Flow not reaching desired rate

[lusi]

Hell. I set the flow to 5ml and it is not passing 1ml. What can possibly be the cause of this? The flow for the other injector, which was not in use for months is reaching the desired rate.
Thanks in advance.

[mac]

I'm not an expert but there could be a leak somewhere in your system. Alternatively, there could be a blockage. A possible location of a blockage could be if you are using a capillary column and you have tightened one the column nuts too much. The column may have broken in the ferrule and be blocking the flow of gas. If you are using an FID detector and your jet is too big for your capillary column there may be gas escaping there.

[lusi]

Thanks Mac. I will look for a blockage. Just one point to know the flow rates etc were ok Friday and when the machine was turned back on Monday, this problem arose. Initially I thought it may be a leak, due to cooling. I did the usual stuff and the flow still was not rising.

As for a breakage due to the column, it was not noticeable. And I cleaned out the opening in the oven just in case the ferrule was stuck there. I did not see anything.

As for the FID it was in use. Additionally, the instrument can regulate flow without a column being attached. This is not something we ever did. But I did try it out when this problem occurred.

We have a PTV injector that is not connected, but capped off and are able to regulate its flow. But the SSL I am not sure.

I am wondering whether it can be the flow controller itself. Since we recently had problems with the one now on the PTV. It was on the SSL but since we use this injector the most they were changed around by the service providers.

Also someone said that it may be moisture, but I do not know.

[Don_Hilton]

The machine was turned off on Friday and back on on Monday - did anythin else get changed? And,if gas was turned off at the bottle, are all valves fully open and is there adequate pressure in the bottle to drive flow. (particualy if the bottle is shared with other instruments.)

[aldehyde]

Remove the column from the inlet, install a blank nut or a column nut with about an inch of column capped with a septa so you are just dealing with a sealed inlet. Set it to split mode, set the pressure to 25 PSI and the split to 100:1. Give it about 30 seconds after it comes up to 25.0 PSI and then set it to off. Watch it and see if the pressure drops, it should just sit at 25.0 (maybe it jumps up to 25.4 or 24.7 when you hit off but if it continually drops or rises you have a problem).

Once you are sure the inlet is a) leak tight b) comes to the set point reinstall the column and see if your conditions work as desired.

What kind of GC is it? Are you sure your column dimensions are right? And yeah, make sure the tank isnt empty or turned off at the regulator.

[lusi]

For some odd reason I did not receive an email indicating that I had replies for this query.

Thanks so much for the replied.

Update, I switched the injectors and am now using a PTV. The flow rates are great with that. The method needs tweaking.

In response to Don_Hilton: nothing else changed and there are no other instruments attached to the gas supply.

We have a thermo TRACE GC ULTRA. The column and gas are / were ok.

Just tried aldehyde's suggestion. At 25psi the flow rate is 0.1. When I turned off the pressure just dropped and kept dropping. You did indicate that it the pressure changes I have a problem, can you kind explain a bit more. Thanks.

I leaked checked and came up with nothing. Besides the PTV has the same gas supply. Could it be the flow control?

[aldehyde]

Sounds like you may not have done it correctly. Do you have a blank column nut or a nut with an inch or two of column blocked off with a septa installed into the inlet? You need something to stop the pressure from just escaping out the bottom.

If it got up to 25 psi thats good. 0.1 mL/min is kind of weird, do you have a column configured (ie, does the inlet know the dimensions of the column installed?

[lusi]

I used a blank nut. The pressure is going up, just not the flow.

I can not explain why the flow is so low. Nothing about this equipment has ever been normal.

[Peter Apps]

How are you measuring the flow ?

Peter

[lusi]

I am using the read out on the GC. Actually I think I did mention a while back we had some problems with one of the flow controllers that was attached to the SSL injector. (The service providers concluded this). Since we had to source the part overseas, the service providers switched the flow controllers. So the one originally on the PTV was switched to the SSL. Well right now that supposedly 'bad' controller is working on the PTV and the SSL began to give some problems.

I have been in contact with the service providers and they are due for a visit at the end of the month. They indicate that they will check it out, but can not at this point in time offer any more assistance.

[Peter Apps]

You need to measure the flow with an independent flow meter, and the pressure with an independent pressure gauge. Also check the actual split flow and the septum purge flow. Also check that all the settings with respect to column dimensions, carrier gas type, septum parge and split/splitless agree with reality.

Peter

[lusi]

Thanks Peter. I will look into getting the flow measured independently. The thing is we do not have our own flow meter. However, I do think this level of work is beyond me. I have had only basic training on the equipment - which was about 2-3 days. Including getting to know the software, well part of it anyway.

We have a Thermo TRACE GC ULTRA and no one else in the country has one. In fact there is probably only one other GC in the country (HP) that may be working. So no technical support either.

[Peter Apps]

Hi Lusi

You do not need anything fancy to measure the flows - just a bubble flow meter and a stopwatch. You can make a bubble flow meter from an ordinary burette; pull out the tap and put a small rubber bung in one side of the tap hole, In the other side of the tap hole put a 1-hole bung and connect it to a piece of flexible plastic or rubber pipe. Over the tip of the burette put a rubber pipette bulb filled with soapy water.

Connect the end of the pipe to the flow that you want to measure, squeeze some soapy water up towards where the tap was on the burette until it makes a bubble with the gas coming in. Time how long the bubble takes to move between two volume marks on the burette. Calculate the volume flow.

Good luck, which country are you in ?

Peter

[Don_Hilton]

For instruments that display pressure and flow electronically, they compute flow from pressure based on column geometry. If the instrument is measureing pressure, and the flow is incorrect. Either the column geometry is set incorrectly or the portion of the instrument that does the calculation is not working correctly. Check to be sure that column information is correctly entered. The newer Thermo units have some extra functions that confirm column information by a pressure test procedure (reason to be sure no fittings are leaking) and if that is not working correctly you may need help from Thermo.

[lusi]

I am tempted to believe that the part of the instrument responsible for the calculation may be malfunctioning. I checked the K-factor using the other detector (PTV) and it was fine.

It is just strange that the instrument is up and running and then shut down for two days with nothing at all changing and then this flow problem arose. Because of this the machine was unable to check the K-factor using the SSL.

As for setting the parameters of the column, I am not familiar with that function. We usually just place the column in, leak check manually, condition if needed, and check the K-factor and leak test on the instrument.