5890 series 2 makeup gas not responding

[theller85]

Working on a 5890 series 2. Single FID. Makeup gas is plumbed onto the main carrier gas line. When operating the makeup gas valve(by screwing the inside flat head) I do not get a gas flow response at the FID detector. With the fuel turned on at the machine, but the hydrogen supply turned off, I do not see any change in flow rates when adjusting the makeup gas. My understanding is that the makeup's is t'd into the hydrogen flow line at the valve block. Thus... with helium tank on, hydrogen tank off, machine hydrogen valve open - adjusting the makeup gas should show a change in flow rate. I see my ~1ml/min helium flow from the column but not shit from the makeup.

Is the makeup valve prone to failure? Not quiet sure how to proceed. Thinking about putting in the second valve block... though I'm dreading it.

Suggestions on how to proceed?

[KM-USA]

We used to "tee" into the helium supply for our make-up gas on our 5890 units, but later changed to nitrogen make-up. These front controls for the FID are essentially on-off valves, not control valves.

I would like to know if your 5890 has any EPC controls, like maybe your EPC channel for make up gas is not turned on? Because with certain configurations that could be the case.

Anyway, if you need replacement valve blocks, I've got some cheap, see viewtopic.php?f=7&t=27106&start=15

[Peter Apps]

If the tip of the column is inside the flame tip then helium makeup gas does nothing useful - the hydrogen fuel gas sweeps the column effluent quickly into the flame, and the sensitivity gain that you get with nitrogen carrier depends on its high MW slowing upstream diffusion of reactants in the flame, an effect that you do not get with helium. So you can save yourself some plumbing hassles and some running costs.

Peter

[Peter Apps]

If the tip of the column is inside the flame tip then helium makeup gas does nothing useful - the hydrogen fuel gas sweeps the column effluent quickly into the flame, and the sensitivity gain that you get with nitrogen carrier depends on its high MW slowing upstream diffusion of reactants in the flame, an effect that you do not get with helium. So you can save yourself some plumbing hassles and some running costs.

Peter

[theller85]

Just a thought here, but if you plumbed in zero air into the makeup line and ran the fuel air mixture at 80% hydrogen (above the rich combustion limit), would you not get the some improvement in sensitivity?

I'm guessing I'd have to crack a few books open to find out exactly how to pull that mix off, but it seems reasonable.

Back on subject - I'm going to replace my valve block with a used part on hand. My ignitor isn't working anyway.

Peter - As for the column being inside the detector - Would standard installation methods from the 5890 II manual place me at that location? Or would I need to go longer?

I think it was somewhere around 10cm of column length above the ferrule.

[Bigbear]

As I remember, there is no control valve inside the gas flow box. I think the "flat head screw" you have been messing with is a flow restrictor. Is it colored?
I remember there is a make up gas adapter that is inserted into the detector that the column passes through.

[James_Ball]

On older 5890, the flows on the flow control blocks were adjusted using needle valves with flat head screws inside the center of the flow control knobs. This was before they switched to using flow restrictors and pressure to control the flows.

You should not be using hydrogen as the makeup gas, that should only be the fuel gas, and carrier if you are running hydrogen carrier. Using Hydrogen as makeup will alter the fuel mix ratio.

The makeup gas was there because the FIDs are designed to work with packed columns with flow rates up to 30ml/min. If you have a carrier flow of 5ml/min, you add in some Helium or Nitrogen to bring the total up to 30ml/min, then set fuel and air flows as stated, usually about 30-40ml/min Hydrogen and 400ml/min Air. With this older control block, turn the gasses on one at a time, opening the valves fully, then adjusting the center screws until the desired flows are met. You want to do them one at a time so you don't risk a fire in the flow meter. For makeup, establish the carrier flow then adjust the makeup flow with the carrier going until you are around 30ml/minute. Once all the flows are set, turn them all on and you should be able to easily light the detector, if you have trouble with the flame blowing out, light it without makeup then add it after the flame is lit.

Nitrogen makeup will give more sensitivity than Helium, but the difference is not that much, and only makes a difference if you are trying to detect very low concentrations.