Pressure gauges have various types and usage, and one of this was a differential pressure gauge. A differential pressure gauge is not just used to measure and display a pressure reading but it is also used as an instrument to measure the level of a liquid specifically in gas industries (cryogenic).

It is used to measure the level of a cryogenic liquid, like Liquid Oxygen (LOX), Liquid Nitrogen (LIN), Liquid Argon (LAR) and others, whether in a storage tank or in a transport tank. These are usually in an enclosed tank.

In this post, I will share with you the following:

1. principles about pressure level gauge,
2. how to set up before calibration,
3. how to calibrate a differential pressure level gauge,
4. how to adjust when you encountered an out of specs or not in zero setting,
5. how to read a level gauge with multiple displays.  Some differential pressure level gauge has a 3 different displayed unit with a varying range. This is confusing if you are not familiar with it.

This post will focus only on the analog type differential pressure level gauge for simplicity.

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Principles of a Differential Pressure Level Gauge

How does Differential Pressure Determine the Level of Liquid?

The height or level of a liquid in an enclosed tank can be determined by the differences of two pressures (the high and low side) divided by the specific gravity of the liquid. That is why we are using a differential pressure gauge as the instrument for this job. This is also applicable to an open-end tank or vessel.

The formula below explains how a level is computed when a delta P (ΔP) or differential pressure is available.

Where:

ΔP = P-P1 – the difference of two pressure

Where:

P – Higher Pressure (Wet Leg)

P1- Lower Pressure (Dry Leg)

SΡ – Specific Gravity of the liquid use

Or re-writing the equation, we can determine the pressure by substituting known specific gravity of the liquid multiplied by the level (height) of the liquid. See below equation:

Based on the above equation, we can see that the pressure is dependent on the liquid density or its specific gravity when in use, so different kinds of liquid gas even though they have the same level value will display a different pressure.

It is important that we should determine first the kind of liquid before we can take the exact value of measured pressure with an equivalent value of level.

For more reading about this topic, you may visit this site>>Level Measurement

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Understanding the Units Displayed in a Differential Pressure Level Gauge

The displayed units we normally encounter are the pressure units which are the mbar or the mmH20 which is also equal to mmWC. Others are directly displayed as percentage % units, volume in Liters, or fraction format to show the actual level, some are in a combination of the two. See photo below.

This kind of units or display is very usual and understanding them is very simple.

But what if the display are different, in LOX, LAR or LIN?

There are also Level Gauges that display only a pressure unit in LIN, LOX or LAR ( also known as ArL, OL, and NL) which has an equivalent pressure value depending on the liquid content of the storage tank.

Actually, this is not a pressure unit, but the name of the liquid gas used, namely, Liquid Oxygen (LOX), Liquid Nitrogen (LIN) and Liquid Argon (LAR). Others are CO2 or 02 which are the ones we normally see.

The main unit that is being used is the percentage (%) and it has a pressure value counterpart in mmH20 (mmWC), mbar or in other pressure units. Some level gauge also has a unit of volume, which is in Litre.

Have you encountered a Differential Pressure Level Gauge with multiple ranges or type of liquid gas it can measure?

There is also a level gauge with 3 displayed range or fluid gas used in one face of the gauge with a fraction as its scale division. It may confuse you at first (because it did to me) on its many displays.  See below photo.

Some level gauges have no pressure units, only the fluid that is being used. Also, some pressure level gauge displays only the full range value of its pressure.

This is why it is important to have in hand the specifications of the tank where it is being used before calibration, in order to determine the pressure displayed versus the equivalent level.

This is provided by the manufacturer or supplier of the instruments used. There are conversions or equivalent pressure value that we need to know in order to understand it.

Below is a sample conversion or reading from manufacturer posted on the storage tank.

You should be aware of these specifications because not all storage tanks display this information.

How to Read a Differential Pressure Level Gauge?

There are so many types or model or manufacturer of a differential pressure gauge and I will share in this post the one that I encounter and hopefully, it will be a great help to you.

Differential Pressure Level Gauge with a Normal Units of display

I will first introduce the level gauge with a simple pressure to the percentage (%), volume, or direct pressure equivalent reading (see Figure 1 above). These type of Level gauges have a readout display that is directly converted to level. An example is a percentage (%) which means that at a full range of pressure generated equals a 100% level.

I suggest that before performing a calibration, you should be aware of below information:

1. The maximum range (some is written on its body)
2. How to set the shut-off valves in a 3 valve manifold ( see below)
3. The unit of measurement it is being used and their equivalent conversion units.
4. The user ranges if available.

By having this information readily available, recording calibration data points can be achieved easily.

We will calibrate the level gauge by using Fluke 754 with a pneumatic pump and a Pressure Module as our reference standards.

Calibration of this type of a level gauge is exactly the same as a normal Differential Pressure Gauge as long as it is detached from its location and manifold and thereby directly accessing the input (+ side) of the differential level gauge.

The procedure for calibration is the same for the calibration of a differential pressure gauge. You can read it on this link >>Diffrential Pressure Gauge Calibration

 

Pressure Level Gauge with ArL (LAR), NL (LIN) or OL (LOX) display (for Figure 2 and 3 above)

As explained above, the level is dependent on the specific gravity of the liquid gas that is in use, but during calibration, since we will isolate the level gauge by closing the valves on its manifold or simply removing the level gauge itself, we will use the air (oxygen) as a medium or fluid for calibration or for generating pressure.

The process of reading is still the same as the normal display units above, the only difference is the liquid used. During calibration, we will neglect the liquid gas setting displayed in the unit and substituting 1 as the specific gravity for every pressure value we have (see formula 2 above).

The level gauge output display in this process act as a normal level or differential pressure gauge (unless you use the exact liquid gas they used). So the calculation of level in terms of specific gravity is not needed.

As seen on the table on the photo above (figure 4), every pressure value has a corresponding level value. But since we will be using air as the medium, we only need the full user range setting (the pressure reading) of the level gauge, and from here, we can now determine the exact division of scales in the level gauge and use this as the calibration points directly

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Pressure Level Gauge with Manifold

In this part, I will share with you how to set up and isolate a differential level gauge with a manifold.

Most level gauge of this type when calibrating on-site or in its location, it is integrated on its manifold and cannot be detached, in this regard, an additional step must be followed in order to properly measure or calibrate the unit.

One must be familiar in the opening and closing its isolation valve in order to achieve the correct reading. This is very tricky when you are not familiar with its manifold. You may also damage the level gauge when a sudden rush of pressure is encountered.

Preparation Before Calibration (Check Zero Setting)

a simple demonstration on how to perform differential pressure level gauge zeroing and calibration set up.

Before performing the calibration, close all valves with pressure going to the manifold and execute the procedure below (See figure) to prepare the level gauge for calibration. There are different designs of manifolds (as you see above) but the valve operation is the same, so don’t get confused with this.

1. Close the high-pressure shut-off valve1.
2. Open the equalizing valve.
3. Close the low-pressure shut-off valve 2.

In this status, the level gauge pointer must also be in Zero position.  It is now ready to be calibrated. If not in zero position, perform an adjustment. Read further below.

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3 Calibration Set-Ups for a Differential Pressure Level Gauge

The Set-Up When it is Removed from its Manifold

The Set-up for a pressure level gauge that is removed from its manifold is also the same as a simple differential pressure gauge so the connections are also the same.

In the case where the Level gauge is detached on its manifold, we will just locate the positive side of the level gauge and connect the input pressure of the reference standard. See below photo.

The Set-Up for Level Gauge with Manifold but with a Dedicated Calibration Port

A dedicated calibration port is included in most Differential Pressure (DP) level gauges for ease of calibration without removing the unit from its manifold or assembly.

This makes calibration or verification much easier and faster. You just need to isolate the unit from the pressure source then Perform the “Zero Setting” procedure presented above.

Zeroing will also Isolate the DP level gauge. Once Isolated, remove the seal of the DP level gauge on its side (right side when facing the unit). it has (+) sign in it. See below photo.

There is another seal in the left, the (-) port, just loosened it, no need to remove. Then insert the NPT fittings of the calibrator on the positive port (+). Make sure to close the equalizing valve.

You are now ready for calibration. See below set up.

Set-Up with the 3-Valve Manifold

When manifold is included,  you need to follow an additional procedure.

After performing the preparation and zero-check procedure above, it is now time to connect our standard for calibration into the manifold. Follow below steps.

1. Ensure that the manifold is Isolated from the supply. Remove all the fittings or connected tubes in the manifold input.

   

2. Open both the negative and positive shut-off valves of the manifold
3. Close the equalizing valves, in this status, the level gauge should be in zero position because the pressure in the positive and negative are exposed to ambient and therefore equal.
4. Connect the port of the calibration pump on the positive side of the manifold.

   

5. You can now start to pump and generate pressure.

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DPLG Calibration Procedure

Since this is a Differential Pressure (DP) gauge, we will follow the procedure in calibrating a differential pressure gauge. The only difference is that pressure value must be converted to the level value designated to the level gauge.  Below photo shows pressure to percent (%) conversion. (We need to compute)

1. Set to zero.
2. Generate pressure in an increasing manner until you reached the full scale. This will only require a small pressure so pump very slowly and not to damage the level gauge.
3. Then slowly release the pressure and get the reading. Same set point but in a decreasing manner.
4. Repeat the process two times.
5. Record all readings in your Measurement Data Sheet.

   

Visit my other post to see the complete differential pressure gauge calibration procedure in this link

Returning For Operation – Startup

After calibration and everything is returned for operation, follow the procedure for “Preparation Before Calibration” but in the reverse order. This will ensure that the level gauge is protected from a strong surge of pressure that can damage the unit.

1. Open the low shut-off valve2.
2. Close the equalizing valve.
3. Slowly open the high shut-off valve1.

It is now ready for testing or operation.

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Calibration Adjustment

You can perform below adjustment just in case the readings are out of specifications or if the needle is not in the zero position. (for Samson made level gauges)

Other level gauges can be adjusted using the method for a normal pressure gauge. Visit my other post in this link. 5 Techniques for Pressure Gauge Adjustment and Repair

First, use the designated zeroing port. If this fails, perform below, see photo.

1. Open the level gauge housing.
2. Locate the screw, use an Allen key to turn. (Some unit has Allen key  included in the unit).
3. Slowly turn clockwise or counterclockwise depending on the offset you need. Remember, Use only a very small force, this is very sensitive.
4. Perform another calibration to check the reading.

Conclusion

I have presented what I believe the important aspects during maintenance or calibration of a differential pressure level gauge. We have discussed the principle on how a differential pressure gauge can measure a level that is dependent on the density of the liquid, I have shown you the different types of level gauges display and explained how to understand them, presented the calibration set up, zeroing, and the procedure.

I hope that this could help you in your level gauge calibration.

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Best Regards,
Edwin