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Author Topic:   Water Pressure: Calibration

jimh posted 09-07-2005 09:29 AM ET (US)
Previously I mentioned the notion of converting a speedometer into a water pressure gauge. I think it would be a matter of just re-calibrating the dial face to new units. However, the re-calibration would require a source of water at known pressure.
What would be a good technique for constructing a source of water under a known pressure? Could this be done using a column of water as the source?
Larry posted 09-07-2005 09:44 AM ET (US)
How about a tee fitting to a water pressure gauge or even a blood pressure cuff if you need medical grade calibration?
Peter posted 09-07-2005 10:08 AM ET (US)
Water pressure gauges for outboard motors are so inexpensive I'm not sure its worth the trouble.
Jerry Townsend posted 09-07-2005 01:03 PM ET (US)
Jim - Peter is right on - buy a pressure gauge. The reason is that a speedometer is effectively using a pitot tube - where one is, in effect, measuring the static pressure and the total pressure - and the difference (total pressure - static pressure) is the dynamic pressure which is proportional to velocity. --- Jerry/Idaho
frostbite posted 09-07-2005 01:34 PM ET (US)
1/4" clear plastic tubing makes a cheap and accurate manometer. Every foot of elevation produces .434 psi or about 1/2 lb. per foot. The only downfall is you need 20 ft. of height to create 10 psi.
Give it a shot.
jimh posted 09-07-2005 03:26 PM ET (US)
The reason for using the speedo is to find something to fill the large hole in the console. The speedo was not much use as a speedo; perhaps as a water pressure gauge it could have a second chance at usefulness.
Buckda posted 09-07-2005 03:47 PM ET (US)
Jim -
The best use of that hole would be a second guage for the twin E-Tecs you're going to put on that rig, eh!!!
Binkie posted 09-07-2005 05:39 PM ET (US)
A speedo is a usefull tool for adjusting your trim at WOT, or just about any planing speed. Just watch the needle when you trim in or out, and when your reach the highest speed, thats your correct trim angle. Who know how accurate they are in determining actual MPH.
jimh posted 09-07-2005 06:41 PM ET (US)
Binkie--I know you seem to always disagree with me, but in this case, you are going to have to accept my testimony that my speedo was not useful to me. If you want to debate about the usefulness of the speedo, please start a new thread. This thread is about how to generate known water pressures.
Having a known water pressure will be useful for checking the calibration on any water pressure gauge, as well as for re-calibrating the dial face on a speedometer that is going to be used as a water pressure gauge.
where2 posted 09-07-2005 09:18 PM ET (US)
Now that we have a known water pressure, how do we get the gauge apart without resorting to using a dremel or a hack saw?
jimh posted 09-07-2005 09:21 PM ET (US)
Disassembly of the gauge is left as an exercise for the student. Good question, really. I will have to look at the gauge. Usually there is a way to take things apart; after all, they were put together. It will depend on the adhesives used, I guess.
Teak Oil posted 09-07-2005 10:13 PM ET (US)
Jim my speedo is inoperative, I cut the line and used it to feed my hose for my pressure gauge through the rigging tunnel. The speedo is useless, but it looks good there matching my other OMC gauges LOL.
Are you that short on console space that you cant just leave it in there?
an86carrera posted 09-09-2005 08:48 AM ET (US)
I am glad to see you liked my idea and want to put it to use, Go to McMaster.com or Omega.com and there you can buy a gauge (electronic or mechanical) to any scale and acuracy you would like to calibrate the speedo to PSI. This however is so acedemdic and possibly anal to need a calibrated gauge for this purpose. Again why not just use it for a relative pressure reading which will warn you of system blockage or wear.
Thanks for a great forum!
Len
Quote below:
an86carrera posted 06-21-2005 11:45 AM ET (US)
--------------------------------------------------------------------------------
I have chosen use my pitot sp? tube type speedo for my water pressure indicator. I found this gauge useless with a GPS on the boat.
But now I have a very large centrally located sensitive gauge. My Ficht 200 runs from 10 mph to 45mph at WOT. Although I no longer have a psi reading as a reference tool I find this very to be so obviously in my face that I look at it alot and can quickly notice any blockage in the system.
Also, it seems very sensitive and in the year it's been like this I am watching my water pump wear out on the guage... originally I had 47mph at WOT.

bsmotril posted 06-21-2005 11:58 AM ET (US)
--------------------------------------------------------------------------------
Now that is one creative idea. I'll be buying tubing and a T connector this afternoon. BillS
jimh posted 06-21-2005 12:22 PM ET (US)
--------------------------------------------------------------------------------
The notion of using the speedometer gauge as the water pressure gauge also occured to me. It was more for the concept of "what can I put in that big 3-3/8-inch hole," but I do think the larger gauge would have more resolution.
The next step would be to disassemble the speedometer and recalibrate the dial face.

jimh posted 09-09-2005 09:45 AM ET (US)
What is the relationship between the pressure created, the height of the column of water, and the diameter of the column. frostbite, can you elaborate?
an86carrera posted 09-09-2005 10:15 AM ET (US)
There is no relationship. Doesn’t matter if it is ten inches depth in a lake or ten inches in a 1/32 I.D. tube. Saltwater would make a difference though due to it's being a heavier specific gravity.
Bthom posted 09-09-2005 10:19 AM ET (US)
Hi All,
The pressure created by a column of water is the same regardless of it's diameter, and it's an easily referenced standard with which to check the calibration of any pressure gauge.
A more accurate conversion is 27.68 inches = 1 psi
Hope this helps,
Brian
Chuck Tribolet posted 09-09-2005 11:10 AM ET (US)
Typical saltwater is 2.56% more dense than fresh water. Slightly
more in the Red Sea and some other mostly landlocked saltwater
bays.
But I think it would be best to have a real pressure gauge.
I'd really like a little electronic box that would beep at
me if the water pressure was low and the engine had been
running for more than n (n=5?) seconds. No need to wait for
it to get hot.

Chuck
an86carrera posted 09-09-2005 12:21 PM ET (US)
Chuck, those types of controls are available. Check out either of those two sites Omega is very pricey though. They would have temp or pressure sensors that would alarm at a pressure drop or a certain rate of rise or a set temperature.
Len

jimh posted 09-09-2005 01:53 PM ET (US)
After some more coffee this morning and pondering this on the way to work in traffic, I deduced the same result.
This weekend I will look more closely at the speedometer gauge itself to see if it can be easily opened for re-calibration of the dial face.
jimh posted 09-09-2005 01:56 PM ET (US)
Chuck--an interesting device would be to brew up a small microprocessor control system which would monitor all of the parameters from your typical classic two-stroke (RPM, temperature, water pressure, engine hours) via simple voltage sensors. You could add "smart" gauges to the old motors, and the cost would be less than buying a set of analogue gauges (once you got into production).
newt posted 09-09-2005 08:16 PM ET (US)
Man, thats a great idea to use the speedo as a water pressure guage. I wish I had thought of that when I had my Montauk. Why spend $40-$50 for a new guage that may require a new hole when you have a large speedo guage already installed in the dash?
an86carrera posted 09-09-2005 08:58 PM ET (US)
Jim,
Most of the time the guage bezel (metal ring) has been roll formed to hold the glass onto the plastic 'bucket' if you carefully open the roll form from the back side you can take it off, then the glass. Just be careful with a flat screwdriver working slowly around the bezel, make a few trips around rather than trying to open any one spot all the way at once.
To re-close it roll it first on a hard surface then move to the edge of the surface and slowly roll the back side to it's previous (hopefully) position. Good luck it is difficult to get a nice smooth perimeter again.
If you know someone with a lathe you could spin it much like the manufacturer did. Originally it was rolled into that shape with a roller bearing device.
The other possibility is that it was ultrasonically welded, heat staked or glued on if it is a plastic bezel.
Good Luck!
Len
frostbite posted 09-12-2005 11:19 AM ET (US)
Sorry for the late reply. I agree with bthom and carrera in their posts, tube diameter has no effect on the pressure created.The pressure is there, just not much volume.
A tall, thin column of water can be compared a high voltage line of low amperage.
an86carrera posted 09-12-2005 01:54 PM ET (US)
The key here is that we are talking about pounds per 'SQUARE INCH' thus it is area dependent. smaller tube = smaller area so mo difference with the height of water.
Alot of people have trouble with this, believe me I've been designing pressure switches for ten years now.
LHG posted 09-12-2005 03:33 PM ET (US)
Mercury's water pressure gauges come with a very small diameter plastic tubing, even smaller than a cocktail straw.
The speedometer kits come with much larger rubber tubing.
My guess is that small diameter tubing is much better in giving water pressure gauge readings.
The other thing many do not know is that a speedometer has to be re-calibrated at the factory for a lower unit pickup, as Mercury uses. Compared to the standard after market plastic pitot tube, this more efficient engine pickup will give a higher, incorrect, speed reading.
I would check Teleflex's website for technical information on the pressure information for the speedometer and water pressure gauges they make. That may help with a conversion, if it can be done at all
DillonBW posted 09-12-2005 09:47 PM ET (US)
If you are measuring in pound per SQUARE inch, should your tubing not be square? Or does your gauge automatically correct round to square?
Most gauges have a chrome bezel that have small tabs on the 'backside' and these can be delicatly bent out enough to remove the ring/bezel. Then it is just a case of slipping/glueing a new paper face on. This can be marked to whatever you desire, PSI, Kilos per Centimeter.
If and when you do get it working, run it without a marked face, see where the needle lands when running at idle, half throttle and wide open. Then you can make some thing up like, Too Low, Just Right and Too Much!
Be creative!
Si
Jerry Townsend posted 09-12-2005 11:55 PM ET (US)
OK - let's back up a bit and put on our learning caps. The pressure is INDEPENDENT of the area. The area doesn't make any difference in the pressure - just the height. The lb/in^2 comes from taking the height of the column of water (ft) multiplied by the density of water (lb/ft^3) which gives lb/ft^2 or converted to lb/in^2.
In actuality - measuring velocity using a pitot tube requires measuring the difference in the total pressure and the static pressure. In airplanes, where the difference in elevation is large, that technique must be used. For a speedometer on a boat, I would not be surprised if only the total pressure measurement is made - and assuming the static pressure is standard atmospheric pressure - which requires only one tube and is therefore relatively simple.
Larry - correcting a speedometer for a lower unit presssure tap would be a good move as there could be a good foot of elevation difference - though the error should be relatively small.
A smaller diameter tube would be beneficial - it would make the instrument more responsive - faster responding and unfluctuating to some extent. The reasons for this are pretty deep and not necessary in this discussion. The small diameter tube is also stronger and relatively "unflexible". However, the "flexibility" of the tubing is not really important as the pressures we are talking about are small.
Now, a speedometer converts a "pressure" to show velocity according to the equation P = (constant) X (density) X Velocity squared and divided by 2 X 32.174. But the construction of the Bordon tube (guts of the pressure guage) makes a difference - and we don't have any information regarding the characteristics of the Borden tube. For these reasons is why I suggested to Jim that he just install a pressure guage and forget the speedometer conversion.
But Jimh - if you are really bent on doing this conversion - get several feet of clear tagon tubing, connect one end to the pressure guage, rig the other end on a vertical side of the barn. Put water in the tubing and calculate the pressure at the guage by: P, psi = (height of water in the tubing above the guage, ft) X 62.4 / 144 ----- Jerry/Idaho
jimh posted 09-13-2005 12:36 AM ET (US)
By my calculation
PSI = 0.433 POUNDS/FOOT of water column height
To get 10 PSI requires
10 POUNDS X 1-FOOT/0.433 POUNDS = 23 FEET
And thus 20 PSI --> 46 FEET (Better have a tall barn)
Re the Mercury hose diameter difference: this could be as simple as making it easy to identify what hose hooks to what gauge.
frostbite posted 09-13-2005 11:55 AM ET (US)
For people with short barns, if you have a supply of mercury handy, you can use that instead of water. Figure .491 psi per inch of elevation.
Another method to calibrate your pressure gauge would be to attach a Schraeder valve to the end of the gauge line and use a bicycle pump or air compressor with a built-in pressure gauge.
I think that you will peg the needle on your speedometer before the pressure gets very high, probably less than 5 psi, but that is a guess.
Running smaller diameter lines allow you to use thinner wall thickness for the same pressure. Speedometer tubing may be larger to reduce clogging from salt and mineral deposits.
Smallfrye posted 09-21-2005 11:09 AM ET (US)
If your concerned about the use of the hole remaining after removal of this guage, replace it with a Floscan fuel meter. I believe they are the same size or only slightly smaller.
Water pressure is a critical value on some boats( shallow runners) Why would you go through the significant effort for an untested method of recoring this information ?
Whalerider posted 09-21-2005 12:40 PM ET (US)
I like Smallfrye's answer. On most boats a water pressure gauge is about as useless as a speedo. A Flowscan would be very nice indeed. Heck... look for something usefull to fill the hole with....maybe a speedo that interfaces with your GPS....even a clock might be better than water pressure.
Mark

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Author	Topic: Water Pressure: Calibration
jimh	posted 09-07-2005 09:29 AM ET (US) Previously I mentioned the notion of converting a speedometer into a water pressure gauge. I think it would be a matter of just re-calibrating the dial face to new units. However, the re-calibration would require a source of water at known pressure. What would be a good technique for constructing a source of water under a known pressure? Could this be done using a column of water as the source?
Larry	posted 09-07-2005 09:44 AM ET (US) How about a tee fitting to a water pressure gauge or even a blood pressure cuff if you need medical grade calibration?
Peter	posted 09-07-2005 10:08 AM ET (US) Water pressure gauges for outboard motors are so inexpensive I'm not sure its worth the trouble.
Jerry Townsend	posted 09-07-2005 01:03 PM ET (US) Jim - Peter is right on - buy a pressure gauge. The reason is that a speedometer is effectively using a pitot tube - where one is, in effect, measuring the static pressure and the total pressure - and the difference (total pressure - static pressure) is the dynamic pressure which is proportional to velocity. --- Jerry/Idaho
frostbite	posted 09-07-2005 01:34 PM ET (US) 1/4" clear plastic tubing makes a cheap and accurate manometer. Every foot of elevation produces .434 psi or about 1/2 lb. per foot. The only downfall is you need 20 ft. of height to create 10 psi. Give it a shot.
jimh	posted 09-07-2005 03:26 PM ET (US) The reason for using the speedo is to find something to fill the large hole in the console. The speedo was not much use as a speedo; perhaps as a water pressure gauge it could have a second chance at usefulness.
Buckda	posted 09-07-2005 03:47 PM ET (US) Jim - The best use of that hole would be a second guage for the twin E-Tecs you're going to put on that rig, eh!!!
Binkie	posted 09-07-2005 05:39 PM ET (US) A speedo is a usefull tool for adjusting your trim at WOT, or just about any planing speed. Just watch the needle when you trim in or out, and when your reach the highest speed, thats your correct trim angle. Who know how accurate they are in determining actual MPH.
jimh	posted 09-07-2005 06:41 PM ET (US) Binkie--I know you seem to always disagree with me, but in this case, you are going to have to accept my testimony that my speedo was not useful to me. If you want to debate about the usefulness of the speedo, please start a new thread. This thread is about how to generate known water pressures. Having a known water pressure will be useful for checking the calibration on any water pressure gauge, as well as for re-calibrating the dial face on a speedometer that is going to be used as a water pressure gauge.
where2	posted 09-07-2005 09:18 PM ET (US) Now that we have a known water pressure, how do we get the gauge apart without resorting to using a dremel or a hack saw?
jimh	posted 09-07-2005 09:21 PM ET (US) Disassembly of the gauge is left as an exercise for the student. Good question, really. I will have to look at the gauge. Usually there is a way to take things apart; after all, they were put together. It will depend on the adhesives used, I guess.
Teak Oil	posted 09-07-2005 10:13 PM ET (US) Jim my speedo is inoperative, I cut the line and used it to feed my hose for my pressure gauge through the rigging tunnel. The speedo is useless, but it looks good there matching my other OMC gauges LOL. Are you that short on console space that you cant just leave it in there?
an86carrera	posted 09-09-2005 08:48 AM ET (US) I am glad to see you liked my idea and want to put it to use, Go to McMaster.com or Omega.com and there you can buy a gauge (electronic or mechanical) to any scale and acuracy you would like to calibrate the speedo to PSI. This however is so acedemdic and possibly anal to need a calibrated gauge for this purpose. Again why not just use it for a relative pressure reading which will warn you of system blockage or wear. Thanks for a great forum! Len Quote below: an86carrera posted 06-21-2005 11:45 AM ET (US) -------------------------------------------------------------------------------- I have chosen use my pitot sp? tube type speedo for my water pressure indicator. I found this gauge useless with a GPS on the boat. But now I have a very large centrally located sensitive gauge. My Ficht 200 runs from 10 mph to 45mph at WOT. Although I no longer have a psi reading as a reference tool I find this very to be so obviously in my face that I look at it alot and can quickly notice any blockage in the system. Also, it seems very sensitive and in the year it's been like this I am watching my water pump wear out on the guage... originally I had 47mph at WOT. bsmotril posted 06-21-2005 11:58 AM ET (US) -------------------------------------------------------------------------------- Now that is one creative idea. I'll be buying tubing and a T connector this afternoon. BillS jimh posted 06-21-2005 12:22 PM ET (US) -------------------------------------------------------------------------------- The notion of using the speedometer gauge as the water pressure gauge also occured to me. It was more for the concept of "what can I put in that big 3-3/8-inch hole," but I do think the larger gauge would have more resolution. The next step would be to disassemble the speedometer and recalibrate the dial face.
jimh	posted 09-09-2005 09:45 AM ET (US) What is the relationship between the pressure created, the height of the column of water, and the diameter of the column. frostbite, can you elaborate?
an86carrera	posted 09-09-2005 10:15 AM ET (US) There is no relationship. Doesn’t matter if it is ten inches depth in a lake or ten inches in a 1/32 I.D. tube. Saltwater would make a difference though due to it's being a heavier specific gravity.
Bthom	posted 09-09-2005 10:19 AM ET (US) Hi All, The pressure created by a column of water is the same regardless of it's diameter, and it's an easily referenced standard with which to check the calibration of any pressure gauge. A more accurate conversion is 27.68 inches = 1 psi Hope this helps, Brian
Chuck Tribolet	posted 09-09-2005 11:10 AM ET (US) Typical saltwater is 2.56% more dense than fresh water. Slightly more in the Red Sea and some other mostly landlocked saltwater bays. But I think it would be best to have a real pressure gauge. I'd really like a little electronic box that would beep at me if the water pressure was low and the engine had been running for more than n (n=5?) seconds. No need to wait for it to get hot. Chuck
an86carrera	posted 09-09-2005 12:21 PM ET (US) Chuck, those types of controls are available. Check out either of those two sites Omega is very pricey though. They would have temp or pressure sensors that would alarm at a pressure drop or a certain rate of rise or a set temperature. Len
jimh	posted 09-09-2005 01:53 PM ET (US) After some more coffee this morning and pondering this on the way to work in traffic, I deduced the same result. This weekend I will look more closely at the speedometer gauge itself to see if it can be easily opened for re-calibration of the dial face.
jimh	posted 09-09-2005 01:56 PM ET (US) Chuck--an interesting device would be to brew up a small microprocessor control system which would monitor all of the parameters from your typical classic two-stroke (RPM, temperature, water pressure, engine hours) via simple voltage sensors. You could add "smart" gauges to the old motors, and the cost would be less than buying a set of analogue gauges (once you got into production).
newt	posted 09-09-2005 08:16 PM ET (US) Man, thats a great idea to use the speedo as a water pressure guage. I wish I had thought of that when I had my Montauk. Why spend $40-$50 for a new guage that may require a new hole when you have a large speedo guage already installed in the dash?
an86carrera	posted 09-09-2005 08:58 PM ET (US) Jim, Most of the time the guage bezel (metal ring) has been roll formed to hold the glass onto the plastic 'bucket' if you carefully open the roll form from the back side you can take it off, then the glass. Just be careful with a flat screwdriver working slowly around the bezel, make a few trips around rather than trying to open any one spot all the way at once. To re-close it roll it first on a hard surface then move to the edge of the surface and slowly roll the back side to it's previous (hopefully) position. Good luck it is difficult to get a nice smooth perimeter again. If you know someone with a lathe you could spin it much like the manufacturer did. Originally it was rolled into that shape with a roller bearing device. The other possibility is that it was ultrasonically welded, heat staked or glued on if it is a plastic bezel. Good Luck! Len
frostbite	posted 09-12-2005 11:19 AM ET (US) Sorry for the late reply. I agree with bthom and carrera in their posts, tube diameter has no effect on the pressure created.The pressure is there, just not much volume. A tall, thin column of water can be compared a high voltage line of low amperage.
an86carrera	posted 09-12-2005 01:54 PM ET (US) The key here is that we are talking about pounds per 'SQUARE INCH' thus it is area dependent. smaller tube = smaller area so mo difference with the height of water. Alot of people have trouble with this, believe me I've been designing pressure switches for ten years now.
LHG	posted 09-12-2005 03:33 PM ET (US) Mercury's water pressure gauges come with a very small diameter plastic tubing, even smaller than a cocktail straw. The speedometer kits come with much larger rubber tubing. My guess is that small diameter tubing is much better in giving water pressure gauge readings. The other thing many do not know is that a speedometer has to be re-calibrated at the factory for a lower unit pickup, as Mercury uses. Compared to the standard after market plastic pitot tube, this more efficient engine pickup will give a higher, incorrect, speed reading. I would check Teleflex's website for technical information on the pressure information for the speedometer and water pressure gauges they make. That may help with a conversion, if it can be done at all
DillonBW	posted 09-12-2005 09:47 PM ET (US) If you are measuring in pound per SQUARE inch, should your tubing not be square? Or does your gauge automatically correct round to square? Most gauges have a chrome bezel that have small tabs on the 'backside' and these can be delicatly bent out enough to remove the ring/bezel. Then it is just a case of slipping/glueing a new paper face on. This can be marked to whatever you desire, PSI, Kilos per Centimeter. If and when you do get it working, run it without a marked face, see where the needle lands when running at idle, half throttle and wide open. Then you can make some thing up like, Too Low, Just Right and Too Much! Be creative! Si
Jerry Townsend	posted 09-12-2005 11:55 PM ET (US) OK - let's back up a bit and put on our learning caps. The pressure is INDEPENDENT of the area. The area doesn't make any difference in the pressure - just the height. The lb/in^2 comes from taking the height of the column of water (ft) multiplied by the density of water (lb/ft^3) which gives lb/ft^2 or converted to lb/in^2. In actuality - measuring velocity using a pitot tube requires measuring the difference in the total pressure and the static pressure. In airplanes, where the difference in elevation is large, that technique must be used. For a speedometer on a boat, I would not be surprised if only the total pressure measurement is made - and assuming the static pressure is standard atmospheric pressure - which requires only one tube and is therefore relatively simple. Larry - correcting a speedometer for a lower unit presssure tap would be a good move as there could be a good foot of elevation difference - though the error should be relatively small. A smaller diameter tube would be beneficial - it would make the instrument more responsive - faster responding and unfluctuating to some extent. The reasons for this are pretty deep and not necessary in this discussion. The small diameter tube is also stronger and relatively "unflexible". However, the "flexibility" of the tubing is not really important as the pressures we are talking about are small. Now, a speedometer converts a "pressure" to show velocity according to the equation P = (constant) X (density) X Velocity squared and divided by 2 X 32.174. But the construction of the Bordon tube (guts of the pressure guage) makes a difference - and we don't have any information regarding the characteristics of the Borden tube. For these reasons is why I suggested to Jim that he just install a pressure guage and forget the speedometer conversion. But Jimh - if you are really bent on doing this conversion - get several feet of clear tagon tubing, connect one end to the pressure guage, rig the other end on a vertical side of the barn. Put water in the tubing and calculate the pressure at the guage by: P, psi = (height of water in the tubing above the guage, ft) X 62.4 / 144 ----- Jerry/Idaho
jimh	posted 09-13-2005 12:36 AM ET (US) By my calculation PSI = 0.433 POUNDS/FOOT of water column height To get 10 PSI requires 10 POUNDS X 1-FOOT/0.433 POUNDS = 23 FEET And thus 20 PSI --> 46 FEET (Better have a tall barn) Re the Mercury hose diameter difference: this could be as simple as making it easy to identify what hose hooks to what gauge.
frostbite	posted 09-13-2005 11:55 AM ET (US) For people with short barns, if you have a supply of mercury handy, you can use that instead of water. Figure .491 psi per inch of elevation. Another method to calibrate your pressure gauge would be to attach a Schraeder valve to the end of the gauge line and use a bicycle pump or air compressor with a built-in pressure gauge. I think that you will peg the needle on your speedometer before the pressure gets very high, probably less than 5 psi, but that is a guess. Running smaller diameter lines allow you to use thinner wall thickness for the same pressure. Speedometer tubing may be larger to reduce clogging from salt and mineral deposits.
Smallfrye	posted 09-21-2005 11:09 AM ET (US) If your concerned about the use of the hole remaining after removal of this guage, replace it with a Floscan fuel meter. I believe they are the same size or only slightly smaller. Water pressure is a critical value on some boats( shallow runners) Why would you go through the significant effort for an untested method of recoring this information ?
Whalerider	posted 09-21-2005 12:40 PM ET (US) I like Smallfrye's answer. On most boats a water pressure gauge is about as useless as a speedo. A Flowscan would be very nice indeed. Heck... look for something usefull to fill the hole with....maybe a speedo that interfaces with your GPS....even a clock might be better than water pressure. Mark