Category Archives: furnace draft measurement

Understanding Digital Displays

Digital display specifications can be confusing. Here is a short application note that describes the terminology and capabilities of a digital display.. 

Digital displays are specified as being either 3-1/2 digits or 4-1/2 digits. In a 3-1/2 digit display the three rightmost digits may read any value between 1 and 9. The left-most digit is limited to 1 or a blank. It is often easier to think in terms of counts: a 3-1/2 digit display can read up to 1999 counts, so it is suitable for displaying readings that have a maximum value of 20, 200 or 2000. Note that the decimal point may be placed within the right most three digits: 19.99, 199.9 or 1999.

Similarly a 4-1/2 digit display may read 19.999, 199.99, 1999.9 or 19999.

To specify the correct number of digits in a display, keep the following in mind: The resolution of the display should be five to twenty times better than the accuracy.

For example, if a 20 psi transducer with 0.25% FS accuracy is used, a 3-1/2 digit display configured to read 19.99 psi will provide a resolution of 0.01 psi when the accuracy of the transducer is 0.05 psi, and this is a good balance between resolution of the display and accuracy of the sensor.

A 4-1/2 digit display specified for the same transducer would read 19.999 psig at full scale, and the last digit would be unusable since it represents 50 times more resolution than the transducer accuracy.

However, if the transducer accuracy is 0.1% FS, then the 4-1/2 digit display would be appropriate because the accuracy of the transducer is now 0.02 psi and the 4-1/2 digit display would have a resolution 20 times greater than the accuracy. A 3-1/2 digit display, by contrast, would only read to 0.01 psi – just twice the accuracy of the sensor.

The diagram below shows how the 3-1/2 and 4-1/2 digital displays are used.

digital display


Check out Validyne products with a Digital Display

PS309 Portable Digital Manometer

PS409 Digital Panel Mount Manometer

P532 Ultra-Low Pressure Transmitter

DR800 Draft Range Transmitter

CD23/223 Carrier Demodulator with LED Display

CD379 Portable Carrier Demodulator with Digital Display


Furnace Draft Measurement

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.