said your setup uses that part, although you seem to think it does ??

[CW2274]

Okay, I believe I see where the confusion is. I thought all that was wanted was the opening of the top of the sensor chamber, that is 4 1/8". What it is down further, I have no idea (I suppose 2.26"). That said, those are removable pieces.

[Phil23]

Interesting & slightly off topic.

Mine is a DFars that I power 27/7 with my own Solar supply.

Comparing the 24H parts with the other...

 [ You are not allowed to view attachments ]
 [ You are not allowed to view attachments ]
 [ You are not allowed to view attachments ]
 [ You are not allowed to view attachments ]

Then take a look at the dust deposit on my Fars.


Gives the impression that the air is mainly drawn in above the first plate below the fan.
have often considered my own home made double sleeve to change this a bit.

[CW7491]

This is the EXACT replacement part for the 24hr VP2 ISS. It DOES NOT have a 2.26" diameter, it is 4 1/8".

This site has better pictures of the SPARS Welded Core sensor chamber that I was referring to when I acknowledged that the 7755 SPARS has a 2.26 inch chamber. Looking at the center (top-down) image, the fan collar is obviously a lot larger (4-1/8") than the sensor chamber (2.26"). I never said your setup uses that part, although you seem to think it does ??

You have it exactly right ginahoy. Good luck with your project.

[ginahoy]

Mine is a DFars that I power 27/7 with my own Solar supply.

Thanks for posting the picture of your radiation shield with 7747 mod.

I'm curious what purpose it serves to have more than one closed bottom plate. I gather from the 7747 instructions that the standard passive radiation shield for the Pro2 has two identical closed plates on the bottom of the stack, and a 3rd closed plate serves as the cap. Seems like a single bottom plate would suffice. The 7747 kit has different cap plates so the original cap gets flipped and moved to the bottom (presumably so the original long screws can be re-used?) and thus the stack ends up with 3 closed plates on the bottom!

Here's why I ask: If the stack needs only a single bottom plate, it would cost less to buy the plates a la carte (1 closed & 2 open plates, versus the 7710 passive shield), although I'd have to find a source for appropriate length screws.

[Phil23]

I'm curious what purpose it serves to have more than one closed bottom plate.
...... a 3rd closed plate serves as the cap.

Often think the same.
On looking at mine when I had it pulled off for cleaning, I do wonder if it would be worth running a few holes in the plate that supports the sensor, which obviously faces down.

[ginahoy]

On looking at mine when I had it pulled off for cleaning, I do wonder if it would be worth running a few holes in the plate that supports the sensor, which obviously faces down.

I noticed the 7755 version (exploded diagram linked in first reply) has an open bottom with a plastic screen. But in that configuration, the sensor is enclosed in the 2.26" sensor chamber. I don't see how a few holes around the sensor could hurt. Since you have a couple of extra bottom plates in the stack, you can always revert to a solid plate if for some reason the mod doesn't work as expected 

[fkapp]

Hi Ginahoy,

I came across this thread and was wondering how the FARS mod worked out?
Did you stay with the daytime FARS or do a phase II upgrade to 24 x 7 FARS?

Thanks
Frank

[ginahoy]

I came across this thread and was wondering how the FARS mod worked out? Did you stay with the daytime FARS or do a phase II upgrade to 24 x 7 FARS?

Hi Frank, the bluetooth sensor fit nicely. Since I didn't have a passive shield to start with, I was going to order the 7710 passive shield but the individual components I needed to complete the stack cost significantly less. Because of the various versions, it took me a while to determine exactly what I needed. Here's a summary of my Scaled Instruments order:
- FARS conversion kit (7747)
- 2 open plates (7342.236)
- 1 closed plate with threaded studs for the bottom (7342.098)
- 1 closed plate with non-threaded studs for the 2nd bottom (7342.099)
- mounting bracket (7353.080)

The 7747 kit came with 1 open ring so my FARS stack has 3 open rings and two closed bottom plates.  The 2nd closed bottom plate protects the sensor from ground reflected radiation. I only needed to find locally (3) long 8-32 stainless screws & washers to attach assembly to mounting bracket. They go through the mounting bracket and FARS stack into the threaded studs on the bottom plate. For the time being, the solar panel that came with the 7747 kit (intended to replace SIM cover) is held in place on top of the bracket with duct tape 

The sensor was previously mounted inside the north facing front porch, but daily highs were always significantly biased on the high side compared to the nearby ASOS station KFHU, especially when the air is calm (KFHU is only 7 miles away, but 380' higher in elevation).  As expected, the FARS temperature readings are more in line with KFHU considering the difference in elevation, so I'm guessing the porch must heat up above ambient. I haven't decided yet if I'll add batteries and charging kit for 24-hr ops.