Hydrogen Fluoride (HF) Gas Measurement Problems

How well-acquinted are you with your testing equipment? You might have just put the sensor to calibration mode at that point.

I bet the EPA and NIOSH protocols for testing gas is you should have 2 calibration gas (1 zero concentration (Could be Nitrogen), 1 with standard known HF concentration), re-evaluate your sensor prior and after actual testing to make your readings quality assured.

HF by USEPA

Have a copy of the testing method for HF, it will be a bit similar to the do's & dont's for testing such gas by a handy sensor--->>>USEPA Method 26

It can be useful to tell us what you use:

type and make of the regulator.

the line to your instrument, size and composition. (permeation)

type of probe you are using in the instrument.

if no probe and you work with spectrum analyzer which one.

what is the pressure applied to the instrument and

how big is the flow through the instrument.

how old is the sample in the cylinder and at what pressure is it operating.

you need to calibrate: is your probe not contaminated by other elements, picked up with earlier measuring.

Is there a procedure to clean the sensing element.

is your flow dry enough.

at what dew point you are working.

there can be a lot of moisture - not necessary in liquid form, that may disturb your process.

We used to run a dry air with a dew point of -130 Celsius for one day, before sending a calibrated gas sample to calibrate.

This is basic start with the details communicated.

If you have access to a sister plant with the same set up, then I'd suggest swapping parts one at a time to find out where the problem lies. e.g. swap your bottles, if that doesn't work, then go to the next component, regulator, tubing, etc... continue on...

Thank you all for your inputs. I think I have found what the problem is. The gas was getting absorbed by the bottle regulator and stem. I had to flow the gas for more than 20 minutes through these to get them passivated before I can get HF out of the bottle. I contacted the gas supplier and also talked to someone who has used HF before and they said that they have encountered similar problems before. To answer your questions dvmdsc:

I am using a stainless steel "dial a flow" regulator and stainless steel stem.

30cm of Viton connects the regulator to the instrument.

Instrument uses NDIR to measure HF

pressure applied to the instrument is 5 psi and flow is 0.5L/min

The cylinder sample is new and its pressure is 100bar.

The gas is reasonably dry (about 30-40ppm H2O)

Any comments on the above or advice to get better measurements are highly appreciated. Also if you have experienced having to "passivate" components like that, how long do you think I need to flow the gas for and how frequent do I need to repeat this process.

Looks pretty well. I would replace the viton by 1/4 O.D. 314L stainless steel, sch 80. You might create some losses there. It will help also when you stop calibrating to hermetically plug the output of the regulator as you stop the flow.

One thing I have noticed when doing HF tests is that when I switch the bottle off (before the High Pressure Regulator), the remaining gas seems to be of higher concentration, about 30% more than what I get if I keep the gas running!

I have repeated that a number of times and it is repeatable.

This has raised an alarm for me as I am concerned that I am loosing some of the gas somehow when it is running normally!

Does anyone have an explanation for that?

permeation

Thank you for your comment. Could you please elaborate more. Where do you think the gas might be permeating through? The main difference (that I can think of!) between the two cases is that gas pressure before the regulator, when the bottle is on, is around 50bar while when I switch the bottle off, the gas pressure will be dropping from 50 bar to 0.

Are you suggesting that the gas is permeating through the input port of the pressure regulator or stem. They are both Stainless Steel, (Regulator is 316L but not sure what S/S grade the stem is) and I bought the regulator specially for use with HF. The regulator has a metal to metal seal to atmosphere. Do you think I could be loosing as much as 30% of the gas through permeation? and the most important question is what can I do about it? If you have a similar experience it will be very useful to know about it.

Many Thanks