pressure guage for methanol

[offonoff]

A test apparatus contains saturated methanol gas being condensed. I'd like to have a pressure gauge tell the pressure of the gas. I have found gauges rated for R-14a, ammonia and other common refrigerants, but nothing explicitly for methanol.

Is there a way to tell if a gauge is fit for use with methanol?

 

[JLSeagull]

The technical aspects for a pressure gauge include the accuracy requirement, range, bourdon tube material, connection type, housing style, dial size/color and window material. Process gauges are usually 1/2 % accuracy. Test gauges require higher precision. I might select type 316 stainless steel for the bourdon tube and socket, 1/2" NPT bottom connection (back is another choice), white dial, etc. The 4.5" diameter is the typical process gauge. Eupopean designs may be 100 mm diameter. I care less whether it is a phenolic or stainless housing provided that it has blowout protection. I like plastic such as Lexan for the window instead of glass but it must be compatible with the fluid.

 

[hacksaw]

just measure the condensing liquid temperature that works in most cases, likely you pressure is going to be too low for a useful reading with a pressure guage

 

[JLSeagull]

For inches or mm H2O, perhaps a manometer or Magnehelic might be a good choice.

 

[offonoff]

Thanks seagull,

So, as far as material compatiability goes, there is only window and bourdon tube material? Is there not sometimes an oil, or lack of oil?

the apparatus is an adsorption chiller, where methanol is being driven off an adsorbant. So, I can't measure the temp at which the methanol is boiling to determine pressure.

Also, The system is sealed, swinging from vacuum to maybe 100 psi (I'm prepared for thrice that just in case). I dont know how to get wires into a sealed system like this (obviously an amateur here) to electrically measure temp. But regardless, I'd like to measure pressure directly (to see if it is holding the vacuum, etc.)

 

[JLSeagull]

Compound gauges exist with ranges from vacuum to 600 psig. The temperature measurement is another issue.

 

[danw2]

>So, I can't measure the temp at which the methanol is boiling to determine pressure.

I don't understand that statment. If you have a threaded port into process into which you can screw of pipe threaded pressure gauge, why can't you put a Tee into that process access port, then put a pressure gauge into one of the Tee legs and a thermometer, or a thermowell for temperature indication (thermocouple/RTD/electronic indicator/bitherm thermomoter) into the other Tee leg?

If the system is so sealed off that you can't measure temperature, how will you measure pressure with a pressure gauge?

>Is there not sometimes an oil, or lack of oil?

Gauges are sometimes 'liquid filled' in order to reduce wear due to pulsation and vibration. The liquid fill
is on the exterior of the bourdon tube, isolated from the process fluid. Although glycerin is a typical liquid
fill, an 'oil' could be used.

If you're referring to 'oil' in the process, then it's a question of whether that oil is compatible with the
gauge's stainless steel wetted parts.

 

[skearse]

Are you looking for a pressure indicator [PI] (like a field gauge) or a transmitter [PT} (wired back to an indicator)?

We handle a lot of methanol, and honestly we don't do anything "special" with our gauges, whether it be a PT or a PI. Just check the materials of construction & temperature rating of the gauge/transmitter, and off you go. Use a liquid filled gauge with a diaphragm seal and you should be all set.

BUT, your statement about not being able to measure temperature is a little confusing-does this mean you don't have a place to install the gauge? Regardless of which you are using (PI or PT), you are still going to need a tap into the process line to install the instrument. If it swings from pressure to vacuum a compound gauge is the way to go. We're typically at lower pressures so our range is normally 30" HgV to 30 psi. Installing an additional tap for a thermowell to get a temperature measurement allows for a second "indirect" measurement as a check for the PI/PT-provided that the methanol is saturated and not subcooled or superheated.

 

[offonoff]

Thanks all (especially Skearse) for the useful replies.

To clarify, I didn't say that i couldn't measure the temperature, I said that i couldn't measure the temperature of the vapor to determine its pressure (because it is super heated).

Of course, thinking about this a little, I realize if I placed the thermometer appropriately (near the liquid bath where the vapor is saturated) then i could determine the pressure. But that would only be a second check. Its much more direct to look at a pressure guage and see "uh-oh"then to look at a temperature gauge and wonder if maybe the methanol picked up water somehow.

thanks again