What is the KET Compact Inline Thermal Flow Sensor?
The KOBOLD KET is a compact inline thermal mass flow sensor for compressed air and other gases that measures mass flow directly, with no moving parts and no inlet straight piping, and connects to energy monitoring systems through Modbus RTU, 4-20 mA and pulse outputs. It ships with an integrated measuring section for ½" to 2" lines.
Its small, compact housing is designed for use within machinery, and its display shows two values at the same time: present flow, total consumption (counter reading) in m³, l or kg, and temperature.
How the KET Measures Gas Flow
The KET operates on the calorimetric (thermal) measuring method. A mechanically protected built-in sensor is heated electrically, and the heat carried away to the passing gas is used to determine mass flow, volume flow and flow velocity. Because the measurement is thermal, mass flow is read directly, without separate pressure or temperature correction of the gas.
Gases, Pipe Sizes and Flow Ranges
The KET measures air, nitrogen, argon, CO2 and oxygen, with nitrous oxide, natural gas, other gases and gas mixtures available on request. Six integrated measuring sections cover ½", ¾", 1", 1¼", 1½" and 2" lines, threaded G ½ to G 2 (BSP) or ½" to 2" NPT, and four speed versions set the full scale for each size.
- Low-Speed (50 m/s), Standard (92.7 m/s), Max (185 m/s) and High-Speed (224 m/s) versions give lower or higher full scales for the same size
- Max version full scales for compressed air (ISO 1217 conditions, 20 °C, 1000 mbar): 50 cfm (90 m³/h) for ½", 103 cfm (175 m³/h) for ¾", 170 cfm (290 m³/h) for 1", 310 cfm (530 m³/h) for 1¼", 430 cfm (730 m³/h) for 1½" and 700 cfm (1195 m³/h) for 2"
Accuracy, Pressure and Temperature Ratings
Standard accuracy is ±1.5% of reading ±0.3% of full scale, with ±1.0% of reading ±0.3% of full scale available as an option. The KET is rated to 232 psi (16 bar), or optionally 580 psi (40 bar), at operating temperatures from -22 to 176 °F (-30 to 80 °C). The housing is polycarbonate, rated IP65.
The 580 psi (40 bar) rating requires the 316L stainless steel measuring section (KET-E), and with NPT threads it is available up to 1"; the standard measuring section is aluminum (KET-A).
Outputs, Power and Communication
Every KET provides an RS 485 digital interface (Modbus RTU), a 4-20 mA analog output and an electrically isolated pulse output that can alternatively be configured as an alarm relay. Power supply is 18 to 36 V DC at 5 W, and the 4-20 mA load (burden) is below 500 Ω. M-Bus, Ethernet or PoE interfaces are optional.
- The 4-20 mA output can be assigned to flow, velocity or temperature
- The pulse output delivers 1 pulse per m³ or per liter, with the pulse weight set on the display; in alarm mode a value, hysteresis and high or low limit are set instead
- M12 connector plugs are supplied when no cable is ordered; Modbus termination is by internal switch or a 120 Ω resistor between pins 2 and 4
Display, Units and Ordering Options
Settings and interfaces are programmed through the display, using two capacitive keys and a password-protected settings menu (factory password 0000). Flow units of m³/h, m³/min, l/min, l/s, ft/min, cfm, m/s, kg/h, kg/min, g/s, lb/min and lb/h are selectable at the keys, and the readout rotates 180° for overhead installation.
Options include improved accuracy of ±1.0% of reading ±0.3% of full scale (code J), oil- and grease-free special cleaning for oxygen service (code R), a silicone-free version with the same cleaning (code S), special measuring ranges (code M), real gas adjustment (code E) and a version without display.
Installing the KET
The KET ships pre-assembled with its measuring block, and the integrated flow straightener means no additional inlet section is necessary. It installs in any mounting position, on an unpressurized system only, with the arrows on the measuring section pointing away from the integrated flow straightener and the connecting nut tightened to 18 to 22 lbf·ft (25 to 30 Nm).
The ambient area around the sensor, outside the pipeline, must not be an explosive area, and installation is to be carried out by authorized professionals.
Maintenance and Recalibration
The sensor unit can be removed from the measuring section and cleaned. Check the sensor head regularly for dirt, dust or oil, which cause measuring deviations; an annual check is recommended, shortened for heavily soiled compressed air. Cleaning is done by carefully moving the head in warm water with a little washing-up liquid, never with a sponge or brush.
Recalibration every 12 months is recommended, with the sensor returned to the manufacturer; a CAL message on the display signals the recorded recalibration date, and calibration certificates are available on request.
KET Flow Measuring Ranges by Size, Version and Gas
Full-scale values below are per measuring section size and speed version. Compressed air is stated at ISO 1217 / DIN 1945 intake conditions (20 °C, 1000 mbar); all other gases are stated in Nm³/h at 0 °C and 1013.25 mbar (DIN 1343). Further gases and mixtures can be supplied on request.
Compressed air full scale by version, cfm (m³/h)
| Size | DN | Low-Speed (50 m/s) | Standard (92.7 m/s) | Max (185 m/s) | High-Speed (224 m/s) |
|---|
| ½" | DN 15 | 12 cfm (20 m³/h) | 26 cfm (45 m³/h) | 50 cfm (90 m³/h) | 65 cfm (110 m³/h) |
| ¾" | DN 20 | 26 cfm (45 m³/h) | 50 cfm (85 m³/h) | 103 cfm (175 m³/h) | 127 cfm (215 m³/h) |
| 1" | DN 25 | 44 cfm (75 m³/h) | 85 cfm (145 m³/h) | 170 cfm (290 m³/h) | 209 cfm (355 m³/h) |
| 1¼" | DN 32 | 82 cfm (140 m³/h) | 156 cfm (265 m³/h) | 310 cfm (530 m³/h) | 377 cfm (640 m³/h) |
| 1½" | DN 40 | 115 cfm (195 m³/h) | 215 cfm (365 m³/h) | 430 cfm (730 m³/h) | 521 cfm (885 m³/h) |
| 2" | DN 50 | 188 cfm (320 m³/h) | 353 cfm (600 m³/h) | 700 cfm (1195 m³/h) | 853 cfm (1450 m³/h) |
Low-Speed version (50 m/s): full scale for other gases, Nm³/h
| Size | DN | Argon | CO2 | Nitrogen | Oxygen | Nitrous oxide (N2O) | Natural gas |
|---|
| ½" | DN 15 | 35 | 20 | 20 | 20 | 20 | 15 |
| ¾" | DN 20 | 75 | 45 | 40 | 45 | 45 | 25 |
| 1" | DN 25 | 120 | 75 | 70 | 75 | 75 | 45 |
| 1¼" | DN 32 | 220 | 140 | 130 | 135 | 140 | 85 |
| 1½" | DN 40 | 305 | 195 | 180 | 185 | 190 | 115 |
| 2" | DN 50 | 505 | 320 | 295 | 305 | 315 | 190 |
Standard version (92.7 m/s): full scale for other gases, Nm³/h
| Size | DN | Argon | CO2 | Nitrogen | Oxygen | Nitrous oxide (N2O) | Natural gas |
|---|
| ½" | DN 15 | 70 | 45 | 40 | 40 | 40 | 25 |
| ¾" | DN 20 | 135 | 85 | 80 | 80 | 85 | 50 |
| 1" | DN 25 | 230 | 145 | 135 | 140 | 140 | 85 |
| 1¼" | DN 32 | 415 | 260 | 240 | 250 | 260 | 155 |
| 1½" | DN 40 | 570 | 360 | 335 | 345 | 355 | 215 |
| 2" | DN 50 | 935 | 590 | 550 | 570 | 585 | 355 |
Max version (185 m/s): full scale for other gases, Nm³/h
| Size | DN | Argon | CO2 | Nitrogen | Oxygen | Nitrous oxide (N2O) | Natural gas |
|---|
| ½" | DN 15 | 140 | 90 | 80 | 85 | 85 | 50 |
| ¾" | DN 20 | 275 | 175 | 160 | 165 | 170 | 105 |
| 1" | DN 25 | 460 | 290 | 270 | 280 | 285 | 170 |
| 1¼" | DN 32 | 830 | 525 | 485 | 505 | 520 | 310 |
| 1½" | DN 40 | 1140 | 720 | 670 | 695 | 715 | 430 |
| 2" | DN 50 | 1870 | 1185 | 1100 | 1140 | 1170 | 705 |
High-Speed version (224 m/s): full scale for other gases, Nm³/h
| Size | DN | Argon | CO2 | Nitrogen | Oxygen | Nitrous oxide (N2O) | Natural gas |
|---|
| ½" | DN 15 | 170 | 105 | 100 | 105 | 105 | 65 |
| ¾" | DN 20 | 335 | 210 | 195 | 205 | 210 | 125 |
| 1" | DN 25 | 555 | 350 | 325 | 340 | 345 | 210 |
| 1¼" | DN 32 | 1005 | 635 | 590 | 610 | 630 | 380 |
| 1½" | DN 40 | 1385 | 875 | 815 | 845 | 865 | 520 |
| 2" | DN 50 | 2265 | 1430 | 1330 | 1380 | 1420 | 855 |
Common Questions about KET - Compact Inline Thermal Flow Sensor
No. With the calorimetric measuring method, the operating temperature and operating pressure of the gas have no influence on the measurement result; only the material data of the gas component are decisive. The sensor is adjusted for the ordered gas type, so the correct gas, or a real gas adjustment, is selected when ordering.
Air, nitrogen, argon, carbon dioxide and oxygen are the standard gas types, selected by order code and adjustable through the KEC service software. Nitrous oxide (N2O) and natural gas have their own order codes and flow tables, and further gases or gas mixtures (with the mixture ratio indicated) are available on request.
The KET is ordered with one of six integrated measuring sections, matched to ½", ¾", 1", 1¼", 1½" or 2" pipe (DN 15 to DN 50), with female G (BSP) or NPT threads. Every size has a 5.31 in (135 mm) long measuring section, which fixes the pipe diameter, so no diameter is entered in the menu.
The standard version measures to ±1.5% of reading ±0.3% of full scale. Ordering option J raises this to ±1.0% of reading ±0.3% of full scale. Air calibration is referenced to 20 °C and 1000 mbar (DIN 1945 / ISO 1217), and calibration certificates verifiable against DKD-certified volume flow measuring devices can be issued on request.
No. A flow straightener is built into the measuring section, so no inlet section is needed ahead of the sensor, and the mounting position is unrestricted. What matters at installation is that the flow direction arrows point away from the straightener, the screwed fixture is pressure tight, and the system is unpressurized during fitting.
Connector plug A (M12, A-coded) carries the 18 to 36 V DC supply, the RS 485 Modbus RTU A and B lines, and the 4-20 mA current signal. Connector plug B carries the galvanically isolated pulse or alarm output. A sensor at the end of a Modbus line is terminated with its internal DIP switch or a 120 Ω resistor.
CAL indicates the recommended recalibration date has been reached; measuring continues. Low Voltage appears below an 11 V supply, when flow, consumption and velocity values are no longer available. Heater Error signals a failed heating sensor, Internal Error a read fault in components such as the EEPROM or A/D converter, and Temp out of Range a gas temperature outside specification.
Volume flow and consumption values shown on the display are related to 68 °F (20 °C) and 1000 hPa by factory setting (ISO 1217 intake condition). Alternatively, 32 °F (0 °C) and 1013 hPa (standard cubic meter) can be entered as the reference. The operating pressure and operating temperature are never entered under reference conditions.
The pulse output transmits total consumption to a counter or energy monitoring system: each pulse represents a set quantity, 0.1 l, 1 l, 0.1 m³ or 1 m³, chosen on the display (kg and cubic feet are also selectable). Maximum frequency is 50 pulses per second, with a 1 second delay, and pulse values that cannot represent full scale are rejected.