Level glass reading


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I am fresh in process control application. I am currently working as junior process engineer. I have a problem in determining correct reading of a level glass attached to a vessel/column. Beside level glass installation, there is float typed level transmitter installation that enables people to monitor in DCS panel.

Here are my question:

1. Is it right that the center of level glass follows the center of float vessel (where float element is placed)?

2. What is level glass height tendency regarding to float vessel height? Is it shorter, normal/same, or taller?

Hope for your explanation..

Best regards,
R. Lakota
Hi there,

Believe it or not but a sight glass or level glass is called a master instrument, because it is one of the only instruments that does a direct measurement of the process. So if your sight glass is working well and there is no blockage the level you see in it will be exactly the same in the main vessel as well as in the float chamber of the float level transmitter.

The sight glass will always be your most accurate reading and we sometimes use it to calibrate our electronic level transmitters. So if you are unsure how to setup the float, set and calibrate it according to the readings of the sight glass. It will be perfectly accurate. If you need more help than this let us know.
Thank you for replying..

I mention a case here to get more obvious explanation. I find in the field, the height of a float chamber is rather smaller than the height of sight glass. I want to compare the actual level reading in sight glass to what displayed in DCS panel. Which one is the correct way?

a. Read the level glass directly with lower position of LG as zero level while upper one as 100 % level

b. Instead of the lower & upper position of LG as reference, use the lower & upper position of float chamber. So, the zero reference of LG is above the lower position of LG.

Hope the next explanation..

R. Lakota
Hi again,

Ok I hope I understand this time what you are tying to ask.

Basically it seems you have a sight glass but the float chamber next to it is smaller in size and you are unsure whether you should use the sight glass or the chamber top and bottom as zero and 100%.

The correct way to do this is to find the design engineer’s internal vessel drawings and have a look at where he have specified zero and 100% level should be. If this is a original design the float chamber and the sight glass will be on the drawing as well as the various distances from tank bottom to the point where Zero and 100% level should be. You will most probably found that the area where the float chamber have been installed is the critical level control area and the level below or above the chamber is not all that important. So once you have found the distances from vessel bottom to the position where zero and 100% should be you can go to the field and mark these points on the float chamber and at the same time mark the same distances on the sight glass. So at the end of the day you should have a zero mark just above the bottom of the float chamber as the true zero position and another mark just below the top of the chamber as the 100% mark. If these markings are then identically marked on the sight glass you can read the actual % level of the sight glass and the DCS should display the exact same value if the float Tx have been setup correctly.

If you do not have design engineering internal vessel drawings make your own markings as I have indicated above on the chamber. To be safe do not use the first 10% from the bottom up as zero and do not use the last 10% from the top down as 100% so keep the markings at about 10% and 90% of the total height of the chamber. Transfer these Z/S markings to the sight glass and recalibrate the float transmitter according to the actual % readings of the sight glass. The DCS will then indicate the exact same % level as what can be red directly of the sight glass in the field, based on the markings you have put on the sight glass.
Thank you, Sam for your explanation. I understand now. I will practice your explanation in the field later..

R. Lakota