THERMOVAC MEMS Vacuum Gauge Guide

Leybold THERMOVAC series MEMS Pirani vacuum transmitters: a complete guide to selection, installation, and troubleshooting

Introduction: When traditional Pirani vacuum gauges cannot meet modern process requirements

In the field of vacuum technology, the accuracy of pressure measurement is directly related to process quality and equipment safety. The traditional Pirani vacuum gauge uses slender hot wires as sensing elements, which are prone to damage due to vibration, impact, or contamination, and have limited response speed. Leybold's THERMOVAC series (TTR 91 N, TTR 96 N, TTR 911 N, TTR 916 N) uses advanced MEMS Pirani solid-state sensors to apply microelectromechanical systems (MEMS) technology to thermal conductivity vacuum measurement. This sensor is composed of heating resistor elements on a silicon chip, which has significant advantages such as anti vibration, anti impact, fast response, and high repeatability. In addition, the coating version (C model) uses Parylene HT coating, which significantly improves its chemical corrosion resistance and is suitable for harsh process environments.

This article is based on the THERMOVAC series product data manual, providing a detailed technical guide from technical principles, model comparison, installation and wiring, gas correction, digital interfaces, fault diagnosis to maintenance and calibration, to help engineers correctly select and use in replacing old models or building new systems.

Principles and advantages of MEMS Pirani technology

1.1 Differences between Traditional Pirani and MEMS Pirani

The traditional Pirani vacuum gauge utilizes the relationship between the heat loss of a hot wire (usually tungsten or platinum) and gas pressure. As the pressure decreases, the number of gas molecules decreases, the heat dissipation of the hot wire deteriorates, the temperature increases, and the resistance changes. This method is simple and reliable, but the hot wire is slender, prone to breakage, sensitive to vibration, and has a large heat capacity, resulting in a slow response.

The MEMS Pirani sensor integrates heating resistors onto tiny silicon membranes and is manufactured using microfabrication technology. Its advantages include:

Extremely high mechanical strength: Silicon chips can withstand severe vibrations and impacts without the risk of filament breakage.

Quick response: The small thermal mass makes the thermal equilibrium time extremely short.

Low power consumption: Operating power consumption<1.2 W.

Good repeatability: The semiconductor process ensures consistency.

Small size: The sensor size is only a few millimeters, which is conducive to integration.

1.2 Measurement principle

THERMOVAC transmitters are based on the principle of thermal conductivity measurement. The heating resistor in the MEMS Pirani sensing unit is subjected to a constant power, and its temperature depends on the thermal conductivity of the surrounding gas. The higher the gas pressure, the higher the thermal conductivity, and the lower the resistance temperature. By measuring the resistance value (or the power required to maintain a constant temperature), the gas pressure can be inferred. The output signal is linearized to provide a voltage proportional to the logarithm of pressure.

1.3 Coating version (C model)

For corrosive process gases such as halogens and acidic gases, the metal and silicon surfaces of standard MEMS Pirani sensors may be corroded. Parylene HT is a high-performance polymer coating with excellent thermal stability (able to withstand 350 ° C for short periods of time) and chemical inertness. The sensors with coated versions (TTR 911 CN, TTR 916 CN) have been coated with Parylene HT, significantly improving their corrosion resistance and making them suitable for harsh chemical processes.

Detailed explanation of product series and models

The THERMOVAC series includes multiple models, covering a wide range of needs from basic analog outputs to advanced digital interfaces.

2.1 TTR 91 N (No Switching Output)

Measurement range: 5.0 × 10 ⁻⁵~Atmospheric pressure (1000 mbar)

Output: Analog voltage (logarithmic characteristic)

Interface: Analog only

Vacuum connection: DN 16 ISO-KF, ⅛ "NPT, DN 16 CF (can be baked to 85 ° C)

Applicable scenarios: General coarse/medium vacuum measurement, connected to a controller (such as PLAY ONE/GRAPHIX ONE)

2.2 TTR 91 SN/TTR 96 N (with switch output)

TTR 91 SN: 2 set point relays (SP1, SP2), DN 16 ISO-KF or NPT

TTR 96 N: 2 set point relays, coated sensor (Parylene HT), suitable for corrosive media

Switch range: 2.7 × 10 ⁻⁴~1000 mbar

Contact capacity: 1 A @ 30 V AC/DC (resistive load)

Contact life: ≥ 100000 times under 1A load; ≥ 2000000 times under 0.2 A load

Applicable scenarios: requiring on-site interlocking, alarm or control (no PLC required)

2.3 TTR 911 N/911 SN/911 CN (digital interface model)

TTR 911 N: Equipped with LCD touch screen display, displaying pressure values, units, and set point status. No digital interface.

TTR 911 SN: with RS 232 interface (+3 set point relays), can be used for communication with PC or PLC.

TTR 911 CN: with EtherCAT or Profibus interface (coated sensor), suitable for industrial fieldbus networks.

Measurement range: standard 5 × 10 ⁻⁵~atmospheric pressure; It can be extended to 1 × 10 ⁻⁵~atmospheric pressure (RS 232/EtherCAT/display version) through a digital interface.

Applicable scenarios: automation integration, remote monitoring, data recording.

2.4 TTR 916 N (coated version with display screen)

Measurement range: 5 × 10 ⁻⁵~atmospheric pressure

Sensor: Coated MEMS Pirani (Parylene HT)

Interface: No digital interface, only analog output+2 set point relays

Display: Touch screen, displaying pressure, set point, status

Applicable scenarios: In corrosive processes, on-site display and local switch control are required

2.5 Special version: Long tube version

For applications that require higher chamber baking temperatures (such as UHV systems), provide a long tube version that separates the sensor from the electronic unit, keeping the electronic unit away from high-temperature areas. The sensor itself can withstand non operational baking at 85 ° C, and the electronic unit should be kept at 0-40 ° C.

Technical specification comparison

Parameters TTR 91 N/91 SN TTR 96 N TTR 911 N/911 SN/911 CN TTR 916 N

Measurement range (mbar) 5 × 10 ⁻⁵~1000 5 × 10 ⁻⁵~1000 5 × 10 ⁻⁵~1000 (standard); 1 × 10 ⁻⁵~1000 (digits) 5 × 10 ⁻⁵~1000

Sensor MEMS Pirani Coating MEMS Pirani/Coating (C) Coating MEMS Pirani

Measurement uncertainty 5 × 10 ⁻⁴~1 × 10 ⁻³: ± 10%;  1×10⁻³~100: ±5%; 100~Atmospheric pressure: ± 25% Same left Same left Same left Same left

Repeatability ± 2% (1 × 10 ⁻ ³~100 mbar) same left, same left, same left

Analog output Vout=log ₁₀ (P_mbar) × 1.286+6.143 (1.1~10 V) Same left, same left, same left

Number of designated points  0 (TTR 91 N) / 2 (TTR 91 SN) 2 2 (SN) / 2 (CN via Profibus) / 3 (SN via RS 232) 2

Digital interface None None RS 232/EtherCAT/Profibus None

Display screen without touch screen (911 N/SN/CN) touch screen

Power supply 9~30 V DC 9~30 V DC 9~30 V DC 9~30 V DC 9~30 V DC

Power consumption<1.2 W<1.2 W<1.2 W<1.2 W<1.2 W

Electrical Connection FCC68/RJ45 FCC68/RJ45 FCC68/RJ45 FCC68/RJ45

Working temperature 0~40 ° C 0~40 ° C 0~40 ° C 0~40 ° C

Baking temperature 85 ° C (non operating) 85 ° C (non operating) 85 ° C (non operating) 85 ° C (non operating)

Weight (DN 16 KF)~170 g~170 g~168 g (not shown)~170 g

Protection level IP40 IP40 IP40 IP40 IP40

Attention: All precision and repeatability data are typical values, measured using nitrogen at ambient temperature and obtained after zero adjustment.

Installation and electrical connection

4.1 Mechanical Installation

Installation direction: arbitrary. Due to the fact that MEMS Pirani sensors are not affected by gravity, the measurement signal is not sensitive to the installation position.

Vacuum connection: DN 16 ISO-KF is standard configuration, using centering rings and O-rings (Viton or other). The CF flange version is used for ultra-high vacuum systems.

Dead volume: approximately 2.8 cm ³ (DN 16 KF version), with minimal impact on system pumping time.

Baking: It can be baked to 85 ° C when not in operation. Note: The electronic unit should not be baked, and the long tube version can separate the sensor from the electronic unit.

4.2 Electrical Connections

All models use FCC68/RJ45 8-pin connectors (standard Ethernet cables). Pin definition (similar to TTR 101):

Pin 1: Power supply+(9~30 V DC)

Pin 2: Power supply GND

Pin 3: Analog output/Setpoint threshold output

Pin 4: Transmitter identification (identify model by resistance value)

Pin 5: Signal common terminal

Pin 6,8: Relay SP2 normally open contact

Pin 7-8: Relay SP1 normally open contact

Cable requirements:

Maximum length: 100 meters

Recommend using shielded cables (Leybold provides prefabricated 5-100 meter cables, part numbers can be found in the ordering information)

Shielding layer single ended grounding (on the controller side)

4.3 Power Supply and Grounding

The power supply voltage is 9~30 V DC, and 24 V DC is recommended.

Power consumption<1.2 W, can be powered by the controller or an independent power source.

The transmitter housing must be grounded through a vacuum system (KF flange using a metal clamping ring).

Simulation output characteristics and calibration

5.1 Logarithmic output formula

The analog output voltage of THERMOVAC transmitter is proportional to the logarithm of pressure (mbar):

Vout = log₁₀(P_mbar) × 1.286 + 6.143

The unit of Vout is volts. This formula is valid within the range of 5 × 10 ⁻⁵ mbar to 1000 mbar.

Theoretical output voltage of pressure (mbar) (V)

5×10⁻⁵ 0.61

1×10⁻⁴ 1.00

1×10⁻³ 2.29

1×10⁻² 3.57

1×10⁻¹ 4.86

1 6.14

10 7.43

100 8.72

1000 10.00

5.2 Integration with PLC

In PLC, the following formula can be used to convert voltage readings into pressure (mbar):

P_mbar = 10^((Vout - 6.143) / 1.286)

If using Torr units, the pressure value needs to be multiplied by a conversion factor (1 mbar=0.75 Torr).

5.3 Zero point adjustment

All THERMOVAC transmitters are calibrated at the factory. Zero drift may occur after long-term use, and it is recommended to regularly (annually) adjust the zero point. Adjustment steps (refer to a similar process in TTR 101):

Install the transmitter on the vacuum system and preheat it with power for at least 10 minutes.

Extract the system to<5 × 10 ⁻⁵ mbar and hold for at least 2 minutes.

Use the accompanying adjustment tool (needle) to press the "ADJ" button (if available), or send adjustment commands through the digital interface.

The transmitter sets the current pressure to zero (5 × 10 ⁻⁵ mbar).

After restoring atmospheric pressure, atmospheric pressure adjustment can be performed (if this function is available).

Note: For models without buttons (such as TTR 91 N), zero adjustment needs to be completed through a controller or digital interface.

Gas type dependence

The MEMS Pirani sensor is based on thermal conductivity, so the relationship between the indicated pressure and the actual pressure depends on the thermal conductivity of the gas. The reading deviation of different gases within the range of 1 × 10 ⁻ ³~100 mbar can be converted by a correction factor. Usually, transmitter factory calibration is for nitrogen/air.

Correction factors for common gases (refer to TTR 101 manual):

He, H ₂: 0.8

Ne：1.4

Air, N ₂, O ₂ CO：1.0

Ar, CO ₂: 1.7

Kr、 Water vapor: 2.4

Xe、Freon：3.0

For the coating version (C model), the correction factor is basically the same because the coating thickness is extremely small and does not affect the thermal conductivity characteristics.

Important note: Above 100 mbar, the relationship between thermal conductivity and pressure is no longer linear, and the measurement accuracy significantly decreases (± 25%). For high-precision measurements near atmospheric pressure, a capacitive thin film vacuum gauge should be used.

Set point relay configuration (S model)

TTR 91 SN, TTR 96 N, TTR 911 SN, and TTR 916 N provide 2 or 3 programmable set point relays. Each relay can independently set a threshold and trigger mode (low point trigger or high point trigger).

7.1 Setting method

Using the buttons (TTR 91 SN/96 N): Similar to the operation of TTR 101, press the SP1/SP2 button to enter the setting mode and adjust the threshold with a needle.

Through the display screen (TTR 911 N/916 N): touch screen operation, intuitively set thresholds and hysteresis.

Through the digital interface (TTR 911 SN/CN): write parameters via RS 232, EtherCAT, or Profibus.

7.2 Delay

The default hysteresis is 10% of the threshold. For example, if a low point trigger is set (the relay closes when the pressure is below the threshold), with a threshold of 1 × 10 ⁻ ² mbar, the relay closes when the pressure drops to 1 × 10 ⁻ ² mbar and opens when the pressure rises to 1.1 × 10 ⁻ ² mbar to avoid shaking.

7.3 Contact Capacity and Lifespan

Rated load: 1 A @ 30 V AC/DC resistive

Minimum load: 10 mA @ 5 V (recommended for low-level signals)

Lifespan: 100000 cycles under 1A load; 2000000 cycles under 0.2 A load

Attention: The relay is a solid-state type with no mechanical contacts, but still has a conducting resistance (≤ 100 m Ω). Not suitable for inductive loads (such as solenoid valves), an external intermediate relay is required.

Digital interface and fieldbus

8.1 RS 232（TTR 911 SN）

Interface type: RS 232, 3-wire (Tx, Rx, GND)

Baud rate: configurable (usually 9600 or 19200)

Function: Read pressure value, read/write set point, zero/atmospheric pressure adjustment, diagnosis

Applicable scenario: point-to-point communication with PC or old PLC

8.2 EtherCAT（TTR 911 CN）

Compliant with IEC 61158 Type 3 standard

Integrated into industrial Ethernet networks with high real-time performance

Support CoE (CANopen over EtherCAT) protocol

Need GSD file (can be downloaded from Leybold website)

8.3 Profibus（TTR 911 CN）

Profibus DP slave station

The node address can be set through hardware dip switches or software settings

Automatic transmission rate detection

Provide GSD files

Note: Profibus and EtherCAT versions do not provide display screens, all parameters are set through the bus.

Troubleshooting and Maintenance

9.1 Common Fault Phenomena and Countermeasures

Possible causes and solutions for the phenomenon

No analog output, LED ring not lit, power supply missing or polarity reversed. Check for 9-30 V DC power supply and confirm positive and negative poles

Output signal constant 0 V (<0.5 V) sensor malfunction or connection problem check cable; Replace the sensor (see 9.2)

Abnormal high or low zero drift of output signal; The gas type has not been corrected and zero point adjustment has been performed; Apply gas correction factor

LED ring red flashing sensor error, power off and restart; If it still exists, replace the sensor

The relay does not operate and the set point is not set correctly; The pressure has not reached the threshold check set value; Confirm trigger direction (low/high)

Digital communication failure, cable disconnection; Baud rate mismatch; Node address conflict check for physical connections; Confirm communication parameters; Replace GSD file

The reading fluctuates greatly, and there is airflow impact at the installation location; Installing electromagnetic interference barriers; Check the shielding grounding

9.2 Sensor replacement

Models such as TTR 916 N and TTR 911 CN offer replaceable sensors (part numbers 230650V02 or 230651V02). Replacement steps:

Remove the transmitter from the vacuum system and release it to atmospheric pressure.

Disconnect the power and cables.

Use a special tool (hex wrench) to loosen the fixing screw between the sensor and the electronic unit.

Pull out the old sensor vertically and insert the new sensor (pay attention to the direction).

Retighten the screw (torque approximately 0.5 Nm).

After reinstallation, zero point adjustment must be performed.

Attention: After replacing the sensor, the calibration data of the transmitter may be lost. It is necessary to perform a complete ATM and HV adjustment.

9.3 Cleaning

If the sensor is contaminated with dust, it can be gently blown with dry compressed air or nitrogen. Avoid using liquid cleaning agents or contact with sensor surfaces. For severe pollution, it is recommended to replace the sensor.

9.4 LED ring status indication

Green constantly on: Normal operation, pressure within measurement range.

Green flashing: warming up or initializing.

Red constantly on: Sensor error (such as filament disconnection).

Red flashing: Communication error or set point out of range.

Precautions for replacing old models

If you are using the old THERMOVAC (such as TTR 90, TTR 100) or other brands (such as Pfeiffer, MKS, Granville Phillips) Pirani vacuum gauge, please note when replacing with TTR 91/911 series:

Output characteristics: Old models may output linear voltage (0-10V corresponds to 1e-4~1000 mbar), while TTR series outputs logarithmic voltage. The calibration curve must be reconfigured in the controller.

Power supply voltage: The TTR series supports 9~30 V DC and is compatible with most 15 V or 24 V systems. Check the power supply capability of the old system.

Cable: The TTR series uses RJ45 connectors, which may require adapters or reworked connectors. Leybold provides prefabricated cables (5-100m).

Flange size: TTR standard is DN 16 ISO-KF. If the old system is DN 25 KF or half "pipe, a variable diameter flange or special version (such as CF or NPT) needs to be ordered.

Digital communication: If the old system uses RS 485 or DeviceNet, TTR 911 SN/CN provides RS 232, EtherCAT or Profibus, which may require a protocol converter.

Set point logic: The default triggering method of TTR relay (low point triggering) may be opposite to the old system. You can change it to high point triggering in the settings or modify the PLC logic.

Ordering Information and Accessories

11.1 Transmitter model (some part numbers)

Model and Part Number

TTR 91 N, DN 16 ISO-KF 230035V02

TTR 91 SN, DN 16 ISO-KF, 2SP 230040V02

TTR 96 N, DN 16 ISO-KF, 2SP (coating) 230045V02

TTR 911 N, DN 16 ISO-KF, Equipped with display screen 89654V02

TTR 911 CN, DN 16 ISO-KF, EtherCAT, Coating 230701V02

TTR 911 SN, DN 16 ISO-KF, RS 232 89660V02

TTR 916 N, DN 16 ISO-KF, Equipped with display screen and coating 89656V02

11.2 Accessories

Connecting cables (FCC68 at both ends, 8-core shielded): 5 m (12426)、10 m (230012)、15 m (12427)、20 m (12428)、30 m (12429)、50 m (12431)、75 m (12432)、100 m (12433)

Spiral tube DN 16 ISO-KF (for flexible connections): 230082

Interchangeable sensor (TTR 916 N): 230650V02

Interchangeable sensor (TTR 916 CN): 230651V02