Category Archives: Pressure

Stand-Alone Pressure Transducer or Sensor + Electronics?

Introduction:

Validyne pressure transducers break down into two general categories:

Type 1 – A complete transducer with integral electronics

pressure transducer

 

 

 

 

 

Type 2 – A variable reluctance sensor and supporting carrier demodulator electronics.

pressure transducer

 

 

 

 

 

 

Type 1 category products include models P55, P61, P66, the P895 family and the DR800 and P532 process transmitters.

Type 2 category products include models DP15, DP360/363, DP103 with carrier demodulator models CD15, CD23/223, CD280 and CD17.

The transducers in both categories measure the same pressure ranges – so why would you choose one type over another?

Cost Effective DC Power and DC Signal:

Type 1 category transducers are generally more cost effective per point than are the sensor + electronics (Type 2) category. The Type 1 products come ready for DC power and produce a high-level DC signal, +-/5 Vdc or 4-20 mA. The Type 1 transducers include temperature compensation and are also available with higher accuracy because we can program corrections to sensor errors into the microprocessors in these products.

Type 1 products are generally ‘plug and play’ devices and are ideal for permanent installations.

Type 1 products, however, do not lend themselves to the changing of pressure ranges easily. It is possible to disassemble the sensor on a P55, for example, and replace a damaged diaphragm or install a diaphragm with a new range – but the correction factors and temperature compensation in the microprocessor will not be matched to the new assembly. Validyne can do this – and include new temperature compensation and error correction factors – but this takes time and has a cost.

Easy Range Changing:

The biggest reason to use Type 2 products is for convenient range changing. A DP15, for example, will be easier to disassemble and easier to replace a diaphragm than the Type 1 units. The sensor will be easier to calibrate with the zero and span adjustment ranges built into the external carrier demodulators. If fast frequency is important, the smaller variable reluctance sensors can be more conveniently close-coupled to piping than the larger Type 1 units and the electronics supporting Type 2 sensors have a higher low pass filter frequency available – up to 1 Khz.

Type 2 products are best suited to laboratory settings where pressure ranges are frequently changing, where a digital display is needed and where installation flexibility is important.

Type 2 products, however, do not have built-in temperature compensation, must be calibrated by the user with an appropriate pressure standard and are generally more expensive per measurement point.

Furnace Draft Measurement

Introduction:
One of the most difficult process measurements to make accurately is furnace draft pressure. This is the pressure inside the furnace that optimizes the air/fuel mixture for the most efficient combustion. The air is drawn into the furnace by the slightly lower pressure that results from heat rising up through the stack, the same way a chimney draws air into a fireplace. In an industrial boiler the draft pressures are very low – typically less than 0.25 In H2O – and are controlled by large vents that open or close depending on the furnace pressure to be maintained. The combination of low draft pressures and the extreme environment of heat around the furnace make accurate draft range pressure measurements difficult. The Validyne DR800 is designed especially to meet these requirements and this application note will describe the features that make the DR800 the best choice for industrial furnace draft measurement.

Low Range:
The DR800 is available in a full scale pressure range of just +/-0.25 In H2O. This is the full scale of the sensor and is not the result of over-amplifying of a higher pressure sensor. The variable reluctance sensing technology of the DR800 produces ten times as much signal as a strain gage sensor, further reducing the need for amplification and resulting in much better performance through temperature.

dr800Rugged Housing:
The DR800 has a NEMA 4 housing that includes an industry-standard junction box with 1/2” NPT conduit connections and externally accessible zero and span adjustments. The mass of the DR800 body acts to absorb rapid changes in ambient temperature and is solidly built.

Offset Calibrations and Turn Down:
Most furnace draft measurements can be both positive and negative with respect to the ambient atmosphere. The DR800 signal can be offset from zero to take these pressure changes into account. A typical calibration of -0.05 In H2O = 4 mA and +0.15 In H2O = 20 mA is possible using the zero adjustment and offset jumpers on the circuit board. The span can be turned down 2.5 times so that a span of just 0.1 In H2O is possible on the most sensitive DR800. This small span can start at -0.25 In H2O, for example, up to +0.15 In H2O using the zero elevation and suppression feature.

Selectable Damping:
Furnace air flows are often noisy and the DR800 has selectable damping with time constants from 0.25 seconds to 8 seconds using jumpers on the circuit board. This smooths the signal allowing for better pressure control.

Standard Mounting and Process Connections:
The DR800 pressure connections are 1/4” female NPT with adapters available for 1/2” NPT. The plus and minus ports are mounted on 2-1/8” centers for easy connection to industry standard 3-valve manifolds. A 2” pipe mounting bracket is also available.

FM Approval:
The DR800 is FM-approved for use in hazards locations for Class I, Div 2, Groups B, C and D.

Selecting Accessories for the Recalibrating the P55 Pressure Transducer

Introduction:
The Validyne P55 pressure transducer has as its sensor a variable reluctance pressure sensor that can be re-ranged for different full scale pressure measurements. The sensor can be disassembled, a new sensing diaphragm installed and the unit re-calibrated to the new full scale pressure. Some 23 different full scale pressure diaphragms are available and this application note will describe how to select and order the parts needed to re-range the sensor and interface the signal to a PC.

Sensor Parts:
A typical P55 is shown below, with the external parts identified:

P55 Parts pressure transducer

 

 

 

 

First, remove the two Philips head screws holding the sensor to the P55 electronics housing. These are located on the underside of the housing. The wires from the sensor to the electronics are very short, so take care they do not break.

To disassemble a P55 sensor a torque wrench, T27 Torx socket and a vise are needed. The tools needed to disassemble the sensor are available from Validyne and are shown below:

torquewrench pressure transducer

 

 

 

 

 

The sensor can be disassembled by removing the four 10-32 Torx T27 body bolts. When disassembled, the sensor body pieces separate and the sensing diaphragm and o-rings are removed. These parts are shown below:

boltsorings pressure transducer

 

 

 

 

 

It is good practice to replace the body bolts and o-rings when changing the range of the P55. Various o-ring compounds are available (see ordering chart).

The sensing diaphragm may now be replaced with one of a different range. A typical sensing diaphragm is shown below:

diaphragm pressure transducer

 

 

 

 

 

To re-range a P55 sensor the full scale pressure must be known and the correct diaphragm part number ordered. The part number for a P55 diaphragm starts with 3- and is followed by a two-digit range code. The diaphragm in the photo above is p/n 3-22 and has a full scale range of 5.5 In H2O. The other available range codes for the P55 sensing diaphragm are shown in the chart below with their full scale pressures expressed in various engineering units.

P55Ranges pressure transducer

 

 

 

 

 

 

 

Re-assembly is simply the reverse of dis-assembly, taking care that the torque on the body bolts is 125 In-Lb. The vise is used to stabilize the sensor body during assembly and to allow the torque to be correctly transmitted to the body bolts.

Also be sure that the bleed screws are tightly seated – these use a 5/64” hex wrench, Validyne p/n K950-0781. The sensor is reattached to the housing using the two Phillips head screws.

Calibration Accessories:

The next step is to calibrate the P555 against a pressure standard. Validyne can supply model T140K calibrator kit that includes a pressure pump and reference standard – an example is shown below.

T140K pressure transducer

 

 

 

 

 

 

 

The T140K calibrator kit is available in six different versions covering the available DP15 full scale pressure ranges. To calibrate theP55 connect it the SI58 digital interface and have a voltmeter to observe the analog output signal of the P55 as it appears on the binding posts of the SI58. 

SI58 pressure transducer

 

 

 

 

 

The SI58 connects to any USB port on a PC and is supplied with software that allows changing the internal registers of the P55 to achieve an accurate calibration. Connect the re-ranged P55 to the SI58 and the SI58 to a PC. Connect a multimeter to the SI58 binding posts to observe the P55 output signal.  

P55Cal pressure transducer

 

 

 

 

Load the calibration software and follow the instructions for applying zero and full scale pressures using the T140K calibrator. The software will adjust the P55 microprocessor correction factors to produce an accurate calibration with the new sensing diaphragm.

The SI58 software also allows the user to compensate the P55 through temperatures. The temperature range can be selected by the user as applied by an environmental chamber. 

SI58 Software pressure transducer

 

Simple Manometer Calibrates Pressure Transducers

Introduction: A simple U-tube manometer made from easily obtained materials can be used to calibrate pressure transducers over the range of a few inches of water to a few psi. This application note describes how to construct a manometer and determine the accuracy that can be expected.  Items Needed:

  • 5 to 10 ft Clear Plastic Tubing
  • Ruler or Tape Measure
  • Water

Constructing the Manometer: A length of clear plastic tubing, formed into a U-tube and partially filled with water should be configured as shown below. Depending on the pressure you need to generate, the tubing should be a few inches to a few feet high. The limiting factor is likely to be the amount of vertical space available to form the water column. Connect one end of the manometer to the transducer pressure port (normally the + port). manometer pressure transducers               Secure the ruler or tape measure to the surface behind the U-tube. Raise or lower the free end of the tube to increase or decrease the fluid head applied to the sensor.  Note that the sensor need not be filled with liquid; the air inside the transducer has no place to go and will compress until it is at the same pressure as the fluid column. Determine the applied water column head by measuring the distance between the fluid level in each leg of the U-tube (see sketch above). The pressure will be expressed as In H2O, CM H2O, etc. Accuracy: The accuracy of the pressure generated by a simple U-tube manometer depends on the accuracy of the ruler or tape measure used to determine the height of the fluid column. If the tape measure used over 100 CM, for example, is marked in mm, then the fluid level can be determined to within one part in a thousand (0.1% FS). This is better than twice the accuracy of the 0.25% pressure transducer. Enhancement: For smaller fluid pressures, try slanting the U-tube. You will have to figure the trigonometry, but the fluid distance along the slanting tube will be the hypotenuse of a triangle whose opposite leg is the actual fluid head. So the fluid must travel further along the tube to raise the pressure. This increases the accuracy of the fluid pressure determination.

TempIsolation

Temperature Isolation of Pressure Sensors

Frequently it is necessary to measure the pressure of fluids which at temperatures above
or below the rated operating range of the available pressure transducers. The expense of
a special transducer can often be avoided if the pressure sensor is isolated from the
pressure source by a short length of pipe or tubing. See how Temperature Isolation can help improve the data you are looking for.

The graph below shows the lengths of tubing needed to limit the temperature at the
transducer to a range of 0 deg F to 200 deg F for fluid temperatures between -400 deg F
and +1700 deg F.
The curves are based on the following assumptions:

· The piping is insulated to limit radiant heat transfer to the transducer. The major source of thermal input to the sensor is by conduction through the connecting tubing.

· The pressure media has a coeffieicnt of thermal conductivity less than 0.4 BTU/hr/ft/deg F. This includes a wide range of liquids and gases.

· The ambient air temperature around the transducer is not greater than 100 deg F.

· The heat transfer rate from the bare tubing to still air is 1.44 BTU/sq ft/hr/deg F

There is no flow of fluid through the tubing to the transducer, therefore the only path for heat transfer is conduction. Because the tubing is not insulated and the ambient air temperature is less than 100 deg F, the heat dissipation of the tubing protects the transducer, even to extremes in temperature of the fluid to be measured.

TempIsol-1 temperature isolation

 

 

 

 

TempIsol-2 temperature isolation