It’s in Sight but is it Right?
Sight glasses are a very useful and popular way of measuring the liquid level inside a storage vessel. However the liquid level in the sight glass is not always the same as the level in the vessel. This can happen if the specific gravity (SG) of the liquid in the vessel is not the same as that in the sight glass.
If this is the case, the glass reading will have an error proportional to the difference in the SG of the product on the sight glass and the tank.
Consider the following example from the spirits industry.
A storage vat is originally filled with high strength spirit 90% alcohol, S. G. 0.83
For cask filling the alcohol strength is reduced to 68% ( SG 0.89) by adding water to the vat.
When the vat is filled with water the level increases. This level in the sight glass will also rise.
The sight glass is connected to the base of the vat by a small bore pipe. Because the turbulence when the water is added is not sufficient to promote uniform mixing, the liquid increase in the sight glass is from the base of the vat and is most likely still to be close to the original 90% strength
When the water addition is finished, the vat is roused to ensure the vat content is all at the required strength. The small bore pipe connecting the sight glass to the vessel is not affected by the rousing operation. At the end of the rousing the sight glass is still full of spirit close to the original 90% strength.
When the sight glass reading is steady, the pressure due to the sight glass level and the vat level must be equal or there would be a flow from the high to low pressure area.
Because of the different SG’s of the spirit in the vat and the sight glass, for equal pressure the levels must be different.
In this example, because the specific gravity of the 68% spirit in the vessel at 0.89 is higher than the 90% spirit in the sight glass at 0.83, the level the operator sees in the sight glass is actually about 7% higher than the level in the vat.
To obtain a true reading the glass should either be drained and refilled with the reduced spirit, or purged back into the vat using air introduced at the top of the sight glass to allow the glass to refill with reduced spirit.
Initial fill Spirit at SG of 0.83.
Sight glass and vat readings equal.
Sight glass still full of 90% spirit at SG 0.83,
Vat full of 68% spirit at SG 0.89.
Pressure in the vat per 1000mm = 1000 x 0.089 Barg
For 0.089 Barg in the sight glass, level has to be 0.89 / 0.83 X 1000 = 1072 mm
The error in the sight glass is 72mm or 7.2%
In practise, the sight glass contents will be somewhere between the initial 90% and the final 68%, but to obtain a true reading the spirit in each must have the same SG.