Onyx Isolator Rings have traditionally been supplied with mechanical gauges and mechanical switches to monitor process pressure and pump performance. These do a fine job, but many users are changing over to solid state switches. This paper will discuss some of the advantages of each.

Principle of operation:

Mechanical pressure switches operate with either a piston and spring, or a bourdon tube and mercury vial. In the piston and spring arrangement, when pressure builds up to the point where it crosses the set point, the piston pushes on a snap acting switch to transfer mechanical contacts.

In a mechanical bourdon tube switch, a bourdon tube tilts a rocking beam fitted with a glass vial of mercury. When pressure builds up to the set point, the beam rocks over and mercury in the glass tube rolls from one end to the other, where it submerges electrical contacts completing the circuit.

In contrast, a solid state switch uses a metal diaphragm about the size of a dime with a micro-miniature strain gauge etched onto its dry surface. Increasing pressure changes the impedance of the strain gauge. A digital comparator monitors the output from the strain gauge and compares it to the user specified set point. When the set point is reached, the electronic circuit turns on a solid state relay, completing the circuit.

Accuracy: A typical mechanical Bourdon tube pressure switch has a published repeatability of ±1% compared to the A-B 836 series which has a published repeatability of ±0.2%, a five-fold improvement in performance.

Dead-band adjustment: A Bourdon tube switch with a range of 2 to 60 psi has a minimum dead-band of 3 psi. The A-B switch with the same range has a minimum Dead-Band of 0.30 psi, a ten-fold improvement in performance.

The difference is even more striking in the low range. A Bourdon tube mechanical switch in a compound range has a minimum dead-band of 1 psi compared to the A-B switch in the same range which has a minimum dead-band of 0.1 psi. If you are trying to monitor suction pressure on a pump, this is the difference between best resolution with a mechanical switch of 2.3 feet of water column and the solid state switch with a resolution at 3 inches of water column.

There’s a big difference in field adjustment setting as well. Mechanical switches typically require screw drivers or wrenches, and setting the switch – even with a socalled “dial” – is a trial and error method. In contrast, the solid state switch is a matter of pushing a few buttons and programming in the exact set point you want.

Attitude: Snap acting mechanical switches can be mounted in any position, but mechanical switches with mercury elements MUST be mounted vertical and upright or they will not function. All solid state switches can be mounted in any position without affecting their operation.

Over Pressure: Mechanical switches with a bourdon tube can not be subject to pressure over the maximum range. In contrast a solid state switch can tolerate a substantial over pressure without adverse effects. An A-B switch in a 60 psi range has a 160 psi maximum working pressure; a 150 psi range solid state switch can go to 400 psi without ill effects.

Price: The cost of a mechanical switch by itself is generally lower than a solid state switch; however, the solid state switch has a digital read out for pressure, so the gauge is superfluous. A solid state switch is (almost) always less expensive than a mechanical switch + gauge assembly.

Power requirements: There are three advantages to the mechanical switch:

1. You don’t need to know the operating voltage in advance. A mechanical switch can work on either AC or DC power at almost any voltage. In contrast solid state switches come in two distinct flavors: Low voltage (12-30 volt DC) and high voltage (90 to 250 volts AC) so you have to know what power level you are dealing with before placing an order.

2. Mechanical switches can handle higher electrical loads. The Bourdon tube element example can handle 4 Amps at 120 volts AC, compared to the solid state switch which can handle 2.5 Amps at the same voltage. Solid state DC switches are generally rated at ¼ Amp at 24 VDC.

3. Mechanical Switches use 2-wires, whereas a solid state switch needs a third wire for a common.

Capillary tubing introduces three effects on the performance of pressure-sensing instruments:

1. Temperature effects. Temperature changes cause the liquid inside the capillary tube to expand and contract, changing the volume of the fill fluid. The resulting error is a function of the total volume of the tubing, pressure instrument, and isolator ring. Because the rubber sleeve in the isolator ring has a much lower modulus of elasticity compared to a diaphragm seal, it can absorb most of the volumetric change resulting from temperature differences throughout the usable temperature range for isolator rings. A typical error in gauge reading through a temperature swing from 0°F to 120°F is about ½ psi depending on isolator ring size and gauge type. This is roughly a quarter the error expected with a standard 60 mm stainless steel diaphragm seal.

2. Elevation effects. As you change the elevation of the gauge with respect to the isolator ring, you introduce an elevation error. This error is due to the static pressure of the liquid in the capillary tube. The change in gauge reading caused by elevation changes of the pressure sensing instrument can be calculated in advance using the following equation:

P {psi} P Elevation{feet} sp gr actual gage = − ÷ 2.31∗

Our standard fill fluid is a silicone oil with a specific gravity = 0.967 at 77°F.

Observe polarity: If the gauge is above the isolator ring, then the elevation term in the above equation is positive; if the gauge is below the isolator ring, the elevation term is negative. If the gauge or transmitter has a zero adjust capability, the elevation error can be eliminated completely by re-setting the zero adjust to compensate for the elevation change.

3. Response time. Capillary tubing introduces a response time lag in the instrument reading. This delayed reaction time is influenced by:
• Length of the capillary tube
• Internal diameter of the capillary tube
• Control volume of the pressure-sensing instrument
• Viscosity of the fill fluid, including temperature effects fill fluid viscosity
The control volume of a pressure-sensing instrument such as a gauge or transmitter is defined as the change in volume required to deflect the bourdon tube or sensing diaphragm from zero to 100% reading. The smaller the control volume, the better the performance. Instruments with smaller control volumes exhibit less temperature error and time lag than instruments with greater volume. As a general rule, higher range instruments have a smaller control volume than lower range instruments; for example a 100-psi gauge has a much smaller control volume than a 15-psi gauge. Another general rule is that electronically amplified devices have a smaller control volume than mechanical devices; again, to use an example, an electronic transmitter has about 1/100th the control volume of a bourdon tube gauge of the same range.

Onyx isolator rings are supplied with capillary tubes with ID = 0.075 inches, and the fill fluid is Silicone with a viscosity = 100 centistokes. Typical time lag with 5 feet of capillary and a 4 ½” turret case gauge with a 60 psi range is about 2 seconds.

4. For those users who want a more exact prediction of the performance of a pressure measuring system consisting of an isolator ring, capillary tube and pressure sensing element, you can bring the full capability of modern mathematics to bear on the problem:

Technically:

Where:

Err = Error in reading expressed in inches H2O water column.

E = Expansion factor. If you use degrees F and inches for all the terms in the above equation, then E for Silicone fluid = 0.000600

Rs = Spring rate. For isolator rings, Rs is related to nominal size such that:

RS = 100/Nom size, in

The error as a percent of reading is therefore:

Error%= Err/ Measured Span in inches H20 * 100

Volume of Iso Rings: